THE   PEINCIPLES 
OF  APPLIED   ELECTEOCHEMISTEY 


THE  PRINCIPLES 

OF 

APPLIED    ELECTROCHEMISTRY 


BY 


A.  J.  ALLMAND,  D.Sc. 

FELLOW  OP   THE   CHEMICAL   SOCIETY;  MEMBER  OP   THE   FARADAY   SOCIETY 
OF   THE  BUNSEN   GESELLSCHAFT 


ILLUSTRATED 


•   -.       ' 


NEW    YORK 

LONGMANS,    GREEN,    AND    CO. 

LOXDOX  :   EDWARD   ARNOLD 

1912 


TO 

M.  E.  A. 


PREFACE 

IN  writing  this  volume,  which  is  designed  for  both  technical  men  and 
students,  I  have  treated  the  subject  primarily  from  the  standpoint  of 
the  theory  and  principles  involved,  being  convinced  that  through 
their  thorough  comprehension  the  best  results  are  to  be  obtained  in 
practice.  At  the  same  time  I  have  endeavoured  to  describe  accurately 
and  with  a  certain  amount  of  detail  the  methods  at  present  technically 
used.  Such  a  book  (Grundriss  der  technischen  Elektrochemie)  was 
written  in  1898  by  my  teacher  Professor  F.  Haber,  but,  though  reprinted, 
has  not  been  brought  up  to  date  or  translated  into  English. 

The  present  book  falls  into  two  sections.  Assuming  an  elementary 
knowledge  of  chemistry  and  electricity,  Part  I.  is  chiefly  concerned 
with  the  treatment  of  the  fundamental  phenomena  and  theory  of  the 
electrochemistry  of  aqueous  solutions.  Particular  stress  is  laid  on 
irreversible  effects,  which  though  of  very  great  importance  in  technical 
electrochemistry,  seldom  receive  the  attention  they  deserve.  My 
great  debt  to  Professor  F.  Foerster's  Elektrochemie  wasseriger  Losungen 
will  here,  as  at  certain  points  in  Part  II.,  be  clear  to  all.  Other  chapters 
deal  generally  with  the  electrolysis  of  fused  melts,  electrothermics, 
and  the  discharge  of  electricity  through  gases.  A  chapter  on  Equili- 
brium is  included.  I  have  tried  as  far  as  possible  to  introduce  no  more 
theory  than  "is  applied  at  some  point  or  other  during  the  book,  and 
employ  frequent  illustrative  examples,  numerical  or  otherwise. 

Part  II.  treats  separately  of  the  various  technical  processes  used, 
discussing  inter  alia  primary  and  secondary  cells.  Here  I  have  aimed 
in  the  first  instance  at  an  adequate  description  of  all  those  methods 
actually  worked  at  the  present  time.  Other  processes  are  only  dis- 
cussed if  (1)  they  are  likely  to  be  used  in  the  near  future,  (2)  although 
obsolete,  they  have  considerable  historical  value,  (3)  although  they 
have  never  passed  the  experimental  stage,  the  reasons  for  their  failure 
are  instructive.  Throughout  I  have  attempted  to  show  the  close 
connection  between  theory  and  practice,  to  treat  the  available  material 
critically,  and  to  exclude  '  paper  '  processes.  The  diagrams  are  designed 
to  emphasize  the  main  features  of  the  apparatus  concerned,  not  so  much 

258689 


vi  PKEFACE 

its  details.  Many  working  details  are  omitted  in  the  text  also,  as 
further  are  statistics  and  questions  of  costs.  To  deal  with  such 
matters  would  materially  alter  the  scope  of  the  book.  The  important 
subject  of  power  production  is  however  briefly  discussed  in  Part  I. 

I  owe  to  Professor  F.  G.  Donnan,  F.R.S.,  who  proposed  to  me  in  the 
first  instance  the  writing  of  this  book,  and  who  has  criticised  practically 
the  whole  of  the  manuscript,  my  warmest  thanks  for  his  many  sugges- 
tions. To  Mr.  G.  F.  Horsley,  of  the  United  Alkali  Co.  Ltd.,  I  am  much 
indebted  for  his  valuable  criticism  of  most  of  Part  I.  He  and  Dr.  J.  T. 
Barker  have  read  through  the  proofs,  and  Mr.  F.  D.  Farrow  has  kindly 
verified  the  literature  references. 

For  new  data,  permission  to  use  diagrams,  &c.,  I  owe  acknowledge- 
ments, amongst  others,  to  Professor  P.  Askenasy  ;  Professor  W.  D. 
Bancroft ;  Dr.  J.  Billiter ;  Mr.  V.  Engelhardt ;  Mr.  R.  Finlay  ; 
Professor  F.  Foerster  ;  Professor  P.  A.  Guye  ;  Mr.  W.  E.  Holland  ; 
the  Reason  Manufacturing  Co.  Ltd.  ;  the  American  Electrochemical 
Society  ;  the  Faraday  Society  ;  the  publishing  firms  J.  A.  Barth 
(Leipzig),  L.  Foss  (Leipzig),  W.  Knapp  (Halle),  F.  Vieweg  (Braun- 
schweig). 

In  conclusion  I  need  hardly  say  that  I  shall  be  most  grateful  for  any 
corrections  on  points  of  fact  or  for  suggestions  for  improved  treatment. 

To  those  who  wish  to  study  the  subject  further,  I  recommend,  besides 
the  books  of  Haber  and  Foerster  mentioned  above  and  the  numerous 
volumes  referred  to  at  the  ends  of  the  different  chapters,  the  following  : 

Einfuhrung  in  die  technische  Elektrochemie  (edited  by  Askenasy). 

Die  elektrochemischen  Verfahren  der  chemischen  Gross-Industrie 
(Billiter). 

Laboratory  Electric  Furnaces  (Slade  ;   about  to  be  published). 

Continuous  Current  Engineering  (Hay). 

Alternating  Currents  (Hay). 

The  first  two  books  are  particularly  valuable  for  technical  details  ; 
the  third  treats  of  laboratory  experimental  methods  ;  the  last  two 
deal  with  the  electrotechnical  side  of  the  subject  in  an  elementary 
manner. 

A.  J.  ALLMAND. 
LIVERPOOL, 
September,  1912. 


CONTENTS 


PAGES 

LIST  OF  ABBREVIATIONS      ........      xi 

LIST  OF  SYMBOLS  USED  .         .    xii 


PART    I.— GENERAL  AND  THEORETICAL 
CHAPTEE  I 

INTRODUCTORY POWER 

Chemical    and    Electrochemical    Methods    compared — Electrical    Units — 

Power 3-11 

CHAPTER  II 

EQUILIBRIUM 

General — Homogeneous   Equilibria — Heterogeneous    Equilibria — Effect    of 

Temperature 12-25 

CHAPTEE  III 
FARADAY'S  LAWS — CURRENT  EFFICIENCY 

Phenomena  of  Electrolysis — Faraday's  Laws — Current  Efficiency — Measure- 
ment of  Quantity  of  Electricity — Calculation  of  Current  Efficiencies  26-37 

CHAPTEE  IV 
OSMOTIC    PRESSURE THEORY    OF    SOLUTIONS 

Osmotic  Pressure — Solution  Laws — Determination  of  Molecular  Weight  of 

Dissolved  Substances — Anomalous  Behaviour  of  Electrolytes          .         38-48 

CHAPTEE  V 

IONIC    TRANSPORT    DURING    ELECTROLYSIS 

Mechanism  of  Migration  of  Ions — Quantitative  Eelations  of  Ionic  Migra- 
tion I — Determination  of  Transport  Numbers — Quantitative  Eelations 
of  Ionic  Migration  II — Applications  of  Ionic  Migration  Phenomena  49-57 


viii  CONTENTS 


CHAPTER  VI 
CONDUCTIVITY    OF    ELECTROLYTES THEORY    OP  ELECTROLYTIC  DISSOCIATION 

PAGES 

Specific    Conductivity — Determination   of  Conductivity — Equivalent   Con- 
ductivity— Electrolytic  Dissociation  Theory 58-76 


CHAPTER  VII 
KNKRGY    RELATIONS 

Total  Energy  and  Maximum  External  Work — Reversible  Processes — 
Irreversible  Processes — Relations  in  Reversible  Galvanic  Cells — 
Relations  during  Reversible  Electrolysis — Maximum  Work  and 
Affinity  77-86 


CHAPTER  VIII 
ELECTROMOTIVE    FORCE 

Necessary  Conditions  for  Electrochemical  Reactions — Measurement  of 
Electromotive  Force — Electrolytic  Solution  Pressure — Quantitative 
Relations  at  Ionising  Electrodes — Gas  Electrodes — Oxidation-Reduc- 
tion Electrodes — Concentration  Cells — Measurement  of  Single  Electrode 
Potentials  87-105 


CHAPTER  IX 

ELECTROLYSIS    AND    POLARISATION ENERGY   EFFICIENCY 

Polarisation— Energy  Efficiency— Factors  Affecting  Electrolysis  .         .     106-116 

CHAPTER  X 
CATHODIC   AND    ANODIC    PROCESSES    IN   DETAIL 

A.  Cathodic  Processes.  Evolution  of  Hydrogen — Cathodic  Metal  Deposition 
— Electrolytic  Reduction. — B.  Anodic  Processes.  Discharge  of  Anions 
—Solution  of  Metals— Electrolytic  Oxidation  ....  1 17-147 

CHAPTER  XI 

THE    ELECTROLYSIS    BATH 

Arrangement — Voltage — Technical  Electrodes  and  Diaphragms     .         .     148-157 

CHAPTER  XII 

MOLTEN   ELECTROLYTES 

Phenomena  of  Electrolysis — Metal  Fog — Anode  Effect  ....     158-166 

CHAPTER  XIII 
GENERAL    PRINCIPLES    OP    K1.K' TKnTH  Kl;  MICS 

Electric  Heating — General  Principles  of  Electric  Furnace  Design — Electrical 

Aspects  of  Electric  Furnace  IV  167-182 


CONTENTS  ix 

CHAPTER  XIV 
ELECTRICAL    DISCHARGES    IN    GASES 

PAGES 

Gas  Ions — Different  Types  of  Discharge — Chemical  Effects  .         .         .     183-192 

PART   II.— SPECIAL  AND  TECHNICAL 
CHAPTER  XV 

PRIMARY    CELLS 
General  Considerations — Primary  Cells  in  General  Use — Fuel  Cells      .     195-219 

CHAPTER  XVI 

SECONDARY    CELLS 

General  Considerations — Lead  Accumulator — Iron  Accumulator   .         .     220-244 

CHAPTER  XVII 
COPPER SILVER GOLD 

Copper  Refining,  Theory — Copper  Refining,  Practice — Copper  Extraction 

— Silver  Refining — Gold  Refining — Gold  Extraction       .         .         .     245-280 

CHAPTER  XVIII 

ZINC TIN NICKEL IRON LEAD VARIOUS 

Electrometallurgy  of  Zinc — Electrometallurgy  of  Tin — Electrometallurgy 
of  Xickel — Electrolytic  Refining  of  Iron — Electrometallurgy  of  Lead- 
Bismuth  and  Antimony  281-307 

CHAPTER  XIX 
ELECTROPLATING   AND    ELECTROTYPING 308-317 

CHAPTER  XX 
HYPOCHLORITES    AND    CHLORATES 

General  Theory — Hypochlorites,  Theory — Hypochlorites,  Technical — Chlor- 
ates, Theory— Chlorates,  Technical 318-341 

CHAPTER  XXI 
ALKALI-CHLORINE    CELLS 

General  Theory — Mercury  Cells — Diaphragm  Cells  with  Stationary  Electro- 
lyte—Cells with  Moving  Electrolyte— Comparative  .  .  .  342-385 


x  CONTENTS 

CHAPTER  XXII 
OTHER    ELECTROLYTIC    PROCESSES 

PAGES 

Hydrogen  and  Oxygen — Regeneration  of  Chromic  Acid — Potassium  Per- 
manganate— Perchlorates,  etc.  ..."  ...  386-406 

CHAPTER  XXIII 

METALS    FROM    FUSED    ELECTROLYTES CAUSTIC    SODA   AND    CHLORINE 

FROM   FUSED    SALT 

Sodium — Magnesium — Calcium — Zinc — Aluminium — Acker  Process     .     407-433 

CHAPTER  XXIV 
ELECTROTHERMICS    IN    THE    IRON    AND    STEEL    INDUSTRY 

Pig-iron  Production — Electric  Steel,  General — Arc  Furnaces — Induc- 
tion Furnaces — Comparative — Ferro- Alloys  .....  434-468 

CHAPTER  XXV 
CALCIUM    CARBIDE    AND    CALCIUM    CYANAMIDE 

Carbide,  General  and  Theory — Carbide,  Technical — The  Nitrogen  Problem 

— Cyanamide,  General  and  Theory — Cyanamide,  Technical      .         .     469-485 

CHAPTER  XXVI 
OTHER    ELECTROTHERMAL    PRODUCTS 

Carborundum  and  Allied  Products— Graphite — Alundum,  Silica — Distillation 

Products,  Carbon  Bisulphide,  Phosphorus,  Zinc        ....     486-504 

CHAPTER  XXVII 
THE    OXIDATION    OF    ATMOSPHERIC    NITROGEN 

Theoretical — Early  Attempts  at  Technical  Apparatus — Birkeland-Eyde 
Process — Pauling  Process — Schonherr-Hessberger  Process — Working 
up  of  Furnace  Gases 505-521 

CHAPTER  XXVIII 
OZONE       .  522-527 

APPENDICES 

I.     Transport    Number    of    Anion    for    Different    Aqueous    Salt    Solu- 
tions at  18°  528 

II.     Current  Densities  Used  in  Technical  Practice        .....     528 

III.  V it-Ids  and  Energy  Expenditure  in  Technical  Practice          .         .  530 

IV.  Theoretical  Quantities   of   Electrolytic  Products  resulting  from    the 

passage  of  1  ampere-hour  of  Electricity     .....  531 

INDEX  OF  AUTHORS  AND  FIRMS 532 

SUBJECT  INDEX 537 


LIST  OF  ABBREVIATIONS 


Abhand.  Bunsen  Ges.    . 

Ann.  Chim.  Phys. 

Amer.  Chem.  Jour. 

Ber 

Berg-und  Hutten.  Zeit.  . 
Bull.  Soc.  Chim.   . 

Chem.  Ind.    ..... 

Chem.  Zeit 

Compt.  Rend. 

Ding.  Poly.  Jour.  . 

Drud.  Ann.  .... 

Electr 

Electrochem.  Ind. . 

Electrochem.  and  Metall. 
Elektrochem.  Zeitsch.     . 
EleMrotech.  Zeitsch. 
Engin.  ..... 

Jahrb.  der  Rad.     . 

Jour.  Amer.  Chem.  Soc. 

Jour.  Chim.  Phys. 
Jour.  Four  Elect. 
Jour.  Phys.  Chem. 
Jour.  Prakt.  Chem. 
Jour.  Soc.  Chem.  Ind.    . 

Lieb.  Ann.     .... 

Metall 

Metall.  Chem.  Engin.    . 
Manch.  Mem. 

Philos.  Mag. 

Proc.  Roy.  Soc. 

Trans.  Amer.  Electrochem.  Soc. 

Trans.  Chem.  Soc. 
Trans.  Farad.  Soc. 
Wied.  Ann.  .... 

Zeitsch.  Angew.  Chem.  . 
Zeitsch.  Anorg.  Chem.    . 
Zeitsch.  Elelctrochem. 
Zeitsch.  Phys.  Chem.      . 


Abhandlungen  der  Deutschen  Bunsen 
Gesellschaft. 

Annales  de  Chimie  et  de  Physique. 

American  Chemical  Journal. 

Berichte  der  Deutschen  Chemischen 
Gesellschaft. 

Berg-und  Huttenmannische  Zeitung. 

Bulletin  de  la  Societe  Chimique  de 
Paris. 

Chemische  Industrie. 

Chemiker  Zeitung. 

Comptes  Rendus  des  Seances  de 
1'Academie  des  Sciences. 

Dinglers  Polytechnisches  Journal. 

Annalen  der  Physik  (Fourth  Series). 

Electrician. 

Electrochemical  and  Metallurgical  In- 
dustry. 

Electrochemist  and  Metallurgist. 

Elektrochemische  Zeitschrift. 

Elektrotechnische  Zeitschrift. 

Engineering. 

Jahrbuch  der  Radioaktivitat  und  Elek- 
tronik. 

Journal  of  the  American  Chemical 
Society. 

Journal  de  Chimie  Physique. 

Journal  de  la  Four  Electrique. 

Journal  of  Physical  Chemistry. 

Journal  fur  Praktische  Chemie. 

Journal  of  the  Society  of  Chemical 
Industry. 

Annalen  der  Chemie. 

Metallurgie. 

Metallurgical  and  Chemical  Engineering. 

Memoirs  of  the  Manchester  Literary 
and  Philosophical  Society. 

Philosophical  Magazine. 

Proceedings  of  the  Royal  Society. 

Transactions  of  the  American  Electro- 
chemical Society. 

Transactions  of  the  Chemical  Society. 

Transactions  of  the  Faraday  Society. 

Annalen  der  Physik  (Third  Series). 

Zeitschrift  fur  Angewandte  Chemie. 

Zeitschrift  fur  Anorganische  Chemie. 

Zeitschrift  fur  Elektrochemie. 

Zeitschrift  fur  Physikalische  Chemie. 


LIST  OF  SYMBOLS  USED 

A  '  Free  energy '  decrease. 

C  Concentration  in  grams. 
litre 

[C]  Concentration  in  gram'molecules. 
litre 

[C],,  Concentration  in  gram.equivalents 

litre 

Cp,  Cp  Molecular  specific  heats  at  constant  pressure  and  volume. 

Cal.  Large  (kilo-)  calorie. 

E  Potential,  voltage. 

E.M.F.  Electromotive  force. 

F  Valence  charge  (96540  coulombs). 

I  Current. 

K  Constant ;  particularly  equilibrium  constant. 

L  Latent  heat  of  change  of  state. 

L  Coefficient  of  self-induction. 

M  Molecular  weight. 

Mol.  Gram  molecule. 

P  Osmotic  pressure. 

Q  Quantity  of  heat. 

K  Resistance. 

R  Gas-constant. 

S.G.  Specific  gravity. 

T  Absolute  temperature. 

U  '  Total  energy  '  decrease. 

UA,  Uc  Ionic  mobility  (velocity  under  gradient  1  ^Q\. 
\  c.m./ 

V  Velocity. 

a  Area. 

c  Specific  heat, 

cal  Small  (gram-)  calorie. 

k  Constant ;  velocity  constant. 

I  Length. 

lc,  I*.  Ionic  conductivity. 

TO  Mass. 

n  Transport  number. 

n  Periodicity  (alternating  current). 

p  Pressure. 

q  Heat  effect  in  reversible  process. 

r  Specific  resistance. 

t  Time. 

i*c,  WA  Ionic  velocity  (actual,  under  experimental  conditions). 

/  Volume. 

a  Degree  of  dissociation. 

8  Specific  gravity. 

«  Single  electrode  potential. 

K  Specific  conductivity. 

A  Equivalent  conductivity. 

H  Concentration  in  E""" -equivalents 
c.m. 

0  Temperature  in  degrees  C. 

cos  6  Power  factor. 

(a)  One  ton  is  taken  as  1000  kilos. 

(6)  A  H.P.  year  is  taken  as  8760  H.P.H. 


PART  I 

GENERAL   AND  THEORETICAL 


CHAPTER  I 

INTRODUCTORY— POWER 

1.  Chemical  and  Electrochemical  Methods  Compared 

CHEMICAL  reactions  can  be  divided  into  two  classes— those  which  give 
out  energy  (usually  in  the  form  of  heat),  and  those  which  absorb  energy, 
whilst  they  are  taking  place.  Examples  of  the  former  class  are  the 
combustion  of  fuels,  the  Goldschmidt  Thermite  processes,  the  slaking 
of  lime.  To  the  latter  class  belong  the  reduction  of  metals  from  their 
ores,  and  the  formation  from  their  elements  of  oxides  of  ^nitrogen  or 
metallic  carbides. 

Electrochemical  reactions  can  be  similarly  divided.  Those  which 
proceed  with  liberation  of  energy  are  not  so  important  practically  as 
those  of  the  second  kind,  and  will  occupy  little  space  in  this  book.  The 
reaction  being  electrochemical,  the  liberated  energy  appears  as  electrical 
energy,  and  the  systems  in  which  the  reaction  takes  place  are  known 
as  primary  cells.  The  Daniell  cell  is  an  example.  The  corresponding 
reaction  is 

Zn  +  CuS04  — >  ZnS04  +  Cu 

and,  when  allowed  to  take  place  chemically,  liberates  its  energy  as 
heat. 

On  the  other  hand,  electrochemical  processes  of  the  second  kind— 
those  which  take  place  with  absorption  of  electrical  energy — have 
achieved  very  considerable  technical  significance,  frequently  sup- 
planting a  purely  chemical  process,  and  in  some  cases  furnishing  new 
products  which  could  hardly  be  obtained  in  any  other  way.  Thus 
copper  is  now  chiefly  refined  electrochemically  ;  the  bulk  of  the  world's 
chlorate  production  is  made  electrochemically  ;  the  electrochemical 
production  of  caustic  alkali  and  bleach  becomes  continually  more 
important ;  whilst  the  manufacture  of  aluminium,  of  CaC2  (and  hence 
acetylene),  and  of  nitrates  from  the  air  could  not  have  reached  their 
present  proportions  without  the  introduction  of  electrochemical 
methods. 

In  chemical  processes,  the  necessary  supply  of  energy  is  usually 

B2 


4   /.PWJSEQIptESiOP  ; APPLIED  ELECTROCHEMISTRY     [CHAP. 

introduced  as  heat.  This  often  results  in  considerable  wear  and  tear 
of  plant,  and  in  products  rendered  impure  by  the  fuel  used.  Further, 
it  often  happens  that  the  simplest  and  most  direct  (on  paper)  of  purely 
chemical  methods  for  reaching  a  certain  result  cannot  be  used,  owing 
to  great  reaction  resistances,  or  to  the  impossibility  of  converting  heat 
energy  into  chemical  energy  under  the  given  conditions.  And  conse- 
quently several  successive  reactions  have  to  replace  a  single  direct  one. 
Thus,  to  obtain  aluminium  from  alumina,  the  old  chemical  process 
consisted  in  preparing  A1C13  by  passing  chlorine  gas  over  a  mixture  of 
the  alumina  with  carbon,  forming  a  double  salt  of  A1C13  with  NaCl,  and 
reducing  this  double  salt  by  heating  with  metallic  sodium. 
Electrochemical  processes  differ  in  the  following  respects  :— 

(a)  the  energy  needed  is  usually  introduced  as  electrical  energy,  not 
as  heat ; 

(b)  when  introduced  as  heat,  the  heat  is  produced  from  electrical 
energy  just  where  it  is  needed,  not  by  means  of  furnaces  or  flue  gases  ; 

(c)  the  processes  are  generally  simpler  and  more  direct  than  the 
corresponding  chemical  processes  ; 

(d)  in  consequence  of  (a)  and  (b)  the  products  are  usually  purer  ; 

(e)  the  wear  and  tear  of  plant  is  generally  less. 

On  the  question  of  the  relative  costs  of  chemical  and  electrochemical 
processes,  it  is  impossible  to  generalise.  Sometimes  one  is  the  cheaper, 
sometimes  the  other.  It  is  a  matter  decided  by  numerous  factors 
which  vary  with  each  separate  case,  such  as  the  local  power  charges, 
their  cost  compared  with  that  of  raw  material,  the  relative  charges  for 
labour  and  maintenance  of  plant,  the  purity  of  product  required,  etc. 
Sometimes  the  electrochemical  method  is  more  expensive,  but  gives  a 
purer  product  and  is  therefore  preferably  used. 

Electrochemical  processes  should  always  be  of  as  simple  a  nature  as 
possible.  This  statement  of  course  holds  good  of  any  kind  of  technical 
chemical  operation,  but  particularly  of  electrochemical  ones.  They 
cannot  '  stand  such  hard  knocks '  as  chemical  processes  can,  and 
generally  only  work  satisfactorily  when  run  under  constant  conditions. 
Simplicity  is  therefore  of  importance.  The  raw  materials  used  should 
also  be  as  pure  as  possible.  When  once  impurities  begin  to  accumulate, 
the  efficiency  of  an  electrochemical  process  usually  decreases  very 
rapidly.  The  most  successful  electrochemical  processes  are  those  in 
which  a  constant  supply  of  a  raw  material  of  high  purity  and  constant 
composition  is  assured.  If  impure,  it  will  usually  pay  to  subject  it 
to  a  pr<-l  i  mi  nary  chemical  purification. 

2.  Electrical  Units 

Electrical  energy,  like  all  other  kinds  of  energy,  can  be  divided  into 
two  firtors.  the  i/muili'ti/  f'arlnr  ;IIK]  flu-  n'l<-nsihf  factor.  The  possibility 


i.j  INTRODUCTORY—  POWER  5 

of  a  change  taking  place  in  the  energy  content  of  a  system  is  deter- 
mined by  the  intensity  factor  only,  the  extent  of  the  change  by  both 
factors.  Whether  or  not  a  quantity  of  water  can  move  spontaneously 
from  one  level  to  another  is  determined  in  the  first  instance  by  the 
relative  heights  of  the  two  levels.  If  movement  does  take  place,  the 
change  of  potential  energy  is  expressed  by  the  product  of  the  weight 
of  water  which  has  flowed  down  into  the  difference  in  height  between 
the  two  positions.  Here  difference  in  height  is  the  intensity  factor, 
weight  of  water  the  quantity  factor.  A  possible  transference  of  heat 
energy  from  one  part  of  a  system  to  another  is  decided  by  the  difference 
in  temperature  between  the  two  regions  —  the  intensity  factor  of  the 
heat  energy.  Excluding  the  effect  of  passive  resistances,  it  is  the 
affinity  of  a  chemical  reaction  which  decides  whether  it  will  set  in  or 
not  ;  whilst  the  amount  of  chemical  energy  transferred  in  such  a 
reaction  is  given  by  the  product  of  the  affinity  of  the  reaction  and  the 
quantity  of  matter  which  has  been  transformed. 

Similarly,  electrical  energy  has  its  intensity  factor,  potential 
difference  (E),  and  its  quantity  factor,  quantity  of  electricity.  The 
former  determines  the  direction  of  transference  of  electrical  energy,  the 
product  of  the  two  quantities  determines  the  magnitude  of  the  change. 

From  these  fundamental  conceptions  we  can  directly  pass  on  to 
others.  When  the  energy  content  of  a  system  is  increasing  or  de- 
creasing (i.e.  when  work  is  being  done  on  or  by  the  system),  the  change 
of  energy  per  unit  time  (the  rate  of  consumption  or  production  of  work) 
is  termed  the  power.  In  the  same  way,  when  a  transference  of 
electricity  takes  place  across  a  certain  point,  the  quantity  which  passes 
per  unit  of  time  is  termed  the  current  l  (I).  Finally,  the  rate  at  which 
electricity  can  pass  between  two  points  at  different  electrical  potentials 
is  directly  proportional  to  the  magnitude  of  this  potential  difference, 
and  also  depends  on  the  nature  of  the  path  along  which  the  current 
travels. 

The  relation  may  be  expressed  in  the  form 


where  R  is  called  the  resistance  of  the  conductor  (Ohm's  Law).  If 
the  resistance  be  great,  the  current  passing  for  a  given  difference  of 
potential  will  be  small,  and  vice  versa. 

In  deciding  on  the  units2  to  serve  for  the  measurement  of  these 

1  The  current  flowing  across  any  point,  divided  by  the  area  of  the  conductor  at 
that  point  at  right  angles  to  the  direction  of  the  current,  is  called  the  current 
density. 

-  The  units  here  denned  were  specified  by  the  International  Conference  on 
Electrical  Units  and  Standards,  1908.  They  are  known  as  the  international  units. 
Ohms  and  volts  of  slightly  lesser  magnitude  were  used  some  years  back,  but  need 
no  further  mention  here. 


6      PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY     [CHAP. 

magnitudes,  it  has  been  found  most  convenient  to  define  first  the  units 
of  resistance  and  current,  and  to  deduce  from  these  the  units  of  potential, 
energy,  power  and  quantity  of  electricity.  The  first  primary  unit 
is  that  of  resistance,  the  ohm,  which  is  defined  as  the  resistance  offered 
to  an  unvarying  electric  current  by  a  column  of  mercury  at  the  tempera- 
ture of  melting  ice,  the  column  to  be  14'4521  grains  in  weight,  of  a 
constant  cross-sectional  area,  and  of  a  length  of  106*300  cm.  The 
second  primary  unit  is  that  of  current,  the  ampere.  As  we  shall 
see  later,1  when  electricity  is  passed  through  a  solution  of  a  metallic 
salt  in  water,  the  salt  is  decomposed,  and  in  many  cases  the  metal  is 
deposited  in  the  free  state.  It  has  been  found  that  the  phenomenon  is  a 
quantitative  one,  and  the  ampere  is  defined  as  the  unvarying  electric 
current,  which,  when  passed  through  a  solution  of  AgN03  in  water, 
deposits  silver  at  the  rate  of  O'OOl 11800  gram  per  second.  It  is 
specified2  that  the  AgN03  solution  shall  contain  15-20  grams  salt 
to  100  grams  distilled  water.  The  solution  must  only  be  used  once, 
not  less  than  100  c.c.  at  a  time,  and  not  more  than  30  per  cent,  of 
the  metal  must  be  deposited.  The  current  density  must  not  exceed 
0*02  amp.  /cm.2  at  the  cathode,3  and  0'2  amp.  /cm.2  at  the  anode.3 

From  this  follows  directly  the  definition  of  the  unit  quantity  of 
electricity — the  coulomb — as  that  quantity  of  electricity  which,  when 
passed  through  a  solution  of  silver  nitrate,  will  deposit  O'OOlllSOO  gram 
of  silver. 

The  unit  of  potential  difference  or  electrical  pressure  is  the  volt, 
defined  as  that  potential  difference  or  voltage  which,  when  steadily 
applied  to  a  conductor  whose  resistance  is  one  ohm,  will  produce  a 
current  of  one  ampere.4 

The  unit  of  power  is  the  watt,  and  is  defined  as  the  rate  at  which 
energy  is  expended  by  an  unvarying  electric  current  of  one  ampere, 
flowing  under  an  electric  pressure  of  one  volt.  Finally,  the  unit  of 
electrical  energy,  the  watt-second  or  joule,  is  defined  as  the  energy 
expended  in  one  second  by  an  unvarying  electric  current  of  one  ampere 
flowing  under  an  electric  pressure  of  one  volt.  For  technical  uses,  the 
power  and  energy  units  are  inconveniently  small,  and  others  have  been 
introduced.  Of  power  units  we  have  the  kilowatt  (K.W.),  which  is 
1,000  watts,  and  the  horse-power  (H.P.),  which  is  746  watts.5  Corre- 
sponding to  these  are  the  energy  units,  kilowatt-hour 6  (K.W.H.)  and 

1  P.  26.  2  Cf.  also  p.  32.  =»  Loc.  cit. 

4  For  purposes  of  voltage  comparison,  certain  standard  ccZZ-s,  furnishing  accurately 
known  and  constant  potential  differences,  can  be  conveniently  employed.  The 
best  known  is  tin-  \\Ystmi  normal  element  with  a  potential  of  T0184  volts  at  20°. 
(See  p.  91.)  The  potential  difference  given  at  the  l«-i  minal.s  of  a  primary  cell 
(such  as  th<-  \\Yston  element)  is  known  as  the  electro-motive  Jorce  (E.M.E.)  of  the 

The  horse-power  in  use  in  Germany  is  736  watts. 
6  The  Board  of  Trade  unit  (B.T.U.). 


i.]  INTRODUCTORY— POWER  7 

horse-power-hour  (H.P.H.),  which  represent  the  energy  consumption 
per  hour  of  systems  absorbing  energy  at  the  respective  rates  of  one 
kilowatt  and  one  horse-power. 

In  the  course  of  this  book  we  shall  have  to  consider  the  quantitative 
relations  existing  between  electrical  energy  on  the  one  hand  and  heat 
energy  on  the  other.  These  relations  are  expressed  by  the  equations  :— 

1  joule  =  0*23865  erjam-calorie  (units  of  heat  energy,  written  cat.}. 

1  kilo-joule  (K.W.S.)  =  0-23865  kilogram-calorie  (written  Cal), 
and  inversely  : — 

1  cal.  =  4-189  joules-^  t-.- 

1  Cal.  =  4-189  K.wS.   " 

A  few  simple  calculations  are  here  appended. 

1.  A  current  is  kept  flowing  along  a,- wire  of  160  ohms  resistance  by  means  of 
a  constant  potential  difference  of  20  volts.     The  whole  of  the  electrical  energy 
consumed  is  turned  into  heat  hy  the  wire.     What  is  the  power  used,  and  how 
many  gram-calories  are  produced  per  hour  ? 

E       20 
=  R=160  =    )'125 

Power  used  =  0-125  x  20  watts 

=  2-5  watts. 
Joules  used  =  2-5  per  second 

=  2-5  x  3,600  per  hour. 

2-5  x  3,600 
Calories  produced  per  hour  =  — r\  RC. —  =  2,148  cals. 

2.  A  small  alkali- chlorine  plant  is  driven  by  means  of  three  dissimilar  dynamos. 
The  first — 400  H.P. — gives  4,000  amperes  at   full   load,  the  smaller  ones  give- 
15  K.W.  at  15  volts  and  80  H.P.  at  60  volts  respectively.     The  alkali-chlorine  ceUs 
number  thirty.     Each  takes  2,500  amperes  at  5  volts,  including  all  leads  and 
connections.     How  must  dynamos  and  cells  be  arranged  ? 

Assuming  1  H.P.  =  750  watts,  which  is  accurate  enough  for  the  present  purpose, 
we  calculate  that  the  dynamos  give  respectively  : — 

(a)  400  x  750  =  300,000  watts 

=  4,000  amperes  at  75  volts. 

(6)  15  x  1,000  =  15,000  watts 

=  1,000  amperes  at  15  volts. 

(c)      80  x  750  =  60,000  watts 

=  1,000  amperes  at  60  volts. 

If  (6)  and  (c)  are  run  in  series,  they  will  yield  1,000  amperes  at  75  volts.  But 
this  is  the  voltage  of  (a),  and  hence  the  combination  of  (b)  and  (c)  can  be  run  in 
parallel  with  (a),  the  whole  yielding  4,000  +  1,000  =  5,000  amperes  at  75  volts. 
If  the  cells  were  all  run  in  series,  they  would  need  2,500  amperes  at  5  x  30  =  150 
volts.  To  adjust  them  to  the  dynamo  conditions,  they  must  be  run  in  two  parallel 
series  of  fifteen  cells.  Each  branch  then  requires  2,500  amperes  at  75  volts,  or 
altogether  5,000  amperes  at  75  volts,  which  is  what  the  dynamos  can  supply. 

3.  An  electric-steel  induction  furnace  consumes  170  K.W.  and  produces  4'7 
tons  of  steel  per  day  by  melting  up  together  a  mixture  of  scrap  wrought-iron  and 
washed  pig-iron.     The  heat  content  of  the  melted  product  is  350  Cals.  per  kilo. 


8      PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY     [CHAP. 

What  is  the  electrical  energy  required  per  ton  of  steel  produced  ?     What  is  the 
thermal  efficiency  of  the  furnace  ?     If  one-third  of  the  material  used  -were  added 
molten,  already  containing  275  Cals.  per  kilo.,  what  would  be  the  production  per 
day,  and  the  power  required  per  ton  of  product  ?  ! 
(a)  Energy  used  per  ton  of  steel 


(6)  Heat  equivalent  of  electrical  energy  consumed  in  furnace   per  ton   of 
product 

=  0-239  x  60  x  60  x  870  Cals. 

Heat  content  per  ton  of  product 

=  1,000  x  350  Cals. 

Thermal  efficiency  of  furnace 

1,000  x  350 

=  0-239  x  60  x  60  x  870 

=  47  per  cent. 
(c)  Heat  supplied  with  starting  material 

=  275  x  5  Cals.  per  kilo. 
9 

=  92  Cals.  per  kilo. 
The  current  must  therefore  supply 

350  —  92  =  258  Cals.  per  kilo. 
Production  per  day  under  these  conditions  : 

350 
=  258  x  4-7  =  6-4  tons. 

Electrical  energy  required  per  ton 


3.  Power 

General.—  The  question  of  power  costs  bulks  largely  in  all  technical 
electrochemical  processes  with  few  exceptions.  Such,  is  for  example, 
the  electrochemical  refining  of  the  noble  metals,  where  interest  charges 
on  the  silver  or  gold  under  treatment  are  of  more  importance.  To  a 
lesser  extent,  this  also  applies  to  the  refining  of  copper,  where  a  very 
large  quantity  of  a  valuable  product  is  obtained  with  a  low  energy  ex- 
penditure. But  in  most  processes  the  value  of  the  product  and  the  cost 
of  the  necessary  energy  are  of  the  same  order,  and  power  questions 
become  of  vital  importance. 

It  is  difficult  to  draw  definite  conclusions  which  can  be  applied  to 
our  present  purpose  from  most  of  the  statements  made  in  journals 
and  articles  on  the  cost  of  power-raising  by  different  systems.  In 
considering  the  matter,  we  must  keep  several  points  clearly  in  view. 
The  first  is  the  very  high  value  of  the  load-factor  of  power  used  in  electro- 
chemical industries.  The  load-factor  is  defined  as  the  ratio  of  the 
average  power  consumption  to  the  maximum  power  consumption  needed 
1  J.  W.  Richards,  ElectrocJiem.  Ind.,  5,  168  (7.W7). 


i.]  INTRODUCTORY— POWER  9 

at  any  time.  In  many  cases — for  example,  municipal  electric  light 
and  power  undertakings— it  is  10-20  per  cent.,  and  rarely  exceeds 
50  per  cent.  But  for  electrochemical  purposes  it  is  generally1  very 
high— about  90-95  per  cent.  A  high  load-factor  means  a  cheaper 
supply,  and  power  for  electrochemical  processes  can  thus  be  got  on 
comparatively  advantageous  terms.  Then  many  of  the  available 
estimates  on  power-production  are  vague  as  to  what  they  really  include. 
The  running  costs  of  a  power  plant,  besides  fuel,  include  charges  for 
repairs  and  sundries,  labour  and  management,  which  must  all  be  brought 
into  an  estimate  of  any  value.  But  there  are  also  the  capital  charges 
for  interest  and  depreciation  of  plant,  including  spare  machines.  These 
can  be  very  different  in  different  systems  of  power-production,  and 
all  must  be  taken  into  account  when  making  an  estimate,  comparative 
or  otherwise.  Further,  the  charges  of  the  different  power  companies  are 
of  little  use  in  making  comparisons.  These  companies  must  make 
their  profits,  which  vary  considerably ;  the  published  figures  often 
include  transmission  charges  over  a  long  distance,  and,  unless  otherwise 
explicitly  stated,  generally  hold  for  power  of  low  load-factor  used  inter- 
mittently for  lighting  or  driving  small  machines.  It  is  futile,  as  has 
been  done,  to  compare  the  charges  of  a  power  company  at  £15-£25  per 
H.P.  year  with  the  costs  of  a  water-power  installation  in  the  Alps  or 
Scandinavia  at  10s.  to  20s.  per  H.P.  year,  and  then  to  conclude  that 
electrochemical  industries  have  no  future  in  this  country  because  of 
the  scarcity  of  suitable  water-power. 

Three  systems  of  power-production  must  be  briefly  considered  : — 

1.  Water-driven  turbines. 

2.  Steam  turbines  or  engines. 

3.  Internal-combustion  engines  consuming  gas  (and  occasionally  oil). 
Water-power. — The   cost   of  water-power    can   vary   enormously, 

according  to  local  circumstances  and  the  engineering  difficulties  to  be 
overcome.  The  price  is  mainly  decided  by  the  initial  capital  costs, 
the  maintenance  charges  being  comparatively  low.  These  capital  costs 
vary  between  £3  and  £30  per  H.P.  installed,  and  probably  average  about 
£10.  If  interest  and  depreciation  be  taken  as  15  per  cent.,  the  average 
capital  charges  will  be  £1  10s.  per  H.P.  year.2  Putting  working  charges 
at  15s.  per  H.P.  year,  the  average  total  cost  of  water-power  will  come 
to  £2  5s.  per  H.P.  year  (0'082d.  per  K.W.H.).  Under  exceptionally 
favourable  circumstances  its  cost  is  far  lower,  as  the  following  figures 
show  :— 

Svaelgfos,  Norway,  8s.  3d.  per  H.P.  year. 

Notodden,  Norway,  13s.  Id.  per  H.P.  year. 

Chedde,  Savoy,  18s.  4d.  per  H.P.  year. 

But  as  a  rule  the  figures  will  fall  between  £2-£4  per  H.P.  year.     At 

1  Not  in  certain  electrothermal  processes. 

-  A  year  is  taken  as  365  x  24  =  8,760  hours. 


10    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY     [CHAP. 

Niagara,  power  is  supplied  by  the  different  companies  to  large  con- 
sumers at  prices  between  £2  2s.  6d.  and  £4  3s.  per  H.P.  year. 

Steam-power  can  be  generated  using  ordinary  reciprocating 
steam  engines  or  else  steam  turbines.  Both  have  approximately  the 
same  low  thermodynamic  efficiency,  only  converting  O1-O15  of  the 
heat  value  of  the  fuel  into  mechanical  energy.  This  fraction  can 
occasionally  rise  to  0*18. 

The  capital  cost  of  an  installation  of  not  less  than  1,000  H.P.  will 
average  about  £12  per  H.P.  Fuel  and  other  running  expenses  vary 
considerably,  according  to  local  conditions.  It  is,  however,  fairly 
safe  to  say  that  the  total  power  costs  in  this  country  will  work  out  at 
£5-£8  per  H.P.  year  (0'18d.-0-29d.  per  K.W.H.),  and  under  very  favour- 
able conditions  may  fall  still  lower.  The  prices  charged  by  English 
power  companies  for  power  of  high  load-factor  vary  between  £9  10s. 
and  £20  per  H.P.  year,  though  better  terms  can  doubtless  be  obtained 
by  large  and  regular  consumers.  When  prices  reach  £10-£12  per  H.P. 
year,  they  approach  the  uneconomic  limit  for  most  electrochemical 
industries. 

Gas-power. —  Gas  engines  are  coming  more  and  more  into  use 
at  the  present  time  for  power-raising,  due  partly  to  the  considerable 
improvements  effected  in  them  during  the  last  few  years.  Their  initial 
cost  is  greater  than  steam  plant,  as  are  also  their  running  expenses 
other  than  fuel,  but  on  the  other  hand  their  efficiency  is  from  50-100 
per  cent.,  higher,  varying  between  0'2-0'25,  and  indeed  can  often  be 
still  greater  with  engines  of  large  size.  The  formation  of  producer  gas 
from  coal  is  a  much  more  efficient  process  than  the  generation  of  steam 
in  a  boiler.  Where  blast-furnace  or  coke-oven  gases  are  available  as 
fuel,  their  advantages  over  steam  plant  are  correspondingly  increased. 

The  initial  cost  of  a  gas  plant  of  fair  dimensions  (3,000  H.P.  or  more) 
will  be  about  £15  per  H.P.  installed,  including  producers,  etc.  Assuming 
fuel  to  cost  9s.  to  10s.  per  ton,  and  that  an  ammonia-recovery  plant  is 
operated,  the  total  cost  per  H.P.  year  will  come  to  £3  to  £3  10s.  (O'lld- 
O'l3d.  per  unit),  of  which  some  40  per  cent,  is  accounted  for  by  capital 
charges,  the  remainder  being  about  equally  divided  between  fuel  and 
other  running  expenses. 

Comparative.— The  above  data  would  indicate  that  a  gas-power 
plant  without  doubt  works  more  cheaply  than  a  steam  plant.  Never- 
theless there  is  still  a  great  deal  of  controversy  on  the  point.  The 
truth  seems  to  be  that  for  small  installations  of  1,000  H.P.  or  there- 
abouts, where  ammonia  recovery  hardly  pays,  a  steam  plant  with  its 
lower  first  cost  is  the  more  economical,  whereas  the  reverse  statement 
holds  good  for  plants  of  larger  capacity.  A  gas  plant  should  have  a 
steady  continuous  load.  A  low  load-factor  is  a  point  strongly  in  favour 
of  steam  power.  Another  deciding  circumstance  in  favour  of  a  steam 
plant  may  be  the  demand  for  waste  steam,  with  its  manifold  applications 


L]  INTRODUCTORY— POWER  11 

in  chemical  works.  Finally,  steam  plant  still  lias  a  better  name  for 
proved  reliability.  The  tendency  to  scrap  steam  engines  in  favour  of 
gas  engines  is  nevertheless  a  marked  one  at  present. 

If  we  further  compare  the  figures  for  gas  and  water  power  we  see  that, 
except  in  certain  exceptional  cases,  there  is  very  little  difference.  Many 
water-powers  actually  generate  their  energy  at  a  cost  exceeding  £3  per 
H.P.  year,  and,  where  large  power  companies  are  in  possession,  the  price 
for  electrochemical  purposes  can  rise  still  higher.  This  disposes  of 
the  ancient  argument  that  water-power  is  essential  to  the  success  of 
most  electrochemical  industries.  When  to  that  are  added  the  facts 
that  in  Great  Britain  coal  is  cheap  and  markets  are  near,  whilst  many 
raw  materials  are  readily  available,  we  see  on  the  contrary  that  this 
country  offers  exceptional  facilities  for  electrochemical  enterprise. 


Literature 

(Power.)    Pring.    Some  Electrochemical  Centres. 


CHAPTER  II 

EQUILIBRIUM 

1.  General 

ALTHOUGH  for  practical  purposes  chemical  reactions  are  often  divided 
into  two  classes — '  reversible  '  and  '  irreversible ' * — no  such  distinction 
strictly  exists.  At  room  temperature  hydrogen  and  chlorine  combine 
practically  completely,  giving  HC1,  and  HC1  will  of  itself  furnish  no 
appreciable  quantities  of  hydrogen  and  chlorine.  Hence  this  reaction 
is  stated  to  be  '  irreversible/  But  in  reality  it  is  just  as  much  reversible 
as  is  the  reaction 

CH3COOH  +  C2H5OH  £  CH3COOC2H5  +  H20. 

Minute  traces  of  hydrogen  and  chlorine  do  undoubtedly  exist  in 
the  free  state  after  the  great  bulk  has  combined,  giving  HC1.  The 
difference  between  the  two  reactions  is  only  one  of  degree,  not  one 
of  kind. 

Every  chemical  system,  left  to  itself,  will  finally  arrive  at  a  state  of 
equilibrium,  when  all  the  component  substances,  whether  originally 
present  or  formed  during  the  reaction,  will  coexist  in  quantities  depend- 
ing on  the  temperature  and  pressure  of  the  system  as  well  as  on  the 
amounts  of  the  original  substances  taken.  If  the  reaction  is  to  proceed 
further,  one  or  other  of  the  products  must  be  removed,  thus  disturbing 
the  equilibrium.  In  the  desulphurisation  of  steel,2  the  sulphur  is 
converted  into  a  form  (CaS)  which  is  insoluble  in  the  reacting  system. 
In  the  dehydration  of  ZnCl2  melts ; 3  the  water  is  blown  off  by 
continually  renewing  the  HC1  atmosphere. 

The  reasons  owing  to  which  this  equilibrium  is  often  not  clearly 
manifest  are  (1)  the  exceedingly  small  equilibrium  concentrations  of 
some  of  the  substances  ;  and  (2)  considerable  reaction  resistances  which 
often  prevent  the  equilibrium  setting  in. 

This  second  point  is  very  important.  Thus,  although  the  equilibrium 
state  of  a  system  is  defined  by  the  constancy  of  composition  of  the  system 
over  an  unlimited  length  of  time,  yet  constancy  of  composition  alone 

1  Not  to  be  confounded  with  the  perfectly  definite  conceptions  of  reversible 
and  irreversible  processes  discussed  in  Chapter  VII. 

2  P.  443.  3  P.  422. 

12 


EQUILIBRIUM  13 

must  not  be  taken  as  a  criterion  of  the  equilibrium  state.  Thus  a  H2  -  02 
mixture  is  apparently  perfectly  stable  at  room  temperature  ;  neverthe- 
less it  is  in  reality  very  far  removed  from  its  equilibrium  state,  at  which 
minute  traces  of  the  gases  exist  in  presence  of  a  large  excess  of  water- 
vapour.  To  be  certain  that  a  system  is  in  a  state  of  equilibrium,  it 
must  be  possible  to  reach  that  state  from  both  sides— commencing 
therefore  with  an  excess  of  either  set  of  the  reacting  substances.  Thus  if 
equimolecular  quantities  of  C2H5OH  and  CH3 .  COOH  react,  producing 
CH3 .  COOC2H5  and  water,  we  know  that  the  equilibrium  state  is  reached 
when  two-thirds  of  the  alcohol  and  acid  have  been  transformed  into 
ester  and  water.  And  we  know  this  to  be  true  because,  starting  with 
CH3.COOC2H5  and  water,  CH3COOH  and  C2H5OH  are  formed, 
and  if  equimolecular  quantities  are  taken  the  reaction  ceases  when 
only  one-third  has  been  converted  to  acid  and  alcohol,  two-thirds 
remaining  unchanged.  But  the  system  has  now  the  same  composition 
as  when  alcohol  and  acid  were  originally  taken,  and  hence  is  at 
equilibrium. 

Catalysts.— These  reaction  resistances  are  often  so  great  that  they 
practically  inhibit  the  commencement  of  reactions  which  would  other- 
wise occur.  A  substance  whose  addition  in  such  cases  renders  the 
progress  of  the  reaction  measurable,  or  in  other  cases  increases  the 
velocity  with  which  equilibrium  is  reached,  is  termed  a  catalyst.  Well- 
known  technical  catalysts  are  the  platinum  used  in  the  S03  contact 
process,  the  copper  salts  used  in  the  Deacon  chlorine  process,  the 
sulphur  and  HgSCX  used  respectively  in  the  chlorination  and  sulphona- 
tion  of  certain  organic  compounds,  etc.  We  shall  encounter  many 
cases  in  this  book  (e.g.  in  graphitisation,1  azotisation  of  CaC2,2  electrical 
oxidation,3  etc.,  etc.). 

The  mechanism  by  which  these  catalysts  act  is  often,  as  we  shall 
see,  quite  unexplained.  One  point  however  must  be  emphasised,  i.e. 
that  the  catalyst  does  not  affect  the  final  equilibrium  state,  but  only  the 
velocity  with  which  that  state  is  reached.  A  system  in  equilibrium  can 
usually  be  regarded  as  in  a  stationary  state,  defined  by  the  equality  of 
the  velocities  of  two  opposed  reactions.  Thus,  in  the  above  example,  the 
velocity  at  equilibrium  of  interaction  between  acid  and  alcohol  giving 
ester  and  water  is  equal  to  the  velocity  of  interaction  of  ester  and  water 
giving  acid  and  alcohol.  If  now  the  velocity  of  one  of  these  reactions 
be  increased  more  than  the  velocity  of  the  opposing  reaction,  it  is  clear 
that  the  system  will  no  longer  remain  in  equilibrium.  It  follows  that 
a  catalyst  must  increase  the  velocity  of  the  two  opposing  reactions 
concerned  to  the  same  extent. 

We  will  now  consider  the  conditions  of  equilibrium  in  different 
types  of  constant  temperature  systems.  As  a  first  classification  we  can 
divide  these  into  homogeneous  and  heterogeneous  systems.  A  homo- 
1  P.  493.  -  P.  482.  3  p.  140. 


14    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY     [CHAP. 

geneous  system  is,  both  chemically  and  physically,  of  the  same 
composition  in  all  parts  ;  e.g.  a  pure  crystalline  substance,  or  an 
aqueous  solution,  or  a  space  filled  with  gas  or  vapour.  A  heterogeneous 
system  can  vary  in  chemical  or  physical  composition  in  different  parts, 
e.g.  a  saturated  aqueous  solution  in  contact  with  undissolved  solid. 

2.  Homogeneous  Equilibria 

In  a  homogeneous  system  the  conditions  of  equilibrium  at  constant 
temperature  are  essentially  determined  by  the  Law  of  Mass  Action. 
Suppose  the  equilibrium  point  of  the  following  reaction  be  considered  :— 

WjAi  +  ™2A2  -f  etc.  .  .  .  J.  w/A/  +  n2'A2'  -j-  etc. 

where  A1}  A2,  A/,  A2',  etc.,  represent  the  different  kinds  of  molecules 
involved,  nlt  n2,  w/,  w/,  etc.,  denote  the  respective  numbers  of  the  same 
taking  part  in  the  reaction,1  and  [CJ  [C2]  [C/]  [C2']  the  molecular  con- 
centrations or  gram-mols.  per  unit  volume  of  the  different  substances.2 
Then  the  following  equation  holds3  :  — 


where  K,  a  constant  for  any  particular  equilibrium  at  a  particular 
temperature,  is  termed  the  equilibrium  constant  of  the  reaction  at  that 
temperature.  We  see  that  K  is  great  if  [C/]  [C2'J,  etc.,  are  great  com- 
pared with  [CJ  [C2],  etc.,  and  that  therefore  a  high  value  of  K  corresponds 
to  the  reaction  going  more  or  less  completely  from  left  to  right.  If 
therefore  K  be  known,  and  if  in  the  equilibrium  reaction  mixture  all 
but  one  of  the  values  [d]  [C2]  [C/]  [C2'],  etc.,  can  be  directly  determined, 
the  remaining  one  can  be  calculated.  And  whatever  the  proportions 
of  the  different  substances  in  the  initial  reaction  mixture,  they  will 
finally  adjust  themselves  so  as  to  satisfy  the  above  equation. 

We  will  discuss  numerically  two  simple  examples. 

The  reaction 

CH3  .  COOH  +  C2H5  .  OH  ^  CH3  .  COOC2H5  +  H20 
was  investigated  by  Berthelot  and  St.  Gilles.4    They  started  with  acid 
and  alcohol,  water  and  ester  being  absent.     Suppose  that  n  mols.  of 
alcohol  were  added  at  the  start  to  one  mol.  of  acid,  the  whole  occupying 
a  volume  v.     At  equilibrium  suppose  x  mols.  each  of  water  and  ester 

1  With  2H2  +  02^2H20,  we  have  n}  =  2,  n.2  =  1,  w,'  =  2.     With  2KMnO4 
+  10FeSO4  +  8H:>S04->  KjSO4  +  2MnSO4  +  5Fe,(S(),),  -f-  8H,<),  we  have  w,  =  2, 
^=10,  na  =  8,  n,'=  1,  ^'=2,  n./  =  5,  n/  =  8. 

grams 

2  Thus  with  two     .-  of  hydrogen  [C]  =  1  ;   with  4'8  grams  ozone  per  100 


litres  [C]  =  0-001  ;  with  20  grams  KC1  per  350  c.c.  [C]  =     °    x  ls    ??  =  0.7G(} 

/4*t)         tJtjU 

3  This  result  must  be  assium-'l. 
*  Ann.  r/,,,,/.  /'//.yv.  [:{]  65,  ,Wi  (As/*),  66,  5  (A%':>),  68,  225  (1863). 


II.] 


EQUILIBRIUM 


15 


to  be  present.     Then  1  —  x  and  n  —  x  represent  the  quantities  of  acid 
and  alcohol  respectively  present.     We  have  : — 


x     x 


1  —  x     n  —  x         (I  —  x)  (n  —  x) 


K  being  independent  of  the  volume. 

The  following  experimental  results  were  obtained  :— 

K 


n 
0-18 
0-33 
0-50 
1-00 
2-00 
8-00 

With  the  ratio 


X 

0-171 
0-293 
0-414 
0-667 
0-858 
0-966 


3-9 
3-3 
3-4 
4-0 
4-5 
3-9 


[acid] 


varying  initially  between  6  : 1  and  1  :  8, 


[alcohol] 

the  final  state  of  the  system  was  throughout  determined  by  the  above 
equation,  in  which  K  is  about  3'8-3'9.     With  more  refined  experimental 
methods,  the  agreement  would  doubtless  have  been  more  exact. 
The  equilibrium 


of  fundamental  importance  in  the  S03  '  contact  '  process,  was  investi- 
gated by  Bodenstein  and  Pohl.  They  led  different  mixtures  of  S02  and 
oxygen  or  S02  and  air  over  platinum  black  at  definite  temperatures, 
when  partial  conversion  to  S03  took  place.  The  gases  issuing  from 
the  reaction  chamber  were  rapidly  drawn  off,  cooled,  and  analysed. 
That  equilibrium  was  established  was  proved  by  starting  with  S03  and 
measuring  the  extent  of  dissociation  into  S02  and  02.  We  can  write 


The  results  of  Table  I.  are  for  experiments  at  727°  C.  (1,000°  abs.). 

TABLE  I 


T° 

p  in 
mm.  of 
mercury 

S02  :  Oo    N2 

o/ 

/o 

S02  :  S03  :  02 

K.103 

1003 

770 

0-42                 0 

59-6 

0-132  :  0-195  :  0-673 

3-49 

1000 

760 

1-24                 0 

52-2 

0-309  :  0-338  :  0-353 

3-59 

1000 

765 

2-44                  0 

42-3 

0-481  :  0-355  :  0-164 

3-67 

1000 

758 

3-36                  0 

37-1 

0-566  :  0-333  :  0-101 

3-52 

1001 

760 

7-94                 0 

20-8 

0-775  :  0-203  :  0-22 

3-67 

997 

764 

2-46                 3-76 

35-8 

0-233  :  0-130  :  0-182 

3-58 

1001 

760 

3-10                 3-76 

32-3 

0-283  :  0-136  :  0-68 

3-52 

1000 

758 

l-06(asS03):l  :0 

54-4 

0-273  :  0-325  :  0-402 

3-43 

16      PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

The  various  columns  contain  respectively  (1)  temperature  ;  (2)  total 
pressure  ;  (3)  composition  of  reaction  mixture  (in  the  last  experiment  S03 
was  used  instead  of  S02)  ;  (4)  percentage  of  original  S02  (S03)  present 
as  S03  in  equilibrium  mixture  ;  (5)  fractional  concentration  of  equili- 
brium mixture  ;  (6)  equilibrium  constant,  referred  to  1000°  abs.,  and, 
when  necessary,  corrected  for  slight  temperature  differences. 

K  is  calculated  as  follows,  the  concentrations  being  expressed  in   .         [C]. 

At  0°C.  and  760  ram.  of  mercury,  one  mol.  of  any  gas  occupies  22*42  litres.  At 
higher  temperatures  the  volume  is  correspondingly  greater.  If  the  fractional  con- 
centrations in  column  5  be  denoted  by  c,  we  have 

rci  -    .  JL  .  ?L3  .  _L_ 

760       T  22-42' 
and 

K  _  *so,^o,        P  273        1 

c*SOs  760  T      22-42 
Thus  in  the  second  case 

(0-309)2  .  (0-353)     760      273          1 


__ 


_  __ 

(0-338)2  760     1,000     22-42 


, 
The  agreement  between  the  different  experiments  is  excellent 


3.  Heterogeneous  Equilibria 

Phases.— A  heterogeneous  system  is  characterised  by  the  presence 
of  two  or  more  homogeneous  systems  in  contact  and  separated  by  de- 
finite limiting  surfaces.  The  number  of  distinct  kinds  of  homogeneous 
systems,  differing  either  chemically  or  merely  physically  from  one 
another,  which  build  up  a  heterogeneous  system,  is  called  the  number 
of  phases  of  the  system,  and  each  of  the  distinct  kinds  of  component 
homogeneous  systems  is  called  a  phase.  It  will  be  seen  that  the  ideas 
of  size  and  shape  have  no  part  in  the  conception  of  a  phase.  Thus  an 
aqueous  Ag2S04  solution  is  a  single  phase.  If  contained  in  an  enclosed 
space  together  with  some  air,  this  air  will  become  saturated  with 
aqueous  vapour  at  the  pressure  of  the  Ag2S04  solution.  Being  homo- 
geneous in  all  parts,  it  will  constitute  a  second  phase.  Similarly  a 
BaCl2  solution  is  a  separate  phase.  If  the  two  solutions  be  mixed,  we 
shall  get  a  precipitate  containing  AgCl  and  BaS04.  Each  of  these  salts 
will  constitute  a  separate  solid  phase,  and  the  homogeneous  mother 
liquor  will  form  a  third,  liquid,  phase. 

When  a  heterogeneous  system  is  in  equilibrium,  each  separate 
phase,  taken  separately,  is  in  equilibrium  internally,  and  further  all 
the  phases  are  in  equilibrium  with  one  another.  Equilibrium  in  a 
heterogeneous  system  is  therefore  characterised  by  the  superposition 
of  a  number  of  homogeneous  equilibrium  systems,  and  to  each  of  the 
latter,  when  c.ipaUe.  of  ;,  variation  iii  composition,  the  l;i\v  of  mass 


ii.]  EQUILIBRIUM  17 

action  is  applicable.  But  the  state  of  equilibrium  in  the  whole  hetero- 
geneous system  is  independent  of  the  relative  masses  or  sizes,  etc.,  of 
the  different  phases. 

For  instance,  suppose  equilibrium  established  in  a  system  where 
ZnS  has  been  precipitated  from  a  ZnCl,  solution  by  H2S.  The  number 
of  phases  is  three  —  solid  ZnS,  liquid  solution,  and  gas.  The  mass 
action  law  can  be  applied  to  the  liquid  phase,  in  which  ZnCl2,  ZnS,  HC1, 
and  HUS  are  in  equilibrium.  Further,  all  three  phases  are  in  equili- 
brium with  one  another  —  the  liquid  phase  is  in  equilibrium  with  the 
precipitate  and  with  the  H2S  in  the  gas  phase.  But  it  makes  no 
difference  to  the  composition  of  the  liquid  phase  if  the  actual  mass 
of  the  ZnS  or  gas  or  solution  is  altered.  If  the  amount  of  precipitate 
be  doubled,  or  if  the  greater  part  of  the  gas  phase  be  cut  off  from 
the  rest  of  the  system,  the  composition  of  the  solution  will  remain 
unaltered. 

The  classification  of  heterogeneous  equilibria  is  most  easily  done 
by  means  of  the  Phase  Rule.  An  adequate  treatment  of  this  subject 
is  here  impossible,  and  readers  are  referred  to  Professor  Findlay^ 
The  Phase  Rule.  For  our  purpose  we  can  conveniently 
constant  temperature  heterogeneous  equilibria  into  three  classes. 

(a)  No  single  phase  can  vary  in  concentration. 

((b)  One  phase  only  is  capable  of  such  variation. 
(c)  Two  or  more  phases  can  thus  vary. 
Of  class  (a)  we  have  many  instances.  For  example,  a  liquid  in 
contact  with  its  saturated  vapour.  At  a  definite  temperature,  the 
pressure  of  the  vapour  phase  is  fixed,  and  no  variation  is  possible 
except  by  altering  the  temperature.  A  similar  example  is  a  saturated 
solution  in  contact  with  excess  of  solid  solute.  At  a  given  temperature 
the  concentration  of  the  solution  is  constant  —  the  solubility  has  a 
definite  value.  Another  instance  is  a  solid  in  contact  with  its  vapour 
phase  at  the  sublimation  pressure  (e.g.  iodine),  or  the  very  similar  case 
of  a  polymerised  solid  in  equilibrium  with  a  gaseous  phase  consisting 
of  simpler  molecules.  As  an  example  may  be  taken  the  case  of  solid 
paracyanogen  in  equilibrium  with  gaseous  cyanogen  :  — 


At  any  given  temperature  the  cyanogen  gas  pressure  is  constant,  and 
is  known  as  the  dissociation  pressure  at  that  temperature. 
Of  another  type  of  dissociation  a  good  example  is 

CaC03  ;p±  CaO  +  C02. 

Here  two  solid  phases  are  in  equilibrium  with  one  gaseous  phase,  and 
once  more  at  constant  temperature  the  pressure  is  fixed,  and  incapable 
of  alteration  except  by  upsetting  the  equilibrium.  That  this  in  fact 
must  be  so  can  be  shown  as  follows.  Consider  the  state  of  equilibrium 

c 


18    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY  ,  [CHAP. 

in  the  single  homogeneous  gaseous  phase.  In  accordance  with  the 
law  of  mass  action  we  can  write  for  any  temperature 

_  [CaO]  .  [COJ 
[OaOOJ 

where  [CaO]  and  [CaC03]  represent  the  very  low  molar  concentrations 
of  CaO  and  CaC03  in  the  gaseous  phase.  Now  these  low  concentrations 
have  a  perfectly  definite  meaning.  They  are  the  sublimation  pressures 
of  lime  and  calcite,  and  of  course  constant  at  constant  temperature. 

We  can  therefore  replace  -£—  ~=  by  Je,  and  get 


[coj  =   • 

That  is,  the  dissociation  pressure  of  CaC03  is  constant  at  constant 
temperature. 

The  fact  that  in  such  equilibrium  equations  as  the  above,  the  con- 
centrations of  the  solid  phases  taking  part  can  be  considered  as  constant, 
is  generally  expressed  by  saying  that  the  active  mass  of  a  solid  which 
is  taking  part  in  a  reaction  is  constant.  This  excludes  the  case  of 
solid  solutions.1  This  generalisation  enables  us  to  treat  more  complex 
cases  of  equilibria  between  solid  phases  and  gaseous  phase,  the  stipula- 
tion being  that  only  one  gas  is  involved  in  the  reaction. 

For  example  we  can  take  the  equilibria  2 

Si02  +  2C  ^±  Si  +  2CO  (1) 

CaO  +  3C  ^-±  CaC2  +  CO  (2) 

As  in  both  cases  all  the  constituents  but  the  CO  are  solids,  we  can  write 

Kt  =  fc,  [CO]' 


the  CO  equilibrium  pressures  being  therefore  definitely  fixed  if  the 
temperature  be  fixed.  Thus  Rothmund  found  for  the  second  case 
pco  at  1620°  to  be  one-third  of  an  atmosphere.  If,  by  pumping  off, 
the  pressure  be  artificially  kept  below  this  figure,  lime  and  carbon  will 
react,  giving  carbide  and  CO  until  all  are  used  up.  If,  on  the  other 
hand,  CO  gas  be  pumped  in,  and  the  pressure  thus  kept  above  the 
equilibrium  figure,  the  reaction  will  proceed  from  right  to  left,  CO 
being  absorbed.  At  higher  temperatures  the  equilibrium  pressure 

1  A  solid  solution  is  a  solid  phase  containing  two  or  more  constituents  whoso 
composition  can  be  continuously  varied  within  certain  limits.     There  are  no 
boundary  surfaces  of  supermolecular  dimensions  separating  its  different  con- 
stituents :   the  intermixture  is  molecular  just  as  in  liquid  solutions.     Many  of  the 
commonest  alloys  are  solid  solutions  (e.g.  steels,  brasses,  Cu-Ag  mixtures).     We 
shall  meet  several  examples  in  this  book:  e.c,.  pp.  121,  133,  130-139,  241,  492,  etc. 

2  Pp.  469,  487. 


ii.]  EQUILIBRIUM  19 

is  higher,  at  lower  temperatures  lower.  If  therefore  the  pressure  be 
artificially  kept  constant  at  one-third  of  an  atmosphere,  and  the  tem- 
perature raised  above  1620°,  the  lime  and  carbon  will  disappear  and 
carbide  will  be  formed.  But  if  the  temperature  be  lowered  below 
1620°,  the  pressure  being  still  kept  at  the  same  figure,  all  the  carbide 
will  disappear  ;  or  if  lime  and  carbon  be  present,  no  carbide  will  result. 
By  a  similar  variation  of  conditions,  silicon  can  be  oxidised  by  CO 
with  deposition  of  carbon  ;  or  silica  can  be  reduced  by  carbon  with 
evolution  of  CO. 

We  now  come  to  the  second  type  of  heterogeneous  equilibrium, 
where  one  of  the  phases  can  vary  in  concentration,  the  temperature 
remaining  constant.  If  NH4HS  be  gently  heated,  it  dissociates  thus  : 


giving  a  mixture  of  two  gaseous  dissociation  products.  Using  pure 
NH4HS,  the  composition  of  the  gaseous  phase  is  constant,  always 
50  per  cent.  H2S  :  50  per  cent.  NH3.  Hence  at  a  definite  temperature 
there  will  be  a  perfectly  definite  dissociation  pressure  —  in  fact  the 
system  will  behave  like  those  just  discussed.  But  if  an  excess  of  one 
of  the  dissociation  products  —  NH3  or  H2S  —  be  added,  a  new  equilibrium 
state  will  set  in,  in  which  the  composition  of  the  gaseous  phase  will  be 
different  from  what  it  was  previously,  whilst  the  solid  phase  will  still 
be  NH4HS.  Consider  the  equilibrium  in  the  gaseous  phase.  We  have 

[H^tNHJ 

[NH4HS] 

NH4HS  being  a  solid,  we  can  put  [NH4HS]  =  constant.  For 
ordinary  purposes  of  calculation  [H2S]  .  [NH3]  can  be  replaced  by 
*  •  PHas  •  Pxn,>  and  we  thus  get 

Kl   =  PXH.  •    ?H3S 

i.e.  the  product  of  the  partial  pressures  of  the  two  dissociation  pro- 
ducts must  be  a  constant  at  any  given  temperature.  Isambert  x 
made  measurements  at  25'1°  and  obtained  the  following  results  (pres- 
sures in  cm.  of  mercury),  which  show  a  sufficiently  good  agreement 
between  the  values  of  KI  :  — 


13-8             45-8  632 

20-9             29-5  617 

20-8             29-4  612 

25-05            25-05  627 

41-7             14-6  609 

45-3             14-3  648 

45-8             13-8  632 

1  Compt.  Reud.  92,  919  (1881)  ;  93,  731  (1881)  ;  94,  958  (1882). 

r  2 


20    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY     [CHAP. 

The  mean  value  for  KI  is  626.  Suppose  then  that  solid  NH4HS  dis- 
sociates at  25°  into  an  atmosphere  of  H2S  kept  at  a  constant  partial 
pressure  of  60  cm.,  the  partial  pressure  of  the  NH3  gas  will  be 

626        1A 

—  =  :   10-4  cm. 

The  solubility  product   of   difficultly  soluble   salts,  discussed   on 
p.  73,  is  a  very  similar  example  to  the  above. 
As  another  instance  we  may  take  the  reaction 

3Fe  +  4H20  ^=±  Fe304  +  4H2. 

When  steam  acts  on  an  excess  of  heated  iron,  Fe304  and  hydrogen  are 
formed,  and  finally  an  equilibrium  state  is  reached  with  both  hydrogen 
and  steam  present  in  the  gas  phase.  Conversely,  when  hydrogen  acts 
on  an  excess  of  heated  Fe304,  the  metal  and  steam  are  produced,  and 
an  equilibrium  state  is  eventually  reached.  The  iron  and  the  oxide, 
being  solids  of  constant  active  mass,  play  no  part  in  the  gaseous  equi- 
librium ;  and  we  get 

1 


or,  as  partial  pressures  can  be  substituted  for  concentrations, 

10 

£2*-  —  constant. 
PH.O 

Preuner  obtained  the  following  results  at  900°  :  — 

Pll*  PHaO  * 

13-5  nun.  8-8  mm.  1-53 

18-0  12-7  1-42 

37-4  25-1  1-45) 

54-1  35-4  1-53 

71-8  49-3  1-40 

If  therefore  steam  at  any  pressure  be  kept  in  contact  with  heated 
iron  at  900°,  it  will  be  decomposed  until  the  pressure  of  the  hydrogen 
formed  is  1'5  times  the  pressure  of  the  steam  remaining.  If  the  original 
pressure  of  the  steam  be  750  mm.,  the  pressure  in  the  final  equilibrium 
mixture  will  be  hydrogen  450  mm.  :  steam  300  mm;  whilst  if 
hydrogen  gas  be  heated  with  Fe304  at  900°,  steam  will  be  formed, 

the  reaction  ceasing  when  —  Ha   =  :    1'5.      If    the  original    hydrogen 

PH.O 

pressure  is  300  mm.,  the  final  pressures  will  be  H2  180  mm.  :  H20 
120  mm. 

In  the  last  type  of  heterogeneous  equilibrium  which  we  shall  discuss, 
two  01  more  phases  can  vary  in  concentration  at  constant  temperature. 


ii.]  EQUILIBRIUM  21 

Simple  examples  are  an  unsaturated  aqueous  solution  and  the  corre- 
sponding aqueous  vapour  phase,  or  a  solution  of  a  gas  in  a  liquid 
and  the  gaseous  phase  in  equilibrium  with  that  solution.  In  the 
former  case,  the  concentration  of  dissolved  substance  in  the  solution 
can  vary  between  zero  and  the  saturation  quantity,  whilst,  since 
the  vapour  pressure  of  the  water  is  lowered  by  the  solution  in  it 
of  the  dissolved  substance,  the  concentration  of  the  vapour 
phase  can  vary  between  the  limits  of  the  pressure  of  pure  water 
and  the  pressure  of  the  saturated  solution.  Similarly  the  gas 
pressure  in  the  second  case  can  be  arbitrarily  varied  within 
certain  limits,  and  to  each  pressure  corresponds  a  definite  solubility 
of  gas  in  the  liquid. 

Any  equilibria  in  the  separate  phases  can  be  treated  by  the  law  of 
mass  action,  and  we  need  here  only  consider  the  equilibria  between 
the  different  phases.  The  law  governing  these  equilibria  is  very 
simple.  Neglecting  complications  arising  when  the  molecules  of  a 
substance  are  of  different  complexity  in  different  phases  of  the  system, 
it  can  be  stated  as  follows  :  At  constant  temperature,  the  ratio  of  the  mole- 
cular concentration  of  a  'substance  in  one  phase  to  its  molecular  concen- 
tration in  another  phase  is  constant.  If  [CA]  and  [C2]  are  the  equilibrium 
'concentrations  in  the  two  phases, 


l         K 

rTn  = 

mj 

(This  constant  ratio  is  of  course  different  for  every  pair  of 
phases.) 

We  will  illustrate  the  above  law  by  a  few  examples.  Consider  a 
gas  in  equilibrium  with  its  solution  in  a  liquid.  For  [C2],  the  concentra- 
tion in  the  gaseous  phase,  we  can  substitute  p,  the  pressure,  or  partial 
pressure,  if  present  in  a  gaseous  mixture.  The  result  (known  as  Henry's 
Law)  is  that  the  solubility  of  a  gas  in  ariiquid  is  proportional  to  the 
pressure  of  the  gas. 

On  p.  344  is  given  an  instance  involving  the  solubility  of  chlorine 
in  water  at  different  partial  pressures.  On  p.  519  is  considered  the 
equilibrium 

3N02  -f  H20  ^  2HN03  +  NO, 

which  tends  to  set  up  when  nitrous  gases  are  passed  into  water.  The 
concentration  of  HN03  in  the  liquid  phase  is  of  course  directly  deter- 
mined (according  to  the  mass  action  law)  by  the  concentrations  of 
N02  and  NO  in  the  liquid  phase  ;  and  as  the  latter  are  proportional 
to  their  concentrations  in  the  gas  phase,  it  follows  that  the  HN03 
concentration  will  increase  with  increasing  N02  and  decreasing  NO 
partial  pressures. 

Succinic  acid  dissolves  in  both  water  and  ether.  If  therefore 
an  aqueous  solution  be  shaken  up  with  ether,  or  an  ethereal  solution 


22    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY     [CHAP. 

with  water,  some  of  the  acid  will  pass  from  one  solvent  to  the  other 
until  equilibrium  is  reached.  In  accordance  with  the  equation,  what- 
ever the  absolute  concentration  of  the  acid  in  the  two  liquids,  their 
ratio  will  always  be  constant  when  equilibrium  has  been  reached. 
The  following  table  contains  some  experimental  figures  :  — 

C,  =  grams  acid  per  ("'._>  =  grams  acid  per  Cl 

10  c.c.  water.  lOc.c.  ether.  ^  ~  (31 

0-024  0-0046  5-2 

0-070  0-013  5-4 

0-121  0-022  5-4 

K  is  termed  the  partition  coefficient  of  succinic  acid  between  water 
and  ether. 

On  p.  443  we  have  a  somewhat  similar  case,  involving  the  distribu- 
tion of  sulphur  between  liquid  steel  and  liquid  slag  in  a  steel-refining  fur- 
nace. But  the  qualification  caused  by  the  presence  of  different  molecular 
complexes  (here  chemically  different  molecules)  in  the  different  phases 
becomes  very  important.  The  law  is  only  qualitatively,  not  quanti- 
tatively, followed.  This  is  to  a  certain  extent  true  of  most  '  practical  ' 
cases. 

As  a  last  example,  consider  an  aqueous  solution  containing  ri  mols. 
of  solute  dissolved  in  n  mols.  of  solvent.  Let  the  vapour  pressures  of 
pure  solvent  and  of  solution  be  respectively  pQ  and  p.  If  the  molecu- 

lar concentration  |         -  j  of  the  solvent  when  in  the  pure  state  be  [C], 

its  molecular  concentration  in  the  solution  will  be  —  --  [C].  Applying 
the  law,  we  have 


Hence 


Po       ri  -\-  n 


PO  ri  +  n 

Po-p  =       n' 

Po  ri  '+  n 

For  a  dilute  solution  ri  is  small  compared  with  n,  and  we  can  write 

Po-P  =  »_'. 
po  n 

That  is,  the  ratio  of  the  difference  of  vapour  pressures  of  solution  and 


II.] 


EQUILIBRIUM 


23 


solvent  (the  '  lowering  '  of  the  vapour  pressure)  to  the  vapour  pressure 
of  the  pure  solvent  is  equal  to  the  ratio  of  the  number  of  molecules  of 
solute  present  to  the  number  of  molecules  of  solvent  in  which  the 
solute  is  dissolved.  This  is  an  exceedingly  important  result,  which 
we  shall  employ  in  Chapter  IV. 

4.  Effect  of  Temperature 

We  must  finally  briefly  discuss  the  effect  of  temperature  varia- 
tion on  equilibrium.  All  chemical  reactions  can  be  divided  into  endo- 
thermic, those  which  absorb  heat,  and  exothermic,  those  which  liberate 
heat.  And  the  manner  of  variation  of  the  equilibrium  constant  of  a 
reaction  with  temperature  depends  essentially  on  whether  the  reaction 
is  exothermic  or  endothermic.  Consider  any  chemical  system  in  equi- 
librium, and  imagine  its  temperature  to  be  raised.  We  can  readily 
see  that,  if  the  equilibrium  changes  with  rise  of  temperature,  that 


ioo 


100% 
2NO 


FIG.  1. 


FIG.  2. 


reaction  must  commence  which  absorbs  heat,  not  the  reverse  reaction 
which  gives  out  heat.  Were  this  not  so,  it  is  evident  that  more  heat 
would  be  added  to  the  system,  the  temperature  would  rise,  and  the 
reaction  would  proceed  more  and  more  rapidly,  the  temperature 
continually  rising.  Such  a  state  would  be  quite  unstable,  and  could 
correspond  to  no  conceivable  equilibrium.  We  conclude  therefore 
that  increased  temperature  favours  endothermic  reactions  and  the 
production  of  endothermic  compounds,  and  that,  conversely,  exothermic 
reactions  and  products  are  favoured  by  low  temperatures.  (These 
statements  apply  to  equilibrium  states  only,  not  to  reaction  velocities.) 
If  the  reaction  has  practically  no  heat  effect,  the  equilibrium  will  be 
almost  independent  of  temperature,  and  the  equilibrium  constants 
of  those  reactions  which  are  accompanied  by  large  heat  effects  (positive 
or  negative)  will  be  most  sensitive  to  temperature  changes. 

Fig.  1  shows  how  the  equilibrium  between  a  typical  exothermic 
compound    (NH3)    and   its   constituents    changes   with   temperature. 


24    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY     [CHAP. 

The  curve  gives  the  relation  between  temperature  and  percentage 
combination  or  decomposition.  At  low  temperatures  the  exothermic 
ammonia  is  stable  ;  at  higher  temperatures  it  dissociates  into  its 
constituent  gases.  Low  temperatures  are  therefore  favourable  for 
the  synthesis  of  ammonia  from  its  elements.1  Fig.  2  shows  how  the 
composition  of  a  system,  consisting  of  an  endothermic  compound 
such  as  NO  in  equilibrium  with  its  constituents,  changes  with  tempera- 
ture. Rise  of  temperature  favours  formation  of  the  compound.  NO 
is  produced  to  a  considerable  extent  at  the  temperature  of  the  electric 
arc.2 

Similarly  the  formation  of  HC1  from  hydrogen  and  chlorine,  of 
S03  from  S02  and  oxygen,  and  of  CuO  from  copper  and  oxygen  give 
out  heat  ;  at  higher  temperatures  HC1,  S03  and  CuO  can  all  be  made 
to  dissociate.  On  the  other  hand,  the  production  of  CaC2,  whether 
from  its  elements  or  from  lime  and  carbon,  takes  place  with  absorption 
of  heat,  and  high  temperatures  are  necessary  for  its  formation.3  The 
hydrolysis  of  chlorine,  or  of  cuprous  salts,  and  the  formation  of  cuprous 
and  aurous  salts  in  aqueous  solution  according  to  the  equations 

CuCl2  +  Cu  —  >  2CuCl 
AuCl3  -{-  2Au  :  —  *  SAuCl 

all  take  place  with  absorption  of  heat  and  are  favoured  by  a  rise  in 
temperature.4  The  esterification  of  C2H5OH  and  CH3COOH5  takes 
place  with  only  a  very  small  heat  effect,  and  the  value  of  the  equilibrium 
constant  K  is  consequently  almost  independent  of  temperature. 

The  equation  expressing  the  relation  between  the  equilibrium 
constant  K,  the  absolute  temperature  T,  and  the  heat  evolved  in 
the  reaction  Q  is 


K2  and  Kx  are  the  equilibrium  constants  at  the  temperatures  T2and  T,, 
and  Q  (expressed  in  calories)  is  assumed  to  be  independent  of  the 
temperature,  or,  if  able  to  vary,  to  be  the  mean  value  over  the  range 
of  temperature  T2  to  TI.  From  this  equation,  knowing  K  at  two  different 
temperatures,  we  can  calculate  Q  ;  or,  knowing  Q  and  the  value  of  K 
for  one  temperature,  we  can  calculate  it  for  another  temperature. 
For  example,  Thompson  •  worked  on  the  calcium  carbide  equilibrium 

CaO  +  3C  ^±  CaC2  +  CO. 

Amongst  other  measurements,  he  found  the  CO  equilibrium  pressure 
to  be  6'44  mm.  at  1445°  C.  and  0'82  mm.  at  1475°  C.     Q  at  1460°  is 

1  See  p.  479.  2  Chap,  xxvii.  :<  P.  469. 

4  Pp.  245,  246,  247,  273,  319.  5  P.  14.  «  P.  470. 


ii.]  EQUILIBRIUM  25 

-  116,000  cals.  (reaction  going  from  left  to  right).     We  have  therefore 

K2=        116000/ 1_  1        \ 

Kj  4-57     \1±75  +  273         1445  +  273/ ' 


TC         T) 

As  the  lime,  carbon,  and  carbide  are  solids,  we  can  substitute  =2  by  "z 

KI       Pi 
and  get 

g  y2  _  116000    B  30 

g  pi         4-57          1748  X  1718 
whence 


(0*82\ 
— -  )  was  1-86. 


Literature 

Nernst.     Theoretical  Chemistry. 

Mellor.      Chemical  Statics  and  Dynamics. 


CHAPTER  III 

FARADAY'S  LAWS— CURRENT  EFFICIENCY 
1.  Phenomena  of  Electrolysis 

Electronic  and  Electrolytic  Conductors. — Conductors  of  electricity 
can  be  divided  into  two  classes — metallic  or  electronic  conductors 
and  electrolytic  conductors.  Typical  of  the  former  class  are  sub- 
stances like  copper,  graphite,  and  Fe304.  To  the  latter  class  belong 
aqueous  solutions  of  H2S04,  AgN03  or  ammonia,  fused  PbCl2  or 
cryolite,  solid  BaCl2  or  BaS04  some  distance  below  their  melting-points, 
and  finally  hot  gases. 

A  fundamental  difference  exists  between  the  two  kinds  of 
conduction,  obvious  when  the  above  examples  are  considered.  In 
both  cases  the  current  produces  heating  and  magnetic  effects  ;  but 
in  l  metallic  '  conductors  the  electricity  passes  through  without  the 
accompaniment  of  any  ponderable  quantity  of  matter,  whereas  in  electro- 
lytic conductors  or  electrolytes  the  movement  of  the  electricity  is 
always  associated  with  the  movement  of  matter.  When  the  electricity 
leaves  the  electrolyte,  it  cannot  take  the  matter  with  it ;  the  latter  is 
consequently  set  free,  and  a  chemical  effect  is  produced,  which  marks 
the  chief  distinction  betwreen  electronic  and  electrolytic  conduction. 
The  study  of  these  chemical  effects  and  of  the  corresponding  electrical 
effects  necessary  for  their  production  forms  the  largest  and  most 
important  part  of  the  science  of  electrochemistry. 

Electrolysis. — If  two  platinum  plates  be  dipped  into  dilute  sul- 
phuric acid,  and  connected  with  the  two  poles  of  a  battery,  a  current 
passes  (the  sulphuric  acid  being  an  electrolyte)  ;  electrolysis  takes 
place,  and  it  is  noticed  that  at  the  platinum  plate  connected  with  the 
negative  terminal  of  the  battery  hydrogen  gas  is  given  off,  whilst  at 
the  plate  connected  with  the  positive  pole  of  the  current  source  oxygen 
is  evolved.  These  gases  are  produced  at  the  platinum  plates  only, 
jnul  not  along  the  path  of  the  current  through  the  electrolyte.  If  we 
use  instead  silver  nitrate  solution,  we  observe  deposition  of  silver  on 
one  platinum  plate  and  evolution  of  oxygen  at  the  other  ;  and  what- 
ever the  solution,  we  notice  that  chemical  action  only  takes  place  at 

16 


FARADAY'S  LAWS  27 

the  electrodes,  or  points  where  the  current  enters  and  leaves  the  electro- 
lyte. This  is  characteristic  of  electrochemical  actions.  The  electricity, 
whether  positive  or  negative,  while  moving  through  an  electrolytic 
conductor,  is  associated  with  some  form  of  matter.  When  it  leaves 
the  electrolyte  to  continue  its  journey  through  an  electronic  conductor, 
it  cannot  carry  this  accompanying  matter  further,  and  the  latter  is 
therefore  set  free. 

If  now  the  hydrogen  and  oxygen  from  the  H2S04  solution  are  col- 
lected, it  is  found  that  they  stand  almost  exactly  in  the  proportions 
necessary  to  form  water,  the  volume  of  hydrogen  being  twice  that  of 
the  oxygen.  Any  slight  deviation  from  this  exact  ratio  is  due  to  dis- 
turbing causes  discussed  later.  Finally,  if,  after  the  electrolysis  is 
finished,  samples  of  acid  in  the  neighbourhood  of  the  two  electrodes 
are  titrated,  it  is  found  that  the  one  from  the  neighbourhood  of  the 
positive  electrode  where  oxygen  was  evolved  is  stronger  than  the  one 
from  the  neighbourhood  of  the  negative  electrode.1  If,  however,  the 
whole  contents  of  the  electrolysis  vessel  or  electrolytic  cell  are  titrated, 
the  total  amount  of  acid  is  found  to  be  unchanged.  The  effects  of  the 
electrolysis  are  thus  : — 

(1)  to  decompose  water  into  hydrogen  and  oxygen,  the  products 
being  separated,  not  mixed  ; 

(2)  to  concentrate  one  portion  of  the  sulphuric  acid,  at  the  same 
time  diluting  another  portion. 

Suppose  similar  experiments  carried  out  with  various  solutions 
such  as  (a)  CuS04,  (6)  HC1,  (c)  KOH,  (d)  NaN03.  It  will  be  found  that 
the  respective  negative  electrode  products  are  (a)  metallic  copper, 
(6)  hydrogen,  (c)  hydrogen,  (d)  hydrogen  and  NaOH.  The  corresponding 
positive  electrode  products  are  (a)  oxygen  and  H2S04,  (6)  chlorine, 
(c)  oxygen,  (d)  oxygen  and  nitric  acid.  Concentration  changes  are 
also  produced  in  all  cases.  Thus  with  KOH,  the  solution  has  now 
become  stronger  near  the  negative  electrode.  A  consideration  of  these 
results  shows  that  the  negative  electrode  products  are  always  of  a 
metallic  or  basic  nature,  whilst  the  positive-  electrode  products  are 
always  of  a  non-metallic  or  acidic  nature.  This  again  is  a  perfectly 
general  result.  When  a  current  passes  through  an  electrolyte,  the 
nwtallic  or  basic  product  of  the  resulting  chemical  action  is  always  set 
free  at  the  negative  pole,  which  is  termed  the  cathode,2  whilst  the  non- 
metallic  or  acid  product  of  reaction  is  liberated  at  the  positive  pole  or 
anode.2  This  fact  is  utilised  when  it  is  necessary  to  distinguish  between 

1  To  the  engineer,  the  negative  pole  of  his  dynamo  (or  secondary  cell)  is  the 
pole  by  which  negative  electricity  leaves  the  machine.  To  the  electrochemist,  the 
negative  pole  of  his  electrolytic  bath  is  the  pole  by  which  negative  electricity  enters. 
This  distinction  must  be  clearly  borne  in  mind. 

•  If  the  electrolyte  is  divided  by  a  diaphragm  or  other  means  into  cathode  and 
anode  portions,  these  are  termed  catholyte  and  anolyte  respectively. 


28    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY     [CHAP. 

the  two  poles  of  a  current  source.  Wires  from  these  are  brought 
a  little  distance  apart  on  to  a  piece  of  pole  reagent  paper — filter- 
paper  moistened  with  a  solution  of  KI  or  a  little  NaaS04  and  phenol- 
phthalein.  In  the  former  case,  the  positive  pole  is  shown  by  a  brown 
coloration  due  to  iodine.  With  the  Na2S04  and  phenol-phthalein 
a  pink  coloration  is  produced  at  the  negative  pole  by  the  liberated 
alkali. 

2.  Faraday's  Laws 

So  far  we  have  seen  that  electrolysis  has  two  effects — the  formation 
of  two  substances  or  groups  of  substances  of  opposite  polarity  and 
the  production  of  concentration  changes.  The  only  quantitative 
relation  noticed  has  been  that,  in  the  electrolysis  of  dilute  sulphuric 
acid,  hydrogen  and  oxygen  are  liberated  in  the  proportions  in  which 
they  form  water.  Suppose  now  that  an  electrolyte  be  put  in  the  same 
circuit  with  an  ammeter,  and  electrolysed,  the  time  being  noted  and 
the  current  kept  constant.  When  the  electrolysis  is  finished,  let  the 
products  be  measured  (by  weight  or  volume)  and  let  this  procedure 
be  repeated,  using  different  currents  during  different  lengths  of  time. 
It  will  be  found  that  the  resulting  chemical  effect  is  directly  proportional 
to  the  product  of  the  current  into  the  time  of  electrolysis,  that  is,  to  the 
quantity  of  electricity  which  has  flowed  through  the  electrolyte.  This  is  a 
statement  of  Faraday's  First  Law  of  Electrolysis. 

Thus,  if  it  is  found  that  30  grams  of  copper  are  produced  by  10  amperes  flowing 
for  x  hours,  then  the  same  quantity  will  be  produced  by  20  amperes  in  -  hours, 

or  by  three  amperes  flowing  for          hours  ;  and  CO  grams  will  be  produced  by  a 

3 

current  of  10  amperes  flowing  for  2  x  hours,  etc. 

Faraday's  First  Law  expresses  then  the  fundamental  relation 
existing  between  quantity  of  electricity  and  quantity  of  any  particular 
substance  set  free  by  or  taking  part  in  an  electrolytic  process.  In  the 
above  statement  of  it,  the  assumption  is  made  that  one  reaction  only 
takes  place  at  each  electrode. 

The  question  now  arises,  In  what  relation  do  the  quantities  of 
different  substances  liberated  by  the  same  quantity  of  electricity  stand 
to  one  another  ?  Suppose  a  number  of  electrolytic  baths  arranged  in 
series,  so  that  when  a  current  is  passing  the  same  quantity  of  electricity 
flows  through  each  of  them.  Let  one  contain  copper  electrodes  in 
CuS04  solution,  another  silver  electrodes  in  AgN03  solution,  a  third 
platinum  electrodes  in  Na2S04  solution,  a  fourth  platinum  electrodes 

no./» 

in  fairly  strong  HC1.     Let  current  be  passed  through  till  -  -  grams  of 

2 

copper  have  been  deposited  on  the  copper  cathode  in  the  first  vessel, 


in.]  FARADAY'S  LAWS  29 

and  let  the  effects  produced  at  the  other  electrodes  be  measured.    From 

/»O.£» 

the  copper  anode  in  (1),  — -  grams  of  copper  will  have  been  dissolved. 

In  (2),  107*9  grams  of  silver  will  have  been  deposited  on  the  cathode 
and  dissolved  from  the  anode.  1*01  grams  of  hydrogen  will  be  obtained 
at  the  cathode  in  both  (3)  and  (4),  and  further  in  (3)  40  grams  of  NaOH. 

16  98 

Finally,  at  the  anode  of  (3)  we  shall  obtain  —  grams  of  oxygen  and  - 

Z  2 

grams  of  H2S04,  and  at  the  anode  of  (4)  35*5  grams  of  chlorine.  These 
quantities  are  in  all  cases  proportional  to  the  equivalent  weights  of  the 
substances  concerned.  This  relation  is  known  as  Faraday's  Second 
Law  of  Electrolysis,  and  together  with  the  first  law  can  be  combined 
into  the  following  formal  statement : — 

If  a  current  pass  through  an  electrolyte,  bringing  about  chemical 
changes  at  the  electrodes,  the  quantity  of  each  substance  formed  will 
be  directly  proportional  to  its  equivalent  weight  and  to  the  time  of 
passage  of  the  current. 

The  quantity  of  electricity  capable  of  bringing  about  the  transforma- 
tions described  in  the  preceding  paragraph,  and  of  liberating  one 
gram-equivalent  of  product,  is  termed  a  faraday,  and  has  been  found 
by  careful  experiment  to  be  very  nearly  96,540  coulombs  or  ampere- 
seconds.  The  ampere-hour  is  3,600  coulombs.  We  therefore  have 
the  relation 

96,540  coulombs  (ampere-seconds)  =  26'8  ampere-hours 

=  1  faraday. 

Conception  of  Ions. — To  explain  the  regularities  described,  Faraday 
suggested  that  the  neutral  dissolved  molecules  of  an  electrolyte  consist 
of  two  oppositely  charged  parts,  which  he  termed  ions.  The  negatively 
charged  fraction  or  anion  is  composed  of  a  non-metal  or  of  an  acid 
radicle,  whilst  the  positively  charged  part,  the  cation,  consists  of  a 
metal,  of  hydrogen,  or  of  some  positive  radicle,  such  as  NH4.  When 
the  current  passes,  the  positive  ions  are  attracted  towards  the  negatively 
charged  cathode,  where  their  charge  is  neutralised,  and  they  are  set 
free.  The  anions  behave  similarly,  being  discharged  at  the  positive 
electrode,  the  anode.  If  the  further  assumption  be  made  that  the 
quantity  of  electricity — positive  or  negative — associated  with  one 
gram-equivalent  of  every  kind  of  ion  is  always  the  same,  and  equal 
to  96,540  coulombs,  all  the  above  results,  both  qualitative  and  quanti- 
tative, can  be  consistently  explained.  This  conception  of  Faraday's, 
developed  in  accordance  with  later  discoveries,  is  now  universally  used. 
A  unit  positive  charge  is  usually  denoted  by  ',  a  negative  charge  by  '. 
As  the  number  of  charges  per  unit  will  vary  directly  as  the  valency 
of  the  ion,  we  write  K",  CT,  Cu*  (cuprous  ion),  Cu"  (cupric  ion), 


30      PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY     [CHAP. 

OH',  SO/',  FeCy6""  (ferrocyanide  ion),  etc.  Corresponding  neutral 
molecules  can  be  written  K2"S04",  Cu"Cl2",  etc.,  though  the  signs 
representing  the  charges  are  usually  omitted. 


3.  Current  Efficiency 

The  minimum  quantity  of  electricity  needed  for  the  production  of 
a  gram-equivalent  of  an  electrolytic  product  is  26 '8  ampere-hours. 
We  emphasise  the  word  minimum,  because  in  the  average  electrolysis 
more  than  this  amount  is  required. 

This  is  not  due  to  any  breakdown  of  Faraday's  Law,  which  has 
been  shown  to  hold  over  a  wide  range  of  temperatures,  for  electrolytes 
other  than  aqueous  solutions,  and  for  fused  salts.  One  faraday  passed 
through  an  electrolyte  will  always  liberate  one  gram-equivalent  of 
product  at  each  electrode.  But  it  often  happens  that  this  product 
is  not  homogeneous.  Some  of  the  electricity  given  up  at  the  electrode 
may  have  been  associated  with  one  kind  of  ion,  and  some  with  another 
kind  of  ion.  Thus,  in  the  electrolysis  of  brine  solutions  for  the 
production  of  alkali  and  chlorine,  under  certain  circumstances  large 
quantities  of  oxygen  can  be  evolved  at  the  anodes  instead  of 
chlorine. 

Secondly,  and  more  important,  is  the  fact  that  the  primary  pro- 
duct of  electrochemical  action,  when  liberated  from  its  accompanying 
charge  of  electricity,  can  often  react  partly  or  completely  with  bodies 
in  the  neighbourhood  of  the  electrode,  some  thus  being  lost ;  or  it  is 
liberated  in  such  a  form  that  it  cannot  be  easily  collected,  or  it  can 
perhaps  diffuse  rapidly  away,  and  to  some  extent  recombine  with  the 
product  of  electrolysis  at  the  other  electrode.  If  a  KC1  solution  be 
electrolysed  between  copper  electrodes,  neither  potassium  nor  chlorine 
is  obtained  as  a  final  product.  The  former  acts  on  the  water,  producing 
alkali  and  hydrogen,  the  latter  attacks  the  copper  electrode,  giving 
cuprous  chloride.  When  sodium  is  prepared,  as  in  the  Castner  process, 
by  the  electrolysis  of  molten  NaOH,  a  certain  quantity  is  lost  by 
vaporisation ;  more  reacts  with  the  water  which  is  constantly  formed 
during  the  process  ;  a  third  source  of  loss  is  due  to  dissolved  metal 
escaping  to  the  anode,  and  there  combining  with  the  evolved  oxygen. 
Or  the  substance  itself  may  be  chemically  unstable  and  tend  to 
decompose  spontaneously — e.g.  sodium  hyposulphite.  Good  results  are 
then  best  obtained  by  using  a  high  current  concentration,1  this  being 

current 

defined  by  the  ratio  -  — .     The 

volume  of  electrolyte  (catholyte,  anolyte) 

required  concentration  of  product  is  in  this  way  quickly  reached. 
Again,  current  leaks  and  short  circuits  often  cause  wastage,  as  in 
copper  refining. 

1  Tafel,  Ber.  33,  2212  (1900). 


in.]  CURRENT  EFFICIENCY  31 

Consequently,  the  yield  of  material  in  an  electrolytic  process  is 
nearly  always  less  than  that  calculated  from  the  quantity  of  electricity 
used.  The  ratio  of  yield  obtained  to  the  theoretical  yield  calculated 
on  the  basis  of  Faraday's  Law  is  termed  the  current  efficiency,  and  a 
high  current  efficiency  is  of  course  one  of  the  chief  aims  of  an  electrolytic 
process.  Sometimes  it  is  the  first  thing  to  be  considered,  often  its 
importance  may  be  considerably  modified  by  other  factors. 

Technical  current  efficiencies  vary  within  wide  limits.  The  anodic 
oxidation  of  anthracene  to  anthraquinone  gives  almost  100  per  cent 
In  copper  refining,  one  of  95  per  cent,  is  the  average.  Alkali-chlorine 
cells  furnish  50-100  per  cent.,  depending  on  the  type  of  cell  and  the 
concentration  of  alkali  produced.  Hypochlorite  liquors  are  obtained 
at  about  66  per  cent,  current  efficiency  ;  metallic  sodium  and  aluminium 
from  fused  electrolytes  at  about  45  per  cent,  and  70  per  cent,  respectively. 
These  figures  all  refer  to  the  chief  product.  One  should  really  speak 
of  anodic  and  cathodic  current  efficiencies. 


4.  Measurement  of  Quantity  of  Electricity 

The  determination  of  current  efficiency  involves,  therefore,  two 
measurements — the  Weight  or  volume  of  product  and  the  quantity 
of  electricity  used.  The  Matter  may  be  measured  in  two  ways.  The 
current  may  be  observed  at  frequent  intervals,  when  the  mean  figure, 
multiplied  by  the  time,  will  give  the  number  of  ampere-hours  expended  ; 
or  this  last  quantity  may  be  measured  directly. 

For  the  detection  of  current,  and  occasionally  for  the  measurement 
of  small  currents,  galvanometers  are  used.  For  heavier  currents 
ammeters  are  employed.  To  enter  here  into  their  construction  or 
use  is  unnecessary.  It  might,  however,  be  mentioned  that  the  most 
economical  system  for  general  experimental  electrochemical  work  is 
the  employment  of  a  high-resistance  milliamperemeter,  together  with 
a  number  of  external  shunts.  One  instrument  will  thus  cover  an 
enormous  range.  The  shunt  resistances  are  relatively  cheap.  The 
method  of  measuring  current  by  employing  a  high-resistance  volt- 
meter and  a  low  resistance  of  known  value  in  the  main  circuit  is  based 
on  the  identical  principle.  Putting  the  voltmeter  across  the  ends 
of  this  resistance,  we  have 

I-E 
~R 

Coulometers. — For  the  measurement  of  quantity  of  electricity  in 
small-scale  experimental  electrochemical  wrork,  coulometers  are  used. 
In  these  instruments  some  electrochemical  product  is  liberated  by  the 
current  in  easily  measurable  form,  and  from  its  amount,  assuming 
Faraday's  Law,  the  quantity  of  electricity  passed  through  can  be 


32    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY     [CHAP. 

readily  calculated.  In  a  good  coulometer  there  must  be  no  disturbing 
or  secondary  by-reactions,  and  much  investigation  has  been  devoted 
to  this  point.  It  has  been  found  necessary  to  define  carefully  the 
conditions  under  which  the  instruments  are  used. 

The  silver  coulometer  is  the  most  accurate 1  of  all  so  far  described. 
As  worked  out  by  T.  W.  Richards,2  it  is  set  up  as  follows  (Fig.  3). 

The  cathode  consists  of  a  platinum 
crucible  containing  the  electrolyte, 
a  freshly  prepared  10  per  cent. 
AgN03  solution.  The  anode  is  a 
pure  silver  rod,  wrapped  in  filter 
paper  and  suspended  in  AgN03 
solution,  but  separated  from  the 
cathode  by  a  1  mm.  Pukall  porous 
pot.  The  level  of  liquid  inside  this 
pot  is  lower  than  that  outside,  and 
diffusion  of  anode  liquid  to  the 
cathode  is  thus  counteracted. 
Richards  found  this  necessary  in 
consequence  of  substances  formed 
at  the  anode  which  can,  under 
varying  conditions  of  current 
density,  produce  either  too  large 
or  too  small  cathodic  deposits. 
The  anodic  and  cathodic  current 
densities  must  not  exceed  0'2  and 
0*02  amp./cm.2  respectively. 

Where  complete  accuracy  is  not  required,  the  copper  coulometer 
can  be  used.  It  is  cheaper  and  gives  sufficiently  good  results.  Here 
again  certain  complications  occur  which  will  be  fully  considered  later,3 
but  which  are  largely  avoided  in  the  form  described  (Fig.  4).  The 
electrolyte  consists  of  the  following  solution 4 : — 


FIG.  3. — Silver  Coulometer. 


150  grams  copper  sulphate  crystals, 
50  grams  H2S04, 
50  grams  alcohol, 
1  litre  water. 


The  electrodes  number  three,  and  are  conveniently  hung  parallel 
to  the  sides  of  a  rectangular  glass  vessel.  They  can  be  kept  in 
position  by  slots  cut  in  small  pieces  of  wood  which  grip  on  the 
edges  of  the  shorter  sides  of  the  glass  jar.  Two  of  these  electrodes 
are  anodes,  one  the  cathode.  The  last,  a  thin  sheet  of  electro- 


,.. »;. 

3  Pp.  245-247. 


-  Zeitech.  Phys.  Chem.  41,302 
4  F.  Oettel,  Chem.  Zeit.  17,543 


III.] 


CURRENT  EFFICIENCY 


33 


Ccu&w&e 


lytic  copper,  is  suspended  between  the  anodes.  Its  increase  in 
weight  is  the  measure  of  the  quantity  of  electricity  passed  through. 
The  decrease  in  weight  of  the  anodes  is  too  indefinite.  These  anodes 
can  be  made  of  ordinary 

sheet   copper,   and    are   best  Anode* 

enclosed  in  bags  of  parch- 
ment paper  to  keep  impuri- 
ties from  the  electrolyte. 
The  size  of  the  coulometer  is 
determined  by  the  currents 
it  is  designed  to  carry.  To 
get  good  results,  a  cathodic 
current  density  of  0'5-2'5 
amps./d.m.2  (taking  into  ac- 
count both  sides  of  the 
cathode)  must  be  used.  Too 
high  values  furnish  a  deposit 
dark  in  colour,  loose,  and 
easily  rubbed  off.  If,  on  the 
contrary,  the  current  density 
be  less  than  the  lower  limit 
given,  the  weight  of  copper 
deposited  is  too  small. 
During  long-continued  use, 
the  electrolyte  should  be 
stirred,  conveniently  by  a 
slow  stream  of  hydrogen, 

which  also  opposes  oxidising  effects  due  to  dissolved  air.  When  the 
electrolysis  is  finished,  the  cathode  is  withdrawn,  washed  thoroughly 
with  water  and  then  alcohol,  dried  quickly  by  holding  some  distance 
above  a  Bunsen  flame,  and  weighed.  As  31 '8  is  the  equivalent  weight 
of  cupric  copper,  if  ra  be  the  increase  in  weight  of  the  cathode  in 
grams,  and  x  the  number  of  ampere-hours  passed  through, 

96540 


FIG.  4. — Copper  Coulometer. 


x  = 


m 
3F8  '    3600 


In  the  water  coulometer,1  the  volume  of  electrolytic  gas  evolved 
during  the  passage  of  the  current  is  measured.  This  gas  is  produced 
by  the  electrolysis  of  a  15  per  cent.  NaOH  solution  (NaCl-free)  between 
nickel  electrodes.  The  electrolysis  vessel  is  an  ordinary  glass  bottle, 
provided  with  a  rubber  stopper.  This  carries  a  delivery  tube,  dipping 
under  a  graduated  glass  cylinder,  which  has  been  filled  and  inverted 
over  water.  The  passage  of  one  faraday  produces  one  gram  (11*2 
litres)  of  hydrogen,  and  eight  grams  (5 '6  litres)  of  oxygen — altogether 

i  F.  Oettel,  Zeitsch.  Elektrochem.  1,  355  (1894). 


34:     PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 


16*8  litres  of  electrolytic  gas  at  0°  and  one  atmosphere.  One  ampere- 
minute  means  therefore  10*44  c.c.  of  gas.  To  convert  the  volume 
as  actually  read  off  to  standard  conditions,  we  can  use  the  formula 

273  (b  -  p) 


=  v 


(273  +  0)  760 


where  v0-6o  is  the  corrected  volume  and  v  the  observed  volume  in  c.c., 


0.760 


b  the  barometric  pressure  and  p  the  vapour  pressure  of  water  in  mm. 
of  mercury.     Then,  if  x  be  the  ampere-minutes  used, 


10-44 

This  instrument  is  not  capable  of  the  same  accuracy  as  the  copper 
coulometer,  in  fact  it  cannot  be  relied  upon  beyond  +  0*5  per  cent. 
Its  chief  advantage  is  that  it  enables  us  to  follow 
closely  certain  oxidation  or  reduction  reactions  whilst 
actually  proceeding,  and  to  determine  the  yield  over 
any  given  length  of  time  without  interrupting  the 
electrolysis.  By  putting  the  coulometer  in  series 
with  the  electrolytic  bath,  and  comparing  the 
quantities  of  gas  liberated  in  the  two  cases  over 
the  same  period  of  time,  it  can  be  found  what 
portion  of  the  current  is  doing  useful  work  and 
what  fraction  is  being  expended  in  the  undesirable 
production  of  hydrogen  and  oxygen.1 

Wright  Electricity  Meter. — Large  quantities  of 
electricity  can  be  directly  measured  by  means  of 
ampere-hour  or  electricity  meters.  Of  these  instru- 
ments one  only  gives  reliable  and  accurate  results. 
This  is  the  Wright  Electrolytic  Meter,  as  modified  by 
Abegg  and  by  Hatfield,  and  sold  in  this  country  by 
the  Reason  Manufacturing  Co.  of  Brighton.  An  accur- 
ately shunted  fraction  of  the  current  passes  through 
a  ballast  resistance  and  through  the  cell,  shown 
in  essentials  in  Fig.  5.  The  anode  is  of  mercury, 
kept  at  a  constant  level  on  the  ring-shaped  ledge  A, 
by  means  of  the  reservoir  B.  The  cathode  C  is  of 
iridium  foil,  and  the  electrolyte  a  potassium  iodide 
solution  of  mercuric  iodide.  When  the  current 
passes,  droplets  of  mercury  are  produced  at  the 
cathode.  Being  of  iridium,  no  amalgamation  or 
adhesion  occurs,  but  the  mercury  falls  through  the 
funnel  underneath  into  the  U-tube  D.  There  its 
volume  can  be  read  ofi,  and  gives  a  measure  of  the  quantity  of 

»  See  p.  36. 


-I) 


FIG.  5.— Wright 

Electrolytic 

Meter. 


in.]  CURRENT  EFFICIENCY  35 

electricity  passed.  When  the  U-tube  is  completely  filled  the  contents 
siphon  over  into  the  bottom  of  the  apparatus  E,  where  is  another  scale. 
At  the  anode  mercury  dissolves  quantitatively,  and  the  composition 
of  the  electrolyte  remains  absolutely  unchanged.  Local  concentra- 
tion differences  at  the  electrodes  are  ingeniously  nullified,  and  mercury 
is  prevented  from  passing  from  the  anode  to  the  lower  part  of  the 
apparatus  (by  shock  or  otherwise)  by  means  of  the  fence  of  glass  tubes 
F.  Temperature  changes  are  compensated  by  the  ballast  resistance. 
The  apparatus  is  reset  by  simply  inverting  it,  when  the  mercury 
flows  again  into  the  reservoir  B.  Its  accuracy  is  within  1  per  cent., 
though  the  amperage  may  vary  from  10  per  cent,  to  150  per  cent,  of 
the  rated  figure.  It  is  exceedingly  reliable  and  constant,  and  needs 
no  attention.  Hatfield  l  has  also  given  some  interesting  details  of 
an  instrument  in  which  bromine  is  precipitated  and  measured,  which 
promises  to  be  still  more  useful  for  small  quantities  of  electricity. 

Calibration  of  Ammeters. — Large  quantities  of  electricity  are, 
however,  still  usually  measured  by  making  periodical  readings  of  an 
ammeter,  and  the  calibration  of  this  instrument  is  important.  It  can 
be  carried  out  in  several  ways. 

(a)  Most  conveniently  with  a  standard  ammeter.    The  two  instruments 
are  connected  in  series,  together  with  a  variable  resistance,  such  as  the 
type  consisting  of  a  number  of  loosely  packed  carbon  plates  which  can 
be  screwed  up  into  better  contact.      Current  is  passed  through,  and 
simultaneous  readings  made.     A  whole  series  can  be  taken  by  suitably 
regulating  the  current. 

(b)  With  a  standard  high  resistance  voltmeter,  standard  low  resistance^ 
and  adjustable  resistance.     The  use  of  voltmeter  and  known  resistance 

for  current  measurement  has  already  been  mentioned.2    Knowing  the 

-p 

value  of  the  standard  resistance  R,  the  true  current  is  given  by  I  =  - , 

R 

where  E  is  the  voltmeter  reading.  A  comparison  series  is  taken  as  in 
method  (a). 

(c)  With  a  coulometer.     This  method  is  much  less  rapid  and  con- 
venient than  (a)  or  (6),  and  can  only  be  used  for  low-range  ammeters. 
It  has  the  advantage  of  needing  no  standard  instrument  of  any  kind. 
Suppose  a  copper  coulometer  to  be  used.     It  is  connected  in  series  with 
the  ammeter  and  variable  resistance.     Until  conditions  are  constant 
and  the  ammeter  registers  the  required  reading,  the  weighed  cathode 
for  actual  use  is  not  inserted,  a  similar  auxiliary  one  being  used.     When 
all  regulation  is  finished,  the  current  is  interrupted,  the  working  cathode 
rapidly  put  in,  the  electrolysis  restarted  and  the  time  noted.     The 
experiment  is  carefully  watched,  and  any  fluctuations  in  the  ammeter 
reading  neutralised  by  adjusting  the  resistance.     When  sufficient  time 

1  Zeitsch.  Elektrochem.  15,  728  (^909).  •  See  p.  31. 

D2 


36     PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

(previously  calculated)  has  been  allowed  for  the  deposition  of  a  con- 
venient quantity  of  copper,  the  electrolysis  is  again  interrupted  and 
the  time  noted,  the  increase  in  weight  of  the  cathode  determined,  and 
the  current  calculated,  assuming  26  '8  ampere-hours  for  each  31'8  grams 
of  copper.  This  is  repeated  for  a  number  of  ammeter  readings. 

5.  Calculation  of  Current  Efficiencies 

Before  closing  this  chapter,  examples  will  be  given  of  the  calculation 
of  current  efficiencies. 

1.  The  electrolytic  reduction  of  carbonic  acid  to  formic  acid  !  is  being  studied 
on  a  small  scale.  The  catholyte  is  saturated  KoSO4  solution,  through  which 
COo  gas  bubbles.  The  cathode  is  an  amalgamated  zinc  plate,  2*5  d.m.2  in  area 
(one  side).  A  current  of  about  0*25  amp.  is  passed  for  eight  hours.  Water  and 
copper  coulometers  are  included  in  the  circuit.  Occasionally  the  Ho  —  COo  mixture 
which  continually  passes  away  from  the  cathode  is  collected,  and  after  absorbing 
the  COo  by  potash,  the  volume  of  the  residual  hydrogen  is  compared  with  the 
volume  of  the  electrolytic  gas  evolved  in  the  water  coulometer  during  the  same 
period.  On  two  such  occasions,  the  volume  of  the  electrolytic  gas  is  in  each 
case  60  c.c.  ;  the  volume  of  cathodic  hydrogen  10*6  c.c.  and  8'7  c.c.  respectively,  in 
all  cases  as  directly  measured.  The  formic  acid  produced  is  estimated  at  the  end, 
and  amounts  to  1*23  grams.  The  increase  in  weight  of  the  cathode  in  the  copper 
coulometer  is  2'335  grams.  Required  (a)  the  current  efficiencies  during  the  periods 
over  which  the  gas  samples  are  collected  and  (6)  the  current  efficiency  during  the 
whole  run. 

(a)  Of  the  60  c.c.2  of  electrolytic  gas,  40  c.c.  are  hydrogen.  This  quantity 
would  be  liberated  on  each  occasion  in  the  electrolysis  vessel  itself  if  no  gas  were 
to  be  used  in  the  reduction  process.  But  10*6-  c.c.  and  8-7  ~  c.c.  are  the  amounts 
actually  liberated.  Hence  (40  —  10'6)  c.c.  and  (40  —  8'7)  c.c.  respectively  are  used 
in  the  reduction.  The  current  efficiencies  are  therefore 
40  —  10-6 

Per  cent-»  ancl 


40  —  8-7 
100  •  —  £Q  —  =  78-25  per  cent. 

(6)  The  increase  in  weight  of  the   copper   cathode  is  2-335  grams.     The 

2-335 
number  of  ampere-hours  used  is  consequently  _,  _    •  26-8.      (Average  current  is 

tin  -re  fore  -5    o"  *  ~  ~  =  0-246  ampere.)  The  equation  expressing  the  reduction  is 
ol'o          o 

HoC03  -t-  Ho  --  >  HoCOo  -f  H20  (passage  of  2  faradays). 

We  see  that  to  produce  one  mol.  (46  grams)  of  formic  acid,  two  faradays  or 
2  .  26'8  ampere-hours  are  needed.     The  theoretical  yield  of  formic  acid  is  therefore 


* 

x  46  = 


2  x  26-8 
and  the  current  efficiency  is 

1-23 
100  •  Y^Q  =  72  8  per  cent. 

1  Coehn  and  S.  Jahn,  Ber.  37,  2,836  (/.w/). 

-  Note  that  unconnected  readings  can  be  used. 


in.]  CURRENT  EFFICIENCY  37 

2.  A  1,000-ainpere  alkali-chlorine  cell  is  being  tested  over  a  short  run.      The 

cathode   liquors   are   intended    to    contain   120    gr8-'   NaOH.      The   current   is 

litre 

measured  by  a  shunted  ammeter  every  five  minutes.  After  constant  conditions 
are  attained,  the  cell  is  run  for  six  hours,  during  which  time  70'3  litres  of  caus- 
ticised  brine  are  produced,  containing  on  an  average  118'0  grams  NaOH  per  litre. 
The  current  readings  run  thus  970  ...  1,008  .  .  .  1,021  .  .  .  1,002  ...  984  ... 
987  ...  1,011  .  .  .  1,020  .  .  .  1,005  ...  994  ...  1,000  .  .  .  1,007,  etc.,  and  average 
1,002  amperes.  Required,  the  cathodic  current  efficiency. 

The  NaOH  produced  =  70'3  x  118  =  8295  grams.  The  ampere-hours  used 
=  6  x  1002  =  6012  ampere-hours.  The  production  of  one  equivalent  (40  grams) 
of  NaOH  requires  theoretically  26'8  ampere-hours. 

Hence,  the  theoretical  NaOH  production  will  be 

6012 
and  the  current  efficiency 


—  x  40  =  8973  grams, 


8295 
100  •    <       =  92-4  per  cent. 


Literature 

Le  Blanc.     Electrochemistry. 

Lorenz.     Elektrochemisches  PraJctiJcum 


CHAPTER  IV 

OSMOTIC  PRESSURE— THEORY  OF  SOLUTIONS 
1.  Osmotic  Pressure 

IT  is  well  known  that  the  particles  of  a  dissolved  substance  behave 
analogously  to  those  of  a  gas.  If  pure  water  be  carefully  superposed 
on  a  sugar  solution,  sugar  can  be  detected  after  a  certain  time  in  all 
parts  of  the  liquid,  and  the  whole  will  ultimately  attain  a  uniform 
composition.  This  is  quite  similar  to  the  expansion  of  a  gas  into  a 
vacuum  or  from  a  high  to  a  low  pressure.  Or  if  solutions  of  KC1  and 
CuS04  be  brought  into  contact,  diffusion  will  proceed  until  the  two 
bases  and  the  two  acids  are  distributed  equally  throughout  the  whole 
mass,  just  as  gaseous  oxygen  and  hydrogen  will  interdifftise  until  a 
uniform  gaseous  mixture  results.  It  is  natural  to  seek  a  common 
explanation  for  these  parallel  phenomena,  and  to  suppose  the  dissolved 
particles  to  be  in  a  state  of  continual  spontaneous  motion,  like  the 
particles  of  a  gas,  and  therefore  capable  of  exerting  a  pressure,  moving 
against  gravity,  and  diffusing. 

The  existence  of  this  osmotic  pressure  can  be  easily  proved. 
It  is  not  manifest  at  the  boundary  surfaces  of  a  solution,  being  counter- 
acted by  an  internally  directed  pressure  of  the  order  of  1000  atmo- 
spheres, due  to  the  surface  tension  of  the  solvent.  To  demonstrate  its 
existence  the  influence  of  the  solvent  must  be  eliminated.  This  con- 
dition can  be  realised  experimentally  if  we  enclose  the  solution  in  a 
vessel  through  the  walls  of  which  the  solvent  molecules  can  freely  pass, 
but  which  is  impermeable  to  the  particles  of  the  solute.  These  particles 
will  still  exert  their  pressure  on  the  walls,  whereas  the  solvent  will  no 
longer  have  any  effect,  but  will  behave  as  an  indifferent  medium. 

Such  a  semi-permeable  membrane  can  be  readily  prepared  by  forming 
a  layer  of  cupric  ferrocyanide  in  the  interior  of  the  walls  of  an  ordinary 
unglazed  porcelain  pot.  K4FeCy6  solution  is  allowed  to  diffuse  into  the 
walls  from  the  inside  of  the  pot,  and  CuS04  solution  from  the  outside. 
The  precipitate  forms  in  the  structure  of  the  wall,  the  pot  being  subse- 
quently thoroughly  freed  by  washing  from  soluble  salts.  (To  obtain 
a  membrane  which  will  withstand  high  pressures,  additional  precautions 
must  be  taken.) 

38 


OSMOTIC  PRESSURE 


39 


FIG.  6. — Osmometer. 


The  apparatus  is  then  built  up  as  shown  in  Fig.  6.     In  the  top  of 
the  porous  pot,  containing,  for  example,  a  sugar  solution,  a  glass  tube  is 
cemented  with  sealing-wax.      This  is  connected  with  a  manometer, 
and  has  also  an  open  T-piece  a,  which  is  sealed 
off  after  the  glass  tube  has  been  cemented  on. 
The  level  of  the  manometer  liquid  is  indicated 
by  b  b.     Finally  the   pot  is  immersed  in   a 
vessel  of  water. 

What  happens  is  as  follows.  At  the  semi- 
permeable  membrane  equilibrium  is  non- 
existent. There  is  a  resultant  pressure  differ- 
ence equal  to  the  osmotic  pressure  of  the 
dissolved  sugar.  Also  the  walls  are  permeable 
to  the  solvent.  The  conditions  correspond  to 
those  of  a  gas  put  into  communication  with 
a  vacuum.  The  gas  tends  to  expand — to 
lower  its  pressure.  The  solution  tends  to 
dilute — to  lower  its  osmotic  pressure.  The 
gas  expands  by  particles  entering  the  vacuum, 
the  solution  dilutes  by  solvent  entering 
through  the  walls.  This  process  tends  to 
continue  until  there  is  no  difference  of  os- 
motic pressure  between  the  two  sides  of 

the  membrane,  in  this  case  indefinitely,  as  there  is  no  sugar  outside 
the  porous  pot. 

But  there  is  an  opposing  force  acting.  The  gaseous  analogy  is  a 
weighted  piston  closing  a  vacuum.  The  gas  will  only  expand  into  the 
vacuum  until  its  pressure  falls  to  that  exerted  by  the  weight  on  the 
piston.  Here  the  solvent  entering  the  porous  pot  compresses  the  air 
above  the  solution,  and  this  alters  the  level  of  the  manometer  liquid. 
The  increased  pressure  in  the  apparatus  tends  to  force  the  solvent  out 
through  the  porous  wall.  As  the  process  continues,  the  osmotic 
pressure  of  the  solution  decreases,  owing  to  dilution,  and  the  pressure 
in  the  apparatus  increases.  When  these  pressures  are  equal,  no  more 
solvent  will  enter.  Equilibrium  is  attained,  and  the  pressure  indicated 
by  the  manometer  is  equal  to  the  osmotic  pressure  of  the  solution  in 
the  cell — not  equal  to  the  osmotic  pressure  of  the  solution  originally 
present,  but  to  that  of  the  more  dilute  solution  formed  at  the  finish. 
This  difference  can  be  made  very  small  by  choosing  a  manometer  of 
narrow  bore,  and  filling  it  with  a  heavy  liquid. 

2.  Solution  Laws 

It  was  in  1885  that  Van't  Hoff  showed  the  full  theoretical  significance 
of  tliis  phenomenon,  though  its  existence  had  long  been  known  to 


40      PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY     [CHAP. 

botanists.  In  particular,  the  well-known  plant  physiologists  Pfeffer, 
de  Vries,  and  Traube  had  done  much  experimental  work  in  that 
field.  The  excellent  properties  of  the  cupric  ferrocyanide  membrane 
were  first  demonstrated  by  Traube,  and  it  was  to  Pfeffer's  measurements 
on  the  osmotic  pressure  of  sugar  solutions  that  Van't  Hoff  applied  his 
calculations. 

He  showed  that  the  solutions,  if  not  too  concentrated,  behaved 
quantitatively  as  if  the  sugar  were  in  the  gaseous  state,  occupying 
the  same  volume  as  the  sugar  solution — osmotic  pressure  being,  of 
course,  substituted  for  gaseous  pressure.  To  be  more  precise,  he  showed 
that  the  osmotic  pressure  P  of  the  sugar  solution  was  proportional  to 
its  concentration,  or  inversely  proportional  to  its  dilution  v.  Thus 

Pt;  =  constant. 

The  gaseous  analogy  is  simply  Boyle's  Law 

pv  =  k. 

Table  II  contains  some  of  Pfeffer's  results  for  sugar  solutions  at 
room  temperature. 

TABLE  II 

n=%of  Pin  mm.  Corresponding  p 

calculated 
Of  *Z  gas  pressure 

1  535  527  535 

2  1016  1054  508 
2-74  1518  1449  554 
4  2082  2108  521 
6  3075  3162  513 

The  osmotic  pressure  rises  with  increase  of  temperature,  and  Van't 
Hofi  showed  the  coefficient  of  increase  of  pressure  per  1°  C.  to  be  -  -  - 

—the  figure  observed  by  Gay  Lussac  for  perfect  gases  heated  at  constant 
volume.  The  osmotic  pressure  is  thus  proportional  to  the  absolute 
temperature.  Table  III  illustrates  this.  It  contains  Pfeffer's  results 
for  a  1  per  cent,  sugar  solution,  expressed  in  atmospheres.  The  formula 
from  which  the  '  calculated '  values  are  obtained  is 
P  =  0-655  (1  -f  0-00366  0). 

TABLE  III 

A  Observed  Calculated 

pressure  pressure 

6-8  0-664  atm.  0-671  atm.  -{-0-007 

13-7  0-691  0-688  —0-003 

14-2  0-671  0-688  +0-017 

15-5  o-'i.st  0-692  +0-008 

22-0  0-721  0-708  —0-013 

32-0  0-716  0-732  +0-016 

36-0  0-746  0-741  —0-005 


iv.]  OSMOTIC  PRESSURE  41 

For  strengths  of  sugar  solution  other  than  1  per  cent.,  the  equation 
becomes 

P  =  0-655.  n  (1+0-00366  0), 

where  n  is  the  percentage  of  sugar  dissolved.     This  value  n  is  inversely 
proportional  to  the  dilution  v,  and  we  can  put 


Substituting  -  for  0'00366  and  absolute  for  Centigrade  temperature, 
273 

we  obtain 

_  0-655  fcjT 
~ 


or 

Pv  =  £2T. 

The  corresponding  gas  equation  is 

pv=RT 

and  if  one  gram-molecule  of  gas  or  of  dissolved  substance  be  considered 
and  the  same  units  used,  it  is  found  that  k2  =  R. 

For  example,  a  1  per  cent,  sugar  solution  at  7°  C.  had  an  osmotic  pressure  of 
505  mm.  (Pfeffer).     Then 

505       2 

P  =  760  =  3  atmosPnere 
T  =  273  +  7  =  280°. 

The  formula  of  cane  sugar  is  C^Ho-On,  the  molecular  weight  consequently  342. 
One  gram  occupies  100  c.c.  of  solution  ;  342  grams  would  thus  occupy  34-2  litres. 
The  value  of  A>  in  litre  -atmospheres  per  degree  is  therefore 

34-2  x  2 


The  gas  constant  R,  expressed  in  the  same  units,  is  0'0821.  We  can 
consequently  write 

Pw  =  RT. 

The  whole  analogy  between  dilute  solutions  and  perfect  gases  is 
most  strikingly  shown  by  the  following  statement,  so  very  similar  to 
Avogadro's  Law.  At  the  same  osmotic  pressure  and  the  same  temperature, 
equal  volumes  of  solutions  contain  the  same  number  of  dissolved  molecules. 

The  above  results  of  Pfefier  were  obtained  by  rather  crude  methods, 
and  the  agreement  is  often  imperfect.  In  recent  years  the  technique 
of  these  measurements  has  been  fully  worked  out,  more  particularly 
by  Morse  and  his  collaborators  in  America,  and  possible  errors  have  been 
attacked  and  eliminated  in  detail.  Very  high  osmotic  pressures  can 
now  be  measured  with  considerable  exactness.  Some  of  the  results  of 
Morse  and  Frazer  on  sugar  solutions  follow.  In  the  calculation  of 
Pfefier's  approximate  figures  in  the  above  tables,  no  distinction  was 
made  between  weight  and  volume  percentages.  In  Tables  IV  and  V 


!L'     PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY     [CHAP. 

the  theoretical  pressures  are  calculated  on  the  assumption  that  the 
sugar  is  gasified  in  a  volume  equal  to  that  of  the  solvent  present. 
Concentrations  are  expressed  as  gram-molecules  per  1,000  grams  of 
water;  pressures  are  in  atmospheres. 


TABLE  IV 


Concentration 

e 

P  found 

Corresponding 
gas   pressure 

Difference 

0-05 

20-5 

1-25 

1-21 

—  0-04 

0-1 

18-5 

2-44 

2-40 

—  0-04 

0-2 

21-5 

4-80 

4-85 

+  0-05 

0-3 

19-4 

7-23 

7-22 

—  0-01 

0-5 

20 

12-08 

12-07 

—  0-01 

0-8 

17-5 

19-07 

19-14 

+  0-07 

0-9 

20-2 

21-80 

21-74 

-0-06 

1-0 

22-5 

24-34 

24-34 

— 

The  figures  of  Table  V  show  how  Gay  Lussac's  Law  is  obeyed. 


TABLE  V 


Concentration 

0  =  4-5° 

0=10° 

6=  15° 

0-1 

2-40 

2-44 

2-48 

0-2 

4-75 

4-82 

4-91 

0-3 

7-07 

7-19 

7-33 

0-4 

9-43 

9-58 

9-78 

0-5 

11-82 

12-00 

12-29 

0-6 

14-43 

14-54 

14-86 

0-7 

16-79 

17-09 

17-39 

0-8 

19-31 

i  I.-T.-, 

20-09 

0-9 

22-15 

22-28 

22-94 

1-0 

24-83 

25-06 

25-42 

Mean  molecular  } 

osmotic  pressure  j 

24-12 

24-50 

24-98 

Extrapolating,  we  get  23'7  atmospheres  as  the  molecular  osmotic 
pressure  at  0°.  The  temperature  coefficient  of  osmotic  pressure  is 
therefore,  taking  the  value  at  0  =  0°  as  unity, 


24-98  -  24-12 
23-7  X  10 


0-00363, 


whilst  the  coefficient  of  increase  of  gaseous  pressure  per  degree  is 
0-00366. 


iv.]  OSMOTIC  PRESSURE  43 

3.  Determination  of  Molecular  Weight  of  Dissolved  Substances 

From  Osmotic  Pressure  Measurements.  —  It  is  clear  that  if 
we  know  the  osmotic  pressure  and  temperature  of  a  solution,  and 
also  the  concentration  of  the  solute,  we  can  calculate  the  molecular 
weight  of  the  latter.  Using  -the  gas  constant  R,  the  equation 


holds  for  that  volume  of  solution  containing  one  gram-molecule  of  the 
dissolved  substance.  If  n  gram-molecules  are  contained  in  the  volume 
v,  the  equation  becomes 

Pv  =  n  RT. 
f 

Measuring  P  therefore,  and  knowing  v  and  T,  n  can  be  calculated.  But 
if  m  be  the  mass  of  solute  dissolved,  and  M  is  its  molecular  weight,  we 
have 

m=  nM., 

whence  M  can  be  calculated. 

For  example,  a  solution  containing  40  grams  of  dissolved  substance  per  litre 
shows  an  osmotic  pressure  of  2  -74  atmospheres  at  14°  C.  What  is  the  molecular 
weight  of  the  dissolved  substance? 

We  have  P  =  2-74  :  v  =  1-0  :  T  =  287  :  R  =  0-0821. 
Whence,  from  Pi;  =  wRT,  we  get  n  =  0-1163. 
Substituting  in  the  equation 

m  =  Mn, 
where  m  is  40,  we  finally  have 

M  =  344. 

From  Vapour  Pressure  Measurements.  —  But  in  practice  osmotic 
pressure  measurements  are  not  easily  carried  out.  They  are  tedious, 
and  a  membrane  that  will  serve  for  one  series  of  measurements  may 
be  useless  for  another,  owing  to  its  permeability  to  the  dissolved 
substance.  A  cupric  ferrocyanide  membrane  is  in  general  an  efficient 
one,  but  is  useless  for  many  electrolytes,  e.g.  potassium  nitrate.  Other 
methods  of  ascertaining  the  molecular  weights  of  dissolved  substances 
are  generally  used.  We  have  seen  x  that  the  vapour  pressure  of  a 
solution  is  lower  than  the  vapour  pressure  of  the  solvent,  and  that, 
putting 

p0  =  vapour  pressure  of  solvent, 

p  =  vapour  pressure  of  solution, 

n'  =  number  of  molecules  of  solute  dissolved  in 

n  molecules  of  solvent, 
we  have  the  relation 


pQ       "  ri  +  n 
1  P.  22. 


44    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

or.  where  ri  is  small  compared  with  n,  that  is,  when  the  solution  is  a 
dilute  one, 

Po  ~  P  _  n' 
PO  n 

If  M',  M',  are  the  molecular  weight  and  mass  of  the  solute,  and  M,  m, 
the  molecular  weight  and  mass  of  the  solvent,  we  have,  as  before, 

m  =  Mn  and  m'  =  M'w', 
and  the  equation  becomes 

p0  —  p  _  w'  ^  M 
PO       ~  m      M' 

Consequently  by  measuring  p,  m'  and  m,  and  knowing  p0  and  M,  we 
can  calculate  M'.     In  the  particular  case  of  a  1  per  cent,  solution, 

m'        1 

-=—  ,  and  we  get 


or,  expressed  fully,  the  product  of  the  molecular  weight  of  a  dissolved 
substance  into  the  relative  lowering  of  vapour  pressure  of  the  solvent 
in  a  1  per  cent,  solution  is  equal  to  the  molecular  weight  of  the  solvent 
divided  by  one  hundred.  In  accordance  with  this  the  values  of  p 
for  solutions  of  perchlorethane,  turpentine,  cyanic  acid,  benzoic  acid, 
trichloracetic  acid  and  benzaldehyde  in  ether  were  found  to  be 
0*71-0*72  (molecular  weight  of  ether  74).  Similarly  for  water  the 
figure  is  0*185  (theoretical  value  0*18). 

From    Freezing-point    and    Boiling-point    Measurements.—  But 
vapour-pressure  measurements,  like  osmotic-pressure   measurements, 

are  not  readily  carried  out.  In  practice, 
freezing-point  and  boiling-point  determina- 
tions are  made.  In  Fig.  7,  AB  represents 
the  sublimation  curve  of  ice,  and  BC  the 
vaporisation  curve  of  water.  The  point 
B  corresponds  to  the  freezing-point  T. 
Let  DE  represent  the  vapour  pressure 
curve  of  a  solution.  It  cuts  the  curve 
AB  at  D.  At  this  point  and  the  corre- 
8ponding  temperature  T',  the  solution  and 
ice  have  the  same  vapour  pressure,  and 
consequently  T'  is  the  freezing-point  of 

the  solution.  The  freezing-point  of  a  solvent  is  therefore  lowered 
by  the  presence  of  a  solute. 

And  we  can  easily  see  that,  as  in  the  immediate  neighbourhood  of 
the  freezing-point,  the  different  curves  can  be  regarded  as  straight  lines, 


IV.] 


OSMOTIC  PRESSURE 


this  depression  is  proportional  to  the  lowering  of  vapour  pressure  of 
the  solvent.  Thus  if  DE  and  D'E7  (Fig.  8)  denote  the  vapour  pressure 
curves  for  two  different  solutions  of  freezing-points  respectively  D  and 
D7,  BF  and  BF7  will  be  the  two  lowerings  of  vapour  pressure,  and  DG 
and  DG7  the  two  depressions  of  freezing-point.  Then  we  see  at  once 

that 

BF        BD  _PG 

BF"7  ~~  BIT  ~~  D7G7 

Hence  the  freezing-point  of  a  solution  is  lower  than  that  of  the  solvent 
by  an  amount  directly  proportional  to  the  molecular  concentration  of  the 
solute.    If  dT  represents  the  depression 
of  freezing-point, 


m 


M 


Temperature. 

FIG.  8. 


A  similar  proposition  can  be  proved 
for  the  boiling-point,  which  is  higher, 
not  lower,  for  the  solution  than  for 
the  solvent. 

These  relations  were  discovered 
empirically  by  Raoult,  and  employed 
by  him  to  determine  the  molecular 
weights  of  many  substances  in  solution.  Van't  Hoff  succeeded  in 
deducing  them  thermodynamically,  in  showing  their  connection  with 
the  osmotic  pressures  of  the  solutions,  in  actually  calculating  Raoult's 
empirical  constants  for  many  solvents  from  latent-heat  and  freezing- 
point  or  boiling-point  data, — in  a  word,  in  putting  the  whole  subject 
on  a  sound  theoretical  basis. 

If  we  rewrite  the  last  equation,  referring  the  freezing-point  de- 
pression to  a  quantity  of  solution  containing  100  grams  of  the  solvent, 
we  get 

dT-k     ** 

'  100 

=  kr  .  n' 

It  was  the  value  of  the  constant  Iff  which  Van't  Hoff  calculated.  His 
calculation  cannot  here  be  considered  in  detail,  but  it  will  suffice  to 
give  the  final  result,  which  is 

RT2 


or,  putting  R  =  2  cals., 


100  L 


fc7  =  0-02    - 
L 


where  T  is  the  freezing-point  and  L  the  latent  heat  of  fusion  per  gram 
of  the  pure  solvent. 


46    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY     [CHAP. 

Table  VI  shows  how  closely  the  values  of  k'  thus  calculated  agree 
with  the  same  found  by  freezing-point  measurements. 


TABLE  VI 


Substance 

T 

L 

T2 

0-02  -- 
Li 

k'  experi- 
mental 

Water     

273 

80-3 

18-6 

18-4 

Acetic  acid 

290 

43-2 

38-8 

39-6 

Formic  acid 

281-5 

55-6 

28-4 

27-7 

Benzene             

278 

29-1 

53-0 

50-0 

Nitrobenzene 

278-3 

22-3 

69-5 

70-7 

As  an  example  of  a  freezing-point  molecular  weight  determination, 
we  may  take  the  following  : — 

The  freezing-point  of  20  grams  of  water  is  lowered  0-362°  by  dissolving  in 
it  0-3  gram  of  a  substance  A.  What  is  the  molecular  weight  of  A  ? 

Substituting  dT  =  0-362  and  k'  =  18-4  in  the  equation  dT  =  k'n'  we  get 
n'  =  0-0197. 

A  concentration  of  0-3  gram  per  20  c.c.  corresponds  to  m'  =  1-5  grams  per 
100  grams  of  solvent. 

Whence  by  substitution  in 

m'  =  MV 

1-6 

we  get 


M' 


0-0197 


«  =  76. 


The  relation  connecting  an  increase  of  the  boiling-point  of  a  solvent 
and  the  gram-molecular  concentration  of  the  solute  is  very  similar. 
These  equations  have  been  verified  for  many  solvents  and  solutes,  and 
have  proved  of  great  service  in  the  measurement  of  the  molecular 
weights  of  the  latter.  Like  the  osmotic-pressure  equations,  they  only 
hold  strictly  for  dilute  solutions  ;  with  strong  solutions  complications 
arise,  similar  to  those  occurring  in  the  theory  of  highly  compressed 
gases.  But  even  in  the  field  of  dilute  solutions,  there  is  a  large  and 
important  class  of  apparent  exceptions. 


4.  Anomalous  Behaviour  of  Electrolytes 

All  solutions  of  strong  electrolytes  (strong  acids,  bases,  and  their 
salts)  show  an  anomalous  behaviour,  giving  abnormally  large  osmotic 
pressures,  lowerings  of  vapour  pressure  and  of  freezing-point.  Such 
solutions  behave  in  fact  as  if  they  contain  a  larger  number  of  molecules 
than  would  be  calculated  from  the  weighed  amount  of  substance  dis- 
solved. Thus,  instead  of  the  equation 


iv.]  OSMOTIC  PRESSURE  47 

holding  for  the  osmotic  pressure  of  that  amount  of  solution  which 
contains  one  chemical  gram-molecule  of  solute,  the  relation 

Pv  =  iRT 

must  be  used,  where  i  —  the  Van't  Hoff  factor  —  is  always  greater  than 
unity,  though  seldom  exceeding  three.  Similarly  the  lowering  of 
vapour  pressure  is  given  by 


Po 
and  the  lowering  of  freezing-point  by 


dT  =  k  .  i.  - 
n 


Moreover,  for  the  same  solution,  the  same  value  of  i  must  be  introduced 
into  these  various  equations.  With  increasing  dilution  of  the  solution, 
it  becomes  greater,  and  finally  approaches  some  simple  integral  limiting 
value. 

To  illustrate  this  abnormal  behaviour,  we  may  take  vapour  pres- 
sure measurements  of  aqueous  solutions  of  electrolytes.  We  have  seen l 
that  for  a  1  per  cent,  solution,  the  product  of  the  molecular  weight  of 
the  dissolved  substance  into  the  relative  lowering  of  vapour  pressure 
of  the  solvent  should  equal  the  molecular  weight  of  the  solvent  divided 
by  one  hundred.  For  water  the  value  of  this  constant  (p)  should  be 
0-18,  and  has  been  found  by  experiment  to  be  O185  for  non-electrolytes. 
With  electrolytes  the  values  of  Table  VII  are  obtained. 

TABLE  VII 

Solute  p  i 

KAc  0-326  1-76 

LiCl  0-359  1-94 

LiBr  0-367  1-98 

KCNS  0-327  1-77 

Ca(N03)2  0-432  2-34 

CaClo  0-423  2-29 

When  similar  behaviour  is  encountered  in  the  measurement  of 
gaseous  vapour  densities,  the  abnormal  values  are  attributed  to  dissocia- 
tion of  the  normal  molecules  into  simpler  constituents,  e.g.  PC15  into 
PC13  and  Cl.,.  In  precisely  the  same  way  the  abnormal  results  discussed 
above  are  explained  by  the  dissociation  of  the  molecules  of  the  electro- 
lyte into  simpler  constituents,  each  of  which  can  act  as  a  separate 
individual  in  determining  the  osmotic  pressure  or  vapour  pressure  of 

1  ?• 


48        PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY 

the  solution.  This  dissociation  is  greater  in  dilute  solution,  just  as 
the  dissociation  of  gaseous  substances,  when  taking  place  with  an 
increase  in  the  number  of  molecules,  is  greater  at  low  pressures. 

The  factor  i  gives  the  ratio  of  the  total  number  of  particles  in  solution 
to  the  number  that  would  be  expected  to  be  present,  judging  from  the 
molecular  weight  and  quantity  dissolved  of  the  substance.  What 
the  nature  of  these  simpler  constituents  is  will  be  shown  in  Chapter  VI. 


Literature. 

Le  Blanc.     Electrochemistry. 


CHAPTER  V 

IONIC  TRANSPORT  DURING  ELECTROLYSIS 
1.  Mechanism  of  Migration  of  Ions 

IN  Chapter  III  we  dealt  with  the  phenomena  occurring  at  the  elec- 
trodes when  electricity  enters  or  leaves  an  electrolyte,  and  saw  how  pro- 
portionality exists  between  the  quantity  of  electricity  and  the  quantity 
of  matter  with  which  it  is  associated.     In  Chapters  V  and  VI  we 
must  consider  the  mechanism  of  the  passage  of  electricity  through  the 
electrolyte. 

Views  of  Grotthuss. — Our   present  ideas   on  this  subject   sprang 
originally     from     Grotthuss. 

Previously  to  him,  the  view      ftw+|c>ccccccoocco<). 
was    that   when    a    current 
passed   through    an    electro-  I 

lyte,  liberating  positive   and      •»   <lr»  ••••.••••••«  IJO 

negative  constituents  at  the      (e)    ,,  M  M  M  M  M  , 

electrodes,  these  positive  and 

negative    constituents    arose        _       i  I 

.  (d)   IICCCCCCOCCCCC  D 

from    the     same    molecules. 

Each  molecule  of  the  electro-  FIG.  9. 

lyte  was  therefore  separately 

decomposed,  and  furnished  its  decomposition  products  simultaneously 

at  the  electrodes. 

But  according  to  Grotthuss  this  was  incorrect.  On  applying  a 
voltage  to  electrodes  dipping  in  an  electrolyte,  the  molecules  were  first 
polarised,  all  their  positive  parts  becoming  turned  towards  the  negative 
pole,  and  vice  versa,  as  in  Fig.  9  (a).  When  the  voltage  exceeded  a 
certain  value,  decomposition  commenced,  and  the  positive  and  negative 
-ions  nearest  the  negative  and  positive  poles  respectively  were  torn 
:away  from  their  molecules,  the  charge  passing  off  through  the  electrodes, 
and  the  material  part  being  liberated,  as  in  Fig.  9  (6). 

The  next  stage  was  an  interchange  of  the  different  ions  along  the 
whole  series  of  polarised  molecules,  resulting  in  the  neutralisation  of 
the  free  ions  y  y  according  to  Fig.  9  (c).  Lastly  the  molecules  were 

49  E 


50    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY     [CHAP. 

twisted  round  under  the  influence  of  the  electric  field,  and  assumed 
the  position  (d),  similar  to  (a),  ready  for  the  cycle  to  recommence. 

Views  of  Faraday.  —  In  Grotthuss'  conception,  therefore,  the 
opposed  attractive  and  repulsive  actions  of  the  two  poles  form  the 
essential  feature.  Faraday  modified  this  idea  and  made  it  less  rigid. 
He  regarded  the  applied  voltage  as  simply  influencing  the  forces  of  i 
affinity  which  play  between  the  electro-positive  and  electro-negative 
parts  of  the  molecules,  and  causing  the  oppositely  charged  ions  to 
wander  more  markedly  in  one  direction  than  another.  The  essential 
cause  of  the  passage  of  electricity  through  the  electrolyte  lies  as  much, 
therefore,  in  the  nature  of  the  dissolved  substance  as  in  the  presence  of  the  \ 
electrodes,  which  are  regarded  by  Faraday  merely  as  *  doors '  by 
which  electricity  enters  or  leaves  the  electrolyte,  and  only  capable 
of  influencing  the  direction  of  motion  of  the  different  ions. 

According  to  him,  it  is  simplest  and  *  most  philosophical  *  to  state 
that,  during  electrolysis,  such  substances  as  oxygen,  chlorine,  and 
iodine  wander  (migrate  or  are  transported)  towards  the  positive  pole,  and 
bodies  like  hydrogen  and  the  metals  towards  the  negative  pole.  This 
is  virtually  the  view  adopted  to-day  of  the  transport  of  electricity 
through  an  electrolyte. 


2.  Quantitative  Relations  of  Ionic  Migration  I 

The  questions  which  immediately  arise  are  the  following.  With 
what  actual  velocity  will  a  given  ion  migrate  towards  an  electrode  under 
given  conditions  ?  And  do  all  ions  migrate  at  the  same  rate  under  the! 
same  conditions  ?  The  first  question  cannot  be  answered  until  the] 
next  chapter,  when  we  shall  deal  more  fully  with  the  nature  of  the  ionsJ 
and  their  relations  to  the  neutral  molecules.  The  second  point  wild 
now  be  treated. 

Effect  of  Differing  Ionic  Velocities.  —  There  is  no  reason] 
a  priori  for  assuming  that  the  different  ions  will  migrate  at  the  samel 
rate  under  the  same  conditions.  As  a  matter  of  fact  they  do  not.1 
Let  Fig.  10  (a)  represent  the  successive  stages  of  an  electrolysis,  supl 
posing  that  the  ions  do  all  move  at  equal  rates,  and  let  the  verticaH 
dotted  line  represent  a  diaphragm,  dividing  the  cell  into  two  parts.: 
Then  we  see  that  the  diminution  of  concentration  is  the  same  at  anjl 
moment  in  the  two  parts  of  the  cell.  Thus,  when  two  molecules  have 
been  discharged,  the  concentration  in  both  anolyte  and  catholyte  has 
fallen  by  one  molecule ;  with  four  molecules  discharged,  the  concen-j 
tration  has  decreased  by  two  molecules  in  each  compartment.  If  I 
therefore,  the  ions  taking  part  in  the  electrolysis  move  at  the  same  ratd 
under  the  electric  field,  and  if  nothing  takes  place  at  anode  and  cathode 
beyond  the  mere  discharge  of  ions,  the  concentration  changes  produced] 
at  the  electrodes  will  be  identical. 


IONIC  TRANSPORT 


51 


But  suppose,  on  the  contrary,  that  the  oppositely  charged  ions  move 
at  different  rates,  as  actually  occurs.  Let  the  positive  ions  have  twice 
the  velocity  of  the  negative  ions.  Then  Fig  10  (6)  will  represent  the 
course  of  electrolysis.  And  we  see  that  the  diminution  in  concentra- 
tion in  the  anode  liquid  from  which  the  more  rapidly  moving  positive 
ions  are  migrating  is  greater  than  the  diminution  in  the  catholyte. 
Moreover,  the  ratio  of  these  two  diminutions  of  concentration  is  2  : 1, 


(a) 


5  e  ee 
i  eee 

\  eeee 
eeeee 

eeeeee 


e 
ee 

eee 


FIG.  10. 

identical  with  the  ratio  of  the  relative  velocities  of  the  ions.  Thus, 
when  three  molecules  have  been  discharged,  the  catholyte  concentra- 
tion has  decreased  by  one  molecule,  the  concentration  at  the  anode 
by  two  molecules. 

This  is  a  perfectly  general  result.  If  the  cation  moves  five  times 
as  fast  as  the  anion,  the  diminution  in  concentration  in  the  anode  com- 
partment will  be  five  times  as  great  as  that  in  the  cathode  compartment, 
and  we  can  write 

Diminution  in  concentration  at  cathode       velocity  of  anion       u^ 
Diminution  in  concentration  at  anode       ~~  velocity  of  cation  =~uc 

E  2 


52    PRINCIPLES  OF  APPLIED   ELECTROCHEMISTRY    [CHAP. 

In  this  way,  therefore,  by  measuring  the  concentration  changes  at  the 
electrodes  during  electrolysis,  we  can  get  a  value  for  the  ratio  of  the 
velocities  of  the  different  ions. 

If,  for  example,  in  the  electrolysis  of  hydrochloric  acid  at  platinum  electrodes, 
it  were  found  that  the  diminution  of  acidity  in  the  anolyte  corresponded  to  a  nor- 
mality of  0-21  and  in  the  catholyte  to  a  normality  of  0-046,  we  should  have 

ttv      0-21 

— —  4-^fi 

uc      0-046  " 

Transport  Numbers. — It  is  evident  that  the  relative  fractions 
of  the  current  carried  through  the  electrolyte  by  the  different  ions  will 
vary  directly  as  the  velocities  of  the  ions,  if  these  are  of  the  same 
valency.  If  the  anion  moves  four  times  as  quickly  as  the  cation,  it 
will  carry  four  times  as  much  current.  Suppose  a  fraction  nA  of  the 
total  current  carried  by  the  anion,  and  hence  1  —  n^  nc  by  the  cation. 
Then  we  have 

UA      velocity  of  anion      fraction  of  current  carried  by  anion      WA 
uc  ~  velocity  of  cation  ~~~  fraction  of  current  carried  by  cation  ~  nc 

The  values  nA  and  nc  are  termed  the  transport  number  or  migration 
ratio  of  anion  and  cation  respectively  for  the  given  electrolyte.  They 
are  connected  by  the  equation 


and  express  both  the  relative  velocities  of  and  the  relative  fractions 
of  current  carried  by  the  different  ions.  Nothing  is  implied  as  to  the 
influence  of  concentration,  etc.,  and  any  given  figures  must  be  regarded 
as  holding  good  for  one  particular  set  of  conditions  only. 

3.  Determination  of  Transport  Numbers 

The  first  worker  in  this  field  was  Hittorf  (1853-1859),  who  recog- 
nised that  there  were  no  grounds  for  assuming  that  all  the  ions  would 
move  at  the  same  rate.  He  carried  out  many  careful  determinations 
of  transport  numbers,  and  was  able  to  make  important  deductions  on 
the  constitution  of  aqueous  solutions  of  certain  salts.  The  principle 
on  which  his  measurements  were  based  is  essentially  the  one  dis- 
cussed— the  determination  of  concentration  changes  produced  near 
the  electrodes  during  electrolysis.  The  forms  of  apparatus  he  used 
have  now  been  superseded  by  more  accurate  and  convenient  ones. 
Hittorf  had  no  high  voltages  at  his  command,  and  was  unable  therefore 
to  use  long  columns  of  liquid  in  his  experiments.  To  avoid  diffusion 
effects,  he  employed  diaphragms,  the  use  of  which  is  open  to  objection. 
And  he  was  further  unable  to  produce  large  differences  in  concent  nit  ion. 

A  more  modern  but  nevertheless  simple  type  of  apparatus,  used  by 


V-] 


IONIC  TRANSPORT 


53 


Nernst  and  Loeb,1  is  shown  in  Fig.  11.  Suppose  the  transport  numbers 
of  a  silver  nitrate  solution  are  being  determined.  The  cathode,  con- 
tained in  the  bulb  on  the  right,  consists  of  silver  foil.  The  anode  is  a 
spiral  of  silver  wire,  fused  into  a  thin  glass  tube.  It  is  introduced  into 
the  limb  on  the  left  through  the  rubber  stopper  A.  The  level  of  the 
electrolyte  is  indicated  in  the  figure.  The  apparatus  is  put  in  series 
with  a  current  source  and  a  silver  coulo- 
meter,  and  a  suitable  quantity  of  elec- 
tricity, measured  by  the  coulometer, 
passed  through.  When  the  experiment 
is  completed,  the  heavy  anode  liquor 
in  the  bottom  of  the  long  limb  is  blown 
off  through  C  by  means  of  the  tube  at 
B,  and  collected  separately.  The 
'  middle '  liquor,  which  in  an  actual 
experiment  might  occupy  the  space 
between  a  and  b,  and  finally  the  lighter 
cathode  liquor  are  withdrawn  and  col- 
lected, in  each  case  separately.  These 
three  fractions  are  then  weighed  and 
analysed.  Other  types  of  apparatus 
have  been  employed.  That  of  H. 
Jahn  and  Hopfgartner2  might  particu- 
larly be  mentioned. 

Before  considering  some  actual 
determinations  in  detail,  there  are 
certain  points  to  be  noted.  It  is  un- 
desirable for  accurate  determinations 
that  gases  should  be  evolved  at  either 

electrode,  as  the  different  parts  of  the  electrolyte  may  thereby  be 
mixed.  If  the  migration  ratios  are  being  determined  for  a  substance 
like  silver  nitrate  or  copper  sulphate,  obviously  no  question  arises 
of  disturbances  occurring  at  the  cathode.  But  if  an  insoluble 
anode  be  used,  free  acid  will  be  formed,  and  oxygen  evolved.  As 
the  H'  ion  of  the  acid  travels  far  more  quickly  than  any  other  cation, 
the  relations  may  be  obscured  not  only  by  mixing  due  to  the  gas,  but 
also  by  an  appreciable  fraction  of  the  current  being  carried  by  the 
H*  instead  of  by  the  Ag"  or  Cu"  ions. 

The  remedy  is  to  use  a  soluble  anode  of  the  metal  whose  ion  is 
already  in  solution — in  the  present  case  silver  or  copper.  The  compli- 
cation thereby  introduced  must  be  allowed  for  in  the  subsequent  calcu- 
lation. Sometimes  it  is  impossible  to  use  such  an  anode,  as  in  the 
determination  of  the  transport  numbers  of  a  salt  of  an  alkali  metal.  In 

1  Zeibch.  Phys.  Chem.  2,  948  (1888). 
-  Zeitach.  Phys.  Chem.  25,  119  (1898). 


T 


FIG.  11. — Transport  Number 
Apparatus. 


54    PRINCIPLES  OF  APPLIED   ELECTROCHEMISTRY    [CHAP. 

that  case  an  anode  of  amalgamated  zinc  or  cadmium  is  used,  a  device 
employed  by  Hittorf .  Both  zinc  and  cadmium  ions,  more  particularly  the  ] 
latter,  move  very  slowly  in  the  electric  field,  will  not  catch  up  the  alkali 
metal  ions,  and  consequently  will  not  disturb  the  transport  relations. 

Similar  complications  can  occur  at  the  cathode  if  the  transport 
numbers  of  an  acid  or  of  a  salt  of  an  alkali  metal  are  being  determined. 
The  evolved  hydrogen  stirs  up  the  liquid,  and  any  free  OH'  ions  pro- 
duced, which,  like  the  H*  ions,  can  travel  very  quickly,  catch  up  the 
anions  of  the  original  electrolyte  which  are  moving  towards  the  anode, 
and  upset  the  transport  relations.  To  avoid  this,  the  cathode  is 
usually  covered  with  a  saturated  layer  of  some  very  soluble  salt,  such  as 
ZnCl2  or  Cu(N03)2,  the  anion  of  the  salt  being  identical  with  that  in  the 
main  electrolyte.  With  HN03,  Cu(N03)2  would  be  used,  and  ZnCl2 
with  an  alkaline  halide.  This  done,  no  hydrogen  is  formed,  as  zinc 
or  copper  is  deposited  at  the  cathode ;  and  consequently  no  free 
alkali  is  produced,  as  is  the  case  in  the  electrolysis  of  an  alkali  metal 
salt  solution  without  this  precaution. 

Another  method  has  been  introduced  by  Noyes,1  to  counteract  the 
disturbing  influence  of  the  OH'  and  H'  ions.  This  consists  in  their 
continued  neutralisation  by  the  addition  of  measured  quantities  of  acid 
and  alkali  to  catholyte  and  anolyte  respectively.  By  eliminating 
disturbing  effects  in  that  way,  long-continued  experiments  can  be 
carried  out,  resulting  in  greater  concentration  changes  and  more 
reliable  data. 

In  every  experiment,  it  must  be  definitely  shown  that  no  mixing 
or  diffusion  has  taken  place  between  the  anode  and  cathode  liquors ; 
and  consequently  the  middle  layer  must  always  be  unchanged  in 
composition  after  the  experiment,  otherwise  the  results  of  the  deter- 
mination must  be  rejected. 

Examples. — We  append  a  couple  of  examples.  T  he  second  shows 
the  type  of  calculation  which  exact  experiments  furnish. 

1.  In  the  determination  of  the  transport  numbers  of  a  dilute  AgNO:,  solution 
(1*98  grams  AgNO.,in  100  grams  solution)  it  was  found  that  the  increase  of  silver 
in  the  anolyte  was  0-1298  gram  (using  a  silver  anode)  and  the  decrease  of  silver 
in  the  catholyte  0-1300  gram.  The  middle  solution  was  unchanged  in  composition. 
0-2470  gram  of  silver  was  deposited  in  a  silver  coulometer  placed  in  circuit. 
What  are  the  transport  numbers  for  the  given  solution  ? 

As  0-2470  gram  of  silver  has  been  deposited  in  the  silver  coulometer,  the 
same  weight  has  been  deposited  in  the  cathode  compartment  of  the  migration 
apparatus.  But  the  decrease  in  the  catholyte  is  only  0-1300  gram.  Hence 
(0-2470 — 0-1300  =  0-1170)  gram  has  migrated  into  the  cathode  compartment 
from  the  anolyte.  Similarly  0-2470  gram  of  silver  has  been  dissolved  from  the 
anode,  and  as  the  increase  of  silver  in  the  anode  compartment  is  only  0-1298  gram, 
it  follows  that  (0-2470  —  0-1298  =  0-1172)  gram  of  silver  has  migrated  to  the 
cathode.  Taking  a  mean  of  these  values,  we  find  that  as  a  result  of  the  experiment, 
the  electrode  reactions  being  eliminated,  0-1171  gram  Ag  has  been  transferred 

1  Zeitsrli.  Phys.  CJiem.  36, 


v.]  IONIC  TRANSPORT  55 

from  anode  to  cathode  compartment.     If  the  Ag-  ions  had  carried  the  total  current, 
0-2470  gram  of  silver  would  have  been  transported.     Therefore 


Ag      = 


n  .,,, 
- 


g      0-2470 
Hence  n^  =  0'474 

nXOa=  1-0  -0-474  =  0-526. 

2.  The  transport  numbers  of  a  dilute  K->S04  solution  are  determined  by  the 
method  of  Noyes.  The  catholyte  analysed  after  the  experiment  contained  8-4394 
grams  KoS04,  and  weighed  493-12  grams.  Of  this  weight  60-78  grams  represent 
dilute  HoS04  added  to  maintain  neutrality,  and  to  compare  the  final  K2S04  concen- 
tration with  that  at  the  start  of  the  experiment  this  is  subtracted. 

Then  another  correction  must  be  made.  K*  ions  have  entered  the  cathode  com- 
partment, SO/  ions  have  left  it,  hydrogen  gas  has  been  evolved.  The  net  result 
of  these  effects  is  calculated  to  be  a  decrease  in  weight  of  0-14  gram,  and  is  there- 
fore an  additive  correction.  The  corrected  weight  of  the  catholyte  comes  to 
(493-12  —  60-78  +  0-14  =  432-2)  grams.  The  amount  of  K2S04  originally  present 
in  this  was  (432-2  x  0-017247  =  7-4591)  grams.  Hence  the  increase  of  K>S04  in 
the  cathode  compartment  is  (8-4394—7-4591  =  0-9803)  gram,  or,  plus  another 
small  correction,  0-9816  gram.  The  equivalent  weight  of  K.2S04  is  87-13,  and 

0-9816 
hence  the  increase,  expressed  in  equivalent  weights,  is         ,      =  0-01126. 

oV'lo 

During  the  experiment  2*4594  grams  of  silver  were  precipitated  in  the  Silver 
coulometer.  Expressed  as  equivalents,  this  is  -  ^  =  0*02279.  If  WK  were 

unity,  the  increase  in  the  equivalent  concentration  of  the  KoS04  would  have  been 
the  same.     Thus 

0-01126 

n*  =  01)2279  ==0'4941' 

4.  Quantitative  Relations  of  Ionic  Migration  n 

Migration  ratios  have  been  studied  in  recent  years  by  Noyes  and 
by  H.  Jahn  and  his  pupils  (Bein,  Tower,  etc.),  and  many  have  also 
been  measured  by  Steele  and  Denison,  using  a  method  essentially 
different  in  principle  from  the  one  described  above.  The  results 
of  these  investigations  substantially  confirm  Hittorf  's  work  of  half  a 
century  earlier. 

Before  undertaking  any  extensive  measurements,  Hittorf  investi- 
gated the  effect  of  current  strength,  concentration,  and  temperature  on 
the  transport  numbers  of  certain  salts.  He  found  first  of  all  that  the 
influence  of  changes  in  current  strength,  when  diffusion  and  heating 
effects  were  eliminated,  was  negligible.  Thus  with  a  certain  copper 
sulphate  solution,  using  currents  of  ratios  113  :  420  :  958,  he  found  as 
values  of  nCu  —  O291,  0-285,  0-289.  The  quantity  of  any  ion  trans- 
ported in  an  experiment  depends  only  then  on  the  quantity  of  elec- 
tricity which  passes  through  the  electrolyte,  not  on  the  rate  at  which 
it  is  sent  through.  Hittorf  then  showed  that  with  certain  very  simple 
salts,  such  as  the  alkaline  halides,  the  transport  ratios  were  only  very 
slightly  dependent  on  concentration,  but  that  as  a  rule  they  showed 


56     PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

variation  in  the  stronger  solutions,  and  only  became  constant  at  a 
certain  limiting  dilution.  Thus  potassium  chloride  and  silver  nitrate 
solutions  gave  the  following  values  : — 


Parts  of  water 

to  1  part  KC1  nci 

6-6  0-516 

18-4  0-514 

39-4  0-515 


254  0-515 


Parts  of  water 

to  1  part  AgN03  WA* 

2-48  0-532 

5-18  0-505 

14-5  0-475 

49-44  0-474 

104-6  0-474 

247-3  0-476 


Sometimes  the  change  in  the  transport  numbers  with  change  in 
concentration  was  very  marked,  and  we  shall  presently  see  the  im- 
portant deductions  Hittorf  was  able  to  make  in  those  cases.  He  also 
investigated  the  effect  of  temperature  on  transport  numbers.  Owing 
to  his  method  being  insufficiently  exact,  he  could  detect  no  marked 
effect.  From  later  measurements,  however,  it  is  now  known  that,  as 
the  temperature  is  raised,  the  rates  of  migration  of  most  ions,  parti- 
cularly monovalent  ones,  tend  to  become  equal  to  one  another,  and 
consequently  the  transport  numbers  of  the  salts  tend  to  become  more 
nearly  0*5  at  higher  temperatures  (Kohlrausch). 

A  table  of  the  general  numerical  results  obtained  by  different  workers 
is  given  in  Appendix  I.  Here  we  need  only  remark  on  the  low  value  of 
WA  for  acids,  indicating  a  very  high  velocity  for  the  hydrogen  ion,  and 
also  how  high  the  value  of  non  is  compared  with  nx  for  other  anions. 

5.  Applications  of  Ionic  Migration  Phenomena 

All  the  above  considerations  relate  to  electrolytes  containing  one 
salt  only  in  solution.  The  important  question  of  how  the  current  is 
carried  in  solutions  containing  a  mixture  of  two  or  more  salts  will 
be  referred  to  in  the  next  chapter,  after  dealing  more  fully  with  the 
condition  of  the  dissolved  molecules  in  an  electrolyte.  Our  chief 
interest  in  the  subject  of  ionic  migration  lies,  indeed,  in  the  light  it 
throws  on  this  matter  and  in  the  essential  role  it  has  played  in  the 
development  of  the  theory  of  electrolytes,  as  we  now  know  it. 

Application  to  Chemical  Questions. — But,  that  apart,  the  study 
of  transport  relations  has  also  often  led  directly  to  very  important 
deductions  concerning  the  constitution  of  electrolytic  solutions.  Thus 
Hittorf  determined  the  transport  ratios  for  a  solution  of  SnCl4,  which  is 
acid  in  reaction.  He  found  the  value  of  wcl  to  be  practically  identical 
with  the  value  it  would  have  if  the  SnCl4  were  completely  hydrolysed 
according  to  the  equation 

SnCl4  +  4H20  ^±  Sn(OH)4  +  4HC1 

and  concluded  that  such  was  the  case.     His  view  was  later  confirmed 
by  another  (thermochemical)  method. 


v.l  IONIC  TRANSPORT  57 

Again,  when  electrolysing  potassium  ferrocyanide  solutions,  Daniell 
found  that  the  iron  concentrated  at  the  anode,  and  therefore  must 
be  associated  with  negative,  not  positive  electricity.  He  accordingly 

gave  the  salt  the  formula  K4  (FeCy6)  which  it  still  retains,  in  place 
of  the  previously  used  double  salt  formula  4KCy,  FeCy2. 

Similarly  Hittorf  found  that  the  platinum  in  sodium  chloroplatinate 
wandered  to  the  anode  during  electrolysis,  and  proposed  for  the  salt 
the  modern  formula  Na2PtCl6,  in  place  of  the  old  2NaCl,  PtCl4.  And 
by  observing  in  each  case  the  movement  of  the  blue  colour  during 
electrolysis,  it  can  be  demonstrated  that  the  copper  in  an  ammoniacal 
copper  sulphate  solution  is  present  as  a  complex  cation,  but  in  an  alkaline 
copper  tartrate  solution  as  a  complex  anion. 

Finally,  changes  in  constitution  accompanying  changes  in  concen- 
tration can  be  followed  by  the  same  method.  Thus  a  dilute  copper 
bromide  solution  is  blue  ;  as  it  is  concentrated  it  becomes  green, 
brownish  green,  and  finally,  in  very  strong  solutions,  a  dark  brown. 
Denham *  has  shown  this  variation  to  be  accompanied  by  a  simultaneous 
change  in  the  value  of  nCn.  This  is  about  0*44  for  dilute  solutions,  but 
rapidly  decreases,  changes  sign,  and  reaches  a  value  of  about  —  O4  in  a 
5  N.  solution.  It  has  been  shown  that  the  value  of  nc  is  given  by  the 
diminution  of  the  cation  concentration  around  the  anode  (disturbances 
due  to  electrode  reactions  eliminated).  A  negative  transport  number 
can  only  mean  that  the  copper  concentration  increases  at  the  anode 
during  electrolysis.  As  in  the  above  cases,  we  therefore  conclude 
that  in  these  solutions  a  portion  of  the  copper,  greater  the  stronger  the 
solution,  is  present  as  complex  anion. 

It  is  seldom  that  transport  relations  have  any  direct  bearing  on  a 
technical  electrochemical  process.  If  electrolysis  takes  place  in  a 
solution  which  is  kept  at  rest,  concentration  changes  result  near  the 
electrodes,  partly  due  to  unequal  rates  of  migration.  But  such  con- 
centration differences  are  undesirable.  They  increase  the  voltage 
required  by  the  cell,  and  may  further  lead  to  the  production  of  impure 
products  at  the  electrodes.  For  these  reasons  (and  others)  the  electro- 
lyte is  nearly  always  continually  circulated  through  the  cells,  or  in 
some  other  way  mixing  is  effected.  Concentration  changes  are  thus  as 
far  as  possible  destroyed,  and  hence  calculations  involving  transport 
numbers  usually  find  no  application.  In  a  few  cases  this  is  not  so.2 


Literature 

Le  Blanc.     Electrochemistry. 

1  Zeitsch.  Phys.  Chem.  65,  641  (1909). 

-  See  theory  of  alkali-chlorine  cells,  p.  357. 


CHAPTER   VI 

CONDUCTIVITY    OF    ELECTROLYTES—  THEORY    OF    ELECTRO- 
LYTIC   DISSOCIATION 

1.  Specific  Conductivity 

WHEN  a  current  flows  through  an  electrolyte,  a  certain  amount  of 
energy  is  consumed  and  degraded  into  heat,  quite  apart  from  what 
happens  at  the  electrodes.  This  is  due  to  the  resistance  of  the  elec- 
trolyte. Since  all  ways  in  which  electrical  energy  is  consumed  during 
an  electrolysis  are  of  importance  to  the  technical  electrochemist,  the 
subject  of  the  resistance  of  electrolytes,  apart  from  its  wider  theoretical 
significance,  demands  a  thorough  treatment. 

The  resistance  of  an  electrical  conductor  depends  not  only  on  the 
material  of  which  it  is  made,  but  also  on  its  dimensions,  and  before 
the  resistances  of  two  substances  can  be  compared  all  questions  of 
relative  size  or  shape  must  be  eliminated.  The  resistance  of  a  con- 
ductor of  uniform  cross  section  is  given  by 


where  r  is  a  constant,  depending  on  the  particular  material  and  on  the 
units  used,  I  the  length,  and  a  the  cross-section.  If  I  and  a  be  made 
unity,  we  have 


r  is  therefore  the  resistance  of  a  unit  cube  of  the  conductor,  and  is 
termed  the  specific  resistance  of  the  substance.  If  the  ohm  and  the 
centimetre  be  employed,  it  expresses  the  resistance  of  the  material 
in  ohms  per  centimetre  cube. 

But  generally  electrolytes  are  not  so  much  compared  by  means  of 
their  specific  resistances  as  by  their  specific  conductivities.  Certain 
important  theoretical  relations  are  thereby  rendered  clearer.  Just  as 
the  conductivity  of  a  substance  is  the  reciprocal  of  its  resistance,  so 
is  its  specific  conductivity  the  reciprocal  of  its  specific  resistance. 

58 


CONDUCTIVITY  OF  ELECTROLYTES  59 

If  the  latter  be  denoted  by  r  and  the  specific  conductivity  by  K,  we 
have 

1 


where  K  is  given  in  reciprocal  ohms.  Just  as  the  specific  resistance 
of  a  substance  is  the  voltage  which  must  be  applied  to  two  opposite 
faces  of  a  centimetre  cube  in  order  that  a  current  of  one  ampere  may 
flow  across  the  cube,  so  the  specific  conductivity  is  the  current  which 
would  flow  across  such  a  cube  on  application  of  a  potential  difference 
of  one  volt  to  two  opposite  faces.  It  is  in  fact  given  by 

current  per  unit  area  current  density 

potential  fall  per  unit  length        potential  gradient 

2.  Determination  of  Conductivity 

General.— Conductivity  determinations  are  readily  carried  out 
by  means  of  Wheatstone's  Bridge,  shown  in  Fig.  12.  ACB  and 
ADB  are  two  paralleled  circuits  through  which  current  flows  from  a 
source  E.  The  resistance  K  between  A  and  C  is  known  (usually  a 
resistance  box)  and  can  be  regulated  ;  the  resistance  x  is  unknown. 


FIG.  12.— Wheatstone's  Bridge. 

The  branch  CD  containing  a  galvanometer  or  some  other  form  of  current 
detector  is  fixed  at  C,  but  makes  connection  by  means  of  a  sliding 
contact  D  with  the  stretched  uniform  graduated  wire  AB.  This 
usually  consists  of  platinum-indium  or  of  constantan,  a  hard,  un- 
tarnishable,  copper-nickel  alloy,  and  is  one  metre  in  length.  By  moving 
D,  the -relative  sizes  of  the  resistances  AD  and  DB  can  be  altered 
at  will. 

Now  the  two  ends  of  the  circuits  ACB  and  ADB  have  common 
potentials.  Hence,  if  the  potential  at  A  be  higher  than  that  at  B,  there 
will  be  a  gradual  potential  fall  along  both  circuits  in  the  direction  of 
A  to  B,  and  every  point  in  the  circuit  ACB  will  have  a  corresponding 
point  in  the  circuit  ADB  at  the  same  potential  Suppose  now  the 


60     PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

contact  D  is  moved  along  the  wire  AB.  As  long  as  the  potentials  of 
C  and  D  differ,  a  current  will  flow  one  way  or  another  through  the 
circuit  CD,  and  will  be  indicated  by  the  instrument  G.  When  the 
potentials  of  C  and  D  are  identical  no  current  will  flow.  In  making  a 
measurement,  this  point  is  found  by  suitably  regulating  R  and  by  moving 
D,  and  the  reading  on  the  slide-wire  scale  taken.  The  wire  being 
uniform,  the  relative  resistances  of  the  branches  AD  and  DB  are  at 
once  obtained.  Let  them  be  RI  and  R2  respectively,  and  suppose  the 
currents  in  the  branches  ACB  and  ADB  to  be  Ix  and  I2  respectively. 
Then,  if  Ex  and  E2  are  the  potential  falls  AC  =  AD,  and  CB  =  DB, 
we  have 

E!  =  IjR  E2  =  Ijaj 

EI  =  I2Rj  Eg  =  ^Ro 

Or 

J»J 

I2       R 

And  finally 

R!         R 

R2~z  ~x 
from  which  equation  x  is  calculated. 

Thus  if  R  is  48  ohms,  and  the  reading  on  the  slide-  wire  (100  divisions)  is 
72-46,  we  have 

R,  =  72-46  R2  =  (100  -  72'46)  =  27'54, 

and  therefore 

rc=='R==         '48 


=  18'24  ohms. 

Of  Electrolytes.  —  When  measuring  electrolytic  conductivities  by 
this  method,  several  special  precautions  must  be  taken.  The  use  of 
direct  current  would  decompose  the  electrolyte.  Its  actual  resistance 
would  thereby  change  ;  but,  more  important  still,  its  apparent  resist- 
ance would  alter  very  considerably  owing  to  losses  of  potential  at  the 
electrodes  caused  by  decomposition.  These  polarisation  l  effects  would 
entirely  destroy  the  accuracy  of  a  conductivity  determination.  A 
small  induction  coil  (only  needing  two  volts  at  its  primary  terminals) 
which  furnishes  alternating  current  is  consequently  employed.  With 
alternating  current,  a  galvanometer  is  of  course  useless.  A  convenient 
current  detector  is  furnished  by  an  ordinary  telephone.  On  reaching^ 
the  equipotential  point  on  the  slide-wire,  a  very  sharp  minimum  of 
sound  can  be  obtained,  using  a  good  coil.  As  the  movement  of  the 
sliding  contact  tends  to  obscure  this,  it  is  customary  to  connect  the 

1  SteClup.  ix. 


CONDUCTIVITY  OF  ELECTROLYTES 


61 


telephone  to  the  ends  of  the  slide-wire,  and  to  put  the  coil  between  C 
and  the  sliding  contact.     The  final  arrangement,  therefore,  is  as  in 
Fig.  13,  where  F  represents  the  primary  cell,  H  the  induction  coil, 
and   K    the    telephone.      The 
alternating  E.M.F    is    applied 
at    C    and     D,     the    current 
traversing  the  parallel  circuits 
CAD  and  CBD.     R  and  D  are 
adjusted  until    the    telephone 
indicates  a  null  potential  differ- 
ence between  A  and  B.      Then 


AD 
BD 


B 


FIG.  13. — Apparatus  for  measuring 
Electrolytic  Conductivities. 


To  deduce  the  specific  con- 
ductivity of  a  solution  directly 

from  a  measurement  carried  out  on  it,  the  vessel  used  must  be  of  exact 
geometrical  shape,  its  dimensions  very  exactly  determined,  and  the 
electrodes  very  carefully  inserted.  This  is  tedious,  and  there  are 
further  slight  uncertainties  as  to  whether  the  current  is  passing 
through  the  liquid  quite  uniformly  over  the  measured  path,  etc. 
For  these  reasons  it  is  best  to  use  a  more  convenient  vessel  whose 
dimensions  need  not  be  exactly  known,  but  in  which  the  experimental 
conditions  can  be  subsequently  reproduced  without  fail.  Its  conduct- 
ance is  measured,  using  a  solution  whose  specific  conductivity  has 
been  determined  once  for  all.  This  done,  a  further  determination 
and  a  comparison  will  furnish  the  specific  conductivity  of  any  other 
solution.  If  the  conductance  of  the  vessel  filled  with  the  standard 
and  the  unknown  electrolyte  be  respectively  A  and  A7,  and  if  the 
specific  conductivity  of  the  standard  electrolyte  be  known  to  be  KQ, 
we  have  for  K  the  unknown  specific  conductivity 


K  =  -?  .  A'  =  KA'. 
A 

K  is  termed  the  cell  constant  for  the  particular  vessel.  When  once 
determined,  the  specific  conductivity  of  any  liquid  can  be  directly 
calculated  from  a  conductivity  measurement,  using  the  above  equation. 
Various  solutions  are  used  as  standards.  The  commonest  are  N. 
potassium  chloride  and  maximal  or  best  conducting  sulphuric  acid. 
This  is  30  per  cent,  by  weight  and  of  S  =  1*223  at  18°,  at  which  tempera- 
ture the  specific  conductivities  of  these  two  solutions  are  respectively 
0*09822  and  0*7398  reciprocal  ohms  per  centimetre  cube. 

Resistance  vessels  of  various  types  are  used,  two  being  illustrated 
in  Fig.  14.  In  (a)  the  body  is  of  hard  glass,  which  is  only  slightly 


62    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

attacked  by  aqueous  solutions.  The  electrodes  are  of  stout  platinum, 
cemented  into  glass  tubes,  and  lie  horizontally,  one  above  the  other. 
The  tubes  pass  through  the  ebonite  lid,  where  they  are  securely  cemented 
and  are  further  connected  by  sealing-wax  at  A.  In  this  way  the 
electrodes  are  held  in  the  same  relative  positions  to  one  another,  and 


\ 


(a) 


FIG.  14. — Conductivity  Vessels. 

the  cell-constant  is  prevented  from  altering.  Before  use,  the  electrodes 
are  platinised  with  a  solution  containing  3  per  cent,  of  chlorplatinic 
acid  and  -fa  Per  cent-  °*  ^ea(^  acetate,  freed  from  platinum  salts  by 
electrolysis  with  dilute  sulphuric  acid,  and  finally  thoroughly  washed 
with  distilled  water.  Polished  platinum  electrodes  give  a  far  poorer 
sound  minimum  with  the  telephone  when  at  the  right  point  on  the 
bridge,  particularly  with  solutions  of  good  conductivity.  Fig.  14  (b) 
represents  a  type  of  vessel  that  can  be  conveniently  dipped  into  a  large 
working  electrolysis  tank,  when  determinations  must  be  made  on  the 
spot. 

In  carrying  out  precision  conductivity  measurements,  great  care 
must  be  taken  to  purify  the  water  used.  Even  when  all  dissolved  salts, 
ammonia,  and  carbon  dioxide  have  been  removed,  there  still  remains 
a  very  slight  residual  conductivity.  This  is  due  to  the  water  molecules 
furnishing  minute  traces  of  H'  and  OH'  ions  which  carry  the  current. 

Conductivity  of  a  Working  Cell. — The  internal  resistance  of  a 
cell  which  has  no  current  passing  through  it  can  be  measured  in  the 
way  described.  If,  however,  the  internal  resistance  of  a  cell  through 
which  current  is  passing  or  of  a  cell  which  is  actually  furnishing  current 


vi.]  CONDUCTIVITY  OF  ELECTKOLYTES  63 

is  desired,  a  different  arrangement  must  be  used.  The  most  important 
methods  for  effecting  this  are  those  of  Block1  and  of  Nernstand 
Haagn.2  Fig.  15  illustrates  the  former  method  applied  to  a  primary 
cell  which  is  furnishing  current.  A  is  the  cell ;  B,  C,  D,  resistances  of 


FIG.  15. — Conductivity  Measurement  on  a  Working  Cell. 


known  value,  capable  of  regulation.  EF  is  the  slide-wire.  G  and  H 
are  condensers  in  the  telephone  and  induction  coil  circuits  respectively. 
The  direct  current  from  A  travels  along  the  circuit  ABCDFE,  but  can- 
not enter  the  branches  KL  and  MHN  because  of  the  condensers  G 
and  H.  These  condensers,  however,  do  not  hinder  the  passage  of  the 
alternating  current  from  P,  and  thus  the  measurement  of  conductance 
is  carried  out  quite  independently  of  the  direct  current  from  A.  When 
a  balance  is  obtained  by  moving  L  as  usual,  we  have  the  following 
relation  of  resistances  : — 

B  C 


A  +  (EL) 


+  (LF) 


from  which  the  resistance  of  A  is  directly  got. 

Conductivity,  Concentration  and  Temperature.  —  The  technique 
of  conductivity  measurements  has  been  chiefly  worked  out  by 
Kohlrausch,  to  whom  we  also  owe  the  great  bulk  of  our  existing 
data,  and  the  discovery  of  several  very  important  relations  dealing  with 
conductivities.  Besides  depending  on  the  nature  of  the  electrolyte 
and  the  solvent,  the  conductivity  of  a  solution  also  depends  on  its 
concentration  and  on  the  temperature. 

The  general  effect  of  an  increase  of  concentration  is  an  increase  in 
specific  conductivity  up  to  a  maximum  point  and  a  subsequent  decrease. 

1  Zeitsch.  Phys.  Chem.  58,  442  (1907). 

•  ZeiiscTi.  Ph'ys.  Chem.  23,  97  (1897).     Zeitsch.  Elektrochem.  2,  493  (189V). 


64     PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

This  behaviour  is  well  shown  in  Fig.  16.     Its  significance  will  be  seen 
later. 

The  specific  conductivity  of  an  electrolyte  increases  almost  linearly 
with  temperature.     We  can  write 


a  is  0'02-0'025   for   salts  and  bases,   and  O'Ol-  0*016  for  acids,  cor- 
responding to  an  increase   of  conductivity  of   l-2'5  per  cent,   per 


2       3       4       5       6       7       8       9       10      11      12 
Concentration  in  Qram  -Equivalents  per  Litre 

FIG.    16. 

degree.  This  relation  holds  good  for  temperatures  considerably  above 
100°  (Noyes).  For  exactly  comparable  measurements,  identity  of 
working  temperature  must  be  carefully  ensured.  It  is  also  obvious 
that,  from  the  point  of  view  of  lessened  resistance,  it  is  advantageous 
in  a  technical  electrolytic  process  to  work  at  as  high  a  temperature  as 
possible. 

The  values  of  a  few  specific  conductivities  and  specific  resistances 
are  given  in  Table  VIII.     They  hold  good  for  18°. 


Electrolyte 
1  N.  HoS04 
5N.  H~.sn, 
1  N.  KOH 
•2  V.  KOH 
4  N.  NaCl 

1  N.  AgN03 
IN.  ( 

2  N. 


TABLE  VIII 

K  in  reciprocal  ohms 
0-20 
0-68 
0-185 
0-25 
0-197 
0'-008 
0-025 
0-079 


r  in  ohms 
5-0 
1-47 
5-41 
4-0 
5-08 

14-7 

40 

12-7 


3.  Equivalent  Conductivity 

The   consideration  of   specific  conductivities  suffices  for  pr;icti< -;il 
purposes.     \Ylu-n  known,  we  can  t.-ll  wli.-tli.-r  much  J..ul.-  heat  or  not 


VI.] 


CONDUCTIVITY  OF  ELECTROLYTES 


65 


will  be  produced  on  passing  a  current  through  an  electrolyte.  But 
for  a  closer  consideration  of  the  mechanism  of  electrolytic  conduction, 
and  the  wider  question  of  the  constitution  of  electrolytes,  it  is  better 
to  use  a  different  unit  for  expressing  conductivities,  that  of  equivalent 
conductivity,  the  conductivity  due  to  one  equivalent  weight  of  dissolved 
substance.  If  for  a  given  solution  we  denote  by  77  the  number  of 
equivalents  per  c.c.  (it  will  of  course  be  a  small  fraction),  and  if  A 
represents  equivalent  conductivity,  we  have 


or  better,  if  v  is  the  number  of  c.c.  of  solution  containing  one  gram- 
equivalent  —  that  is  the  dilution  in  c.c.  —  we  have 

A  =  KV. 

To  render  the  relations  of  these  different  magnitudes  quite  plain, 
let  us  consider  a  conductivity  vessel  consisting  of  a  tall  vertical  rect- 
angular prism  of  unlimited  length,  open  at  the  top, 
and  one  centimetre  square  in  plan  section  ABCD 
(Fig.  17).  Suppose  two  opposite  vertical  sides  ADE 
and  BCF  to  be  constructed  of  platinum  and  to  act 
as  electrodes,  the  other  two  sides  being  of  some 
non-conducting  material  (glass).  If  one  c.c.  of  elec- 
trolyte be  poured  into  this  vessel,  and  the  conductance 
measured,  the  result  will  be  the  specific  conductivity 
of  the  solution,  as  the  height  of  the  liquid  in  the 
vessel  is  one  cm.  and  the  current  consequently  passes 
across  a  centimetre  cube.  If  more  solution  be  poured 
in,  the  conductance  of  the  vessel  will  continually 
increase,  proportionally  to  the  quantity  added. 
When  finally  the  height  of  liquid  in  the  cell  be- 
comes v  cm.,  v  c.c.  of  electrolyte,  and  therefore  one 
gram-equivalent  of  dissolved  material,  are  present. 
The  conductance  of  the  vessel  will  be  v  times  its 
original  value  (the  specific  conductivity),  equal 
.therefore  to  the  equivalent  conductivity  in  accordance  with  the 
above  equation. 

Equivalent  Conductivity  at  Infinite  Dilution.  —  We  can  see  that 
A  has  a  twofold  dependence  on  v.  Not  only  is  it  directly  pro- 
portional to  the  latter,  but  its  other  factor,  the  specific  conduc- 
tivity K,  decreases  as  v  increases,  except  in  very  strong  solutions.1 
As  a  matter  of  fact  the  work  of  Kohlrausch  and  others  has  shown 


FIG.  17. 


1  See  p.  64. 


66     PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 


that  the  equivalent  conductivity  increases  with  the  dilution,  at  first 
quickly,  then  less  rapidly,  and  finally  in  many  cases  asymptotically 
reaches  a  maximum  and  becomes  practically  constant.  In  other  cases 
the  final  figure  cannot  be  experimentally  observed,  the  solutions  becom- 
ing too  dilute  for  accurate  measurement.  This  maximum  value  is 
termed  the  equivalent  conductivity  at  infinite  dilution,  and  is  denoted 
by  A  oo.  We  can  write  in  fact 

A  =  aAoo 

where  a  is  a  fraction  less  than  unity,  altering  with  the  dilution  and  ex- 
pressing the  ratio  of  the  equivalent  conductivity  to  the  maximum 
possible  equivalent  conductivity,  that  at  infinite  dilution. 

Table  IX  contains  a  number  of  values  of  AQQ  and  A  for  different 
electrolytes  at  various  dilutions  at  18°. 

TABLE  IX 


Normality 
of  solution 

v  in 
litres 

KC1 

AgN03 

H2S04 

HC1 

Acetic 
Acid 

KOH 

00 

130-1 

115-80 

398-5 

395-2 

238-7 

0-0001 

129-07 

115-01 

— 

— 

107 

— 

0-0002 

5'103 

128-77 

114-56 

— 

.  — 

80 

— 

0-0005          2  -10s 

128-11 

113-88 

368 

— 

57 

— 

0-001        [  103 

127-34 

113-14 

361 

377 

41 

234 

0-005 

2-102 

124-41 

110-03 

330 

373 

20-0 

230 

0-01 

10- 

122-43 

107-80 

308 

370 

14-3 

228 

0-02            50 

119-96 

105-60 

286 

367 

10-4 

225 

0-05 

20 

115-75 

99-50 

253 

360 

6-48 

219 

0-1 

10 

112-03 

94-33 

225 

351 

4-60 

213 

0-5 

2 

102-41 

77-5 

205 

327 

2-01 

197 

1 

1 

98-27 

67-6 

198 

301 

1-32 

184] 

2 

0-5 

92-6 

~ 

183 

254 

0-80 

160-8 

The  conducting  power  then  of  a  gram-equivalent  weight  of  elec- 
trolyte increases  with  the  dilution,  finally  reaching  a  maximum  value 
measured  by  AGO,  the  equivalent  conductivity  at  infinite  dilution. 

Ionic  Conductivity. — The  next  step — one  of  first  importance 
—was  taken  by  Kohlrausch.  He  showed  that  these  equivalent 
conductivities  at  infinite  dilution  split  up  into  two  additive  parts, 
which  can  be  referred  to  cation  and  anion  respectively.  Moreover.  1  he 
equivalent  ionic  conductivity  thus  deduced  for  a  particular  ion 
from  measurements  on  certain  salts  is  independent  of  the  salts  and 
characteristic  of  that  ion,  and  if  used  to  calculate  the  equivalent  con- 
ductivity at  infinite  dilution  of  other  different  salts  will  give  correct 


VI.] 


ELECTROLYTIC   DISSOCIATION    THEORY 


67 


results.  That  is,  Kohlrausch  showed  that  A  00  for  every  electrolyte 
is  an  additive  function  of  the  ionic  conductivities  of  the  different  ions. 
Denoting  these  ionic  conductivities  by  /A  and  lc,  we  have 

Aoo  =  1A  +  lc. 


The  values  of  a  number  of  ionic  conductivities  at  18 
bslow  :  — 


are  given 


Li' 

Na- 
if 
Ag' 
NH4' 


33-4 
43-6 

64-7 
54-0 
64 


H* 

OH' 
Cl' 
Br' 
I' 


318  ? 
174  ? 
65-4 
67-6 
66-4 


CIO,' 
NO/ 
*'S04" 


iCa" 
JCu" 
JPb" 


55 

61-8 

68-4 

46 

51-8 

47-3 

61-3 


Table  X  contains  values  of  Aoo  calculated  from  the  above  figures, 
and  values  extrapolated  from  experimental  results.  The  agreement 
is  excellent. 

TABLE  X 


Salt 

t 

*c 

M 

A  oo  extrapolated 

KBr 

67-6 

64-7 

132-3 

132-3 

KOH 

174 

64-7 

238-7 

not  <  234 

KC103 

55-0                  64-7          |         119-7 

119-7 

NaCl 

65-4                  43-6 

109-0 

108-99 

AgNO, 
NH4NO:{ 

61-8 
61-8 

54-0 
64 

115-8 
125-8 

115-8 
not  <  126-1 

iNaoS04 

68-4 

43-6 

112-0 

not  <  110-5 

JMgCL, 

65-4 

46 

111-4 

/not  <  109-4 
\not  <  115-1 

iCad2 

65-4 

51-8 

117-2 

not  <  115-2 

4.  Electrolytic  Dissociation  Theory 

Before  proceeding  further,  a  brief  review  of  what  we  already  know 
of  the  constitution  of  electrolytes  and  the  mechanism  of  conduction 
may  be  given.  We  regard  dissolved  molecules  of  electrolytes  as  com- 
posed of  two  oppositely  charged  halves — the  ions — of  opposed  chemical 
nature.1  When  a  current  passes,  these  ions  travel  towards  the  elec- 
trodes, and  lose  their  electricity,  the  material  part  being  set  free. 
The  velocities  with  which  different  kinds  of  ions  travel  towards  their 
respective  electrodes  under  otherwise  identical  conditions  are  not 
usually  the  same  ;  the  relative  velocities  of  the  oppositely  charged 


Chap.  III. 


Chap.  V. 


F  2 


C8     PKINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

ions  in  any  salt  can  be  determined  by  migration  experiments.  The 
conducting  power  of  a  gram-equivalent  of  an  electrolyte  increases 
with  the  dilution,  finally  reaching  a  maximum.1  (Arrhenius,  one  of 
the  chief  workers  on  conductivities,  divided  the  dissolved  molecules  in 
an  electrolyte  into  two  classes,  the  active,  which  conducted  the  current, 
and  the  inactive,  which  did  not.  The  proportion  of  active  molecules 
increased  with  increasing  dilution,  and  this  fraction  he  termed  the 

coefficient  of  activity,  and  measured  it  by  the  ratio  — — .  It  was  there- 
fore identical  with  a  in  the  equation  A=  aAoo-2)  The  values  of 
Aoo  for  different  electrolytes  are  additive  functions  of  characteristic 
equivalent  ionic  conductivities  belonging  to  the  different  ions.3  Finally 
solutions  of  strong  electrolytes  have  abnormally  high  osmotic  pressures,4 
indicating  that  a  dissociation  of  the  dissolved  molecules  into  simpler 
ones  has  taken  place,  a  dissociation  moreover  which  increases  with  the 
dilution  and  appears  to  reach  a  limiting  value,  in  both  respects  just 
as  the  equivalent  conductivity  does. 

These  different  facts  were  correlated  by  Arrhenius  in  1887  by 
means  of  his  Electrolytic  Dissociation  Theory— a  generalisation  of  the 
utmost  importance  and  fruitfulness.  Several  physicists  (Clausius, 
Helmholtz,  Planck)  had  already  suggested  that  solutions  of  electrolytes 
contained  free  charged  ions,  and  that  only  such  ions  were  capable  of 
conducting  the  current.  At  the  same  time,  the  number  of  ions  so 
split  off  from  the  neutral  molecules  was  regarded  as  comparatively  small. 
Arrhenius  extended  this  conception,  stated  that  the  dissolved  molecules 
were  often  very  largely  dissociated  into  their  charged  ions,  which  could 
then  act  to  a  great  extent  independently,  pointed  out  several  inde- 
pendent means  of  measuring  the  degree  of  this  dissociation,  and  showed 
that  values  obtained  by  these  different  methods  gave  concordant 
results. 

The  first  essential  point  in  Arrhenius'  theory  is  that  the  fraction 
of  the  dissolved  molecules  thus  dissociated  is  often  very  great.  Small 
in  concentrated  solutions,  it  increases  with  the  dilution  (just  as  a  purely 
chemical  dissociation  taking  place  with  increase  in  number  of  molecules 
increases  with  decreased  pressure),  and  finally  approaches  the  limiting 
value  of  100  per  cent.  Binary  salts  formed  from  monobasic  acid  and 
monacid  base — as  KN03,  NaCl,  AgC103 — are  the  most  strongly  dis- 
sociated. The  corresponding  simple  acids  and  bases— HC1,  NaOH, 
NHtOH— vary  enormously.  Salts  such  as  CuS04  and  ZnS04  are 
dissociated  far  less  than  the  simpler  ones  given  above,  whilst  with  more 
complex  examples,  as  Ba(N03)2,  H2S04,  and  H3P04,  the  dissociation 
takes  place  step  by  step— two  ions  only  being  formed  in  moderately 

1  P.  66.  -  P.  66. 

'  P.  67.  4  Chap.  IV. 


vi.]  ELECTROLYTIC  DISSOCIATION  THEORY  69 

strong  solutions  and  a  greater  number  in  dilute  solutions.     Thus  HaS04 
at  first  gives 

H2S04  — >  H-  +  HSO/  ; 
the  dissociation 

HSO/  — >  H-  +  SO/ 
will  set  in  later. 

The  dilution  necessary  to  reach  the  same  degree  of  dissociation- 
say  80  per  cent.— varies  therefore  considerably  with  the  electrolyte. 
It  is  smaller  the  less  complex  the  undissociated  salt  and  the  stronger 
the  component  base  and  acid.  We  have  the  following  approximate 
values  for  18°  : — 

KC1  is  80  per  cent,  dissociated  in  0'4  N.    solution. 
KAc  „  „  0-1  N. 

BaCL,          „  „  0-05  N.        „ 

CdCl,          „  „  0-005  N.       „ 

Like  other  electrical  phenomena,  electrolytic  dissociation,  often 
loosely  termed  ionisation,  is  only  very  slightly  affected  by  temperature. 

Now,  as  current  is  carried  solely  by  these  free  ions,  which  increase 
in  number  with  the  dilution,  it  follows  that  the  conducting  power  of 
a  gram-equivalent  of  electrolyte  will  also  increase  with  dilution  and 
will  tend  towards  a  maximum  value,  corresponding  to  complete  ionic 
dissociation.  The  conductivity  of  any  solution  will  depend  then,  firstly 
on  the  number  of  ions  present,  and  secondly  on  the  velocity  with  which 
they  move  under  the  electric  field.  The  osmotic  pressure  of  an  electro- 
lyte will  be  determined  by  the  total  number  of  osmotically  active 
particles  in  solution,  and,  as  many  molecules  are  dissociated  into  two 
or  more  ions,  will  be  greater  than  the  amount  calculated  not  allowing 
for  dissociation.  This  discrepancy  will  be  more  marked  in  dilute  solutions 
where  .the  dissociation  is  more  complete  than  in  strong  solutions,  which 
corresponds  to  facts.  At  very  great  dilutions,  the  ratio  of  observed 
to  normal  osmotic  pressure  will  be  given  by  the  total  number  of  ions 
the  salt  can  furnish — will  always  therefore  tend  towards  a  small  integral 
number.  With  NaCl  and  CuS04,  it  will  be  two  ;  with  K3FeCy6,  four ; 
etc.,  etc. 

A  last  important  qualitative  agreement  with  theory  is  the  fact 
that  most  of  the  properties  of  dilute  solutions  can  be  regarded  as 
additive  functions  of  the  properties  of  the  component  ions,  instead 
of  as  the  properties  of  the  undissociated  salts.  Thus  a  dilute  AgN03 
solution  has  one  set  of  properties  which  we  associate  with  the  Ag'  ion 
and  another  set  which  we  associate  with  the  N03'  ion,  but  no  properties 
which  we  can  put  down  as  peculiarly  due  to  silver  nitrate.  A  dilute 
solution  containing  equivalent  quantities  of  KC1  and  Na2S04  has  the 
same  properties  as  a  dilute  solution  containing  equivalent  quantities  of 
K2S04  and  NaCl.  The  properties  of  both  solutions  are  the  sum  of 
properties  attributed  to  the  Na',  K',  Cl',  and  S04"  ions.  These  state- 


70     PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

ments  hold  good  for  such  properties  as  colour,  specific  gravity,  refrac- 
tivity,  specific  heat,  etc.,  etc.,  and  go  to  show  that  in  dilute  solutions 
the  ions  act  practically  independently  of  one  another. 

Degree  of  Dissociation.— The  most  important  evidence,  however, 
presented  by  Arrhenius  was  of  a  quantitative  nature.  In  par- 
ticular he  was  conspicuously  successful  in  showing  that  the  extent  of 
dissociation  of  an  electrolyte  calculated  by  two  absolutely  independent 
methods — osmotic  pressure  and  conductivity — was  equal  or  nearly 
so.  Suppose  a  molecule  of  electrolyte  can  dissociate  into  n  ions, 
and  that  the  fraction  dissociated,  its  degree  of  dissociation,  is  a  for  a 
particular  solution.  Let  i  as  before1  be  the  Van't  HofE  factor,  i.e. 

observed  osmotic  pressure  .       ,  ,  ,. 

.      The  degree  of  dissociation  is 

calculated  chemical  osmotic  pressure 

identical  with  Arrhenius'  coefficient  of  activity?  and  we  can  write 


a  = 


•co 


If  now  a  be  the  fraction  of  dissociated  molecules,  the  undissociated 
fraction  will  be  1  —  a.  As  each  molecule  on  dissociating  furnishes  n 
ions,  the  total  number  of  ions  will  be  na,  and  the  total  number  of 
osmotically  active  particles  na  -f  (1  —  a).  Therefore 


_^ 


na  -f  (1  -  a) 


That  is,  a  value  for  i  has  been  calculated  from  conductivity  data  only. 
Conversely,  if  i  be  known  from  osmotic  measurements,  the  degree  of 
dissociation  a  can  be  calculated. 

Table  XI  contains  values  for  i  calculated  from  three  different  series 
of  measurements. 

TABLE  XI 


Substance 

Molecular 
normality 

t  from  osmotic 
pressure 

i  from  freez- 
ing-point data 

i  from 
conductivity 

Cane  sugar 

0-3 

1-00                        1-08 



Acetic  acid 

0-33 

— 

1-04 

1-01 

KC1 

0-14 

1-81 

1-93 

1-86 

LiCl 

0-13 

1-92 

1-94 

1-84 

MgS04 

0-38 

1-25 

1-20 

1-35 

Ca(N03)2 

0-18 

2-48 

2-47 

2-46 

SrClj 

0-18 

2-69 

2-52 

2-51 

K^eCy, 

0-356 

3-09 

— 

3-07 

Chap.  IV. 


2  P.  68. 


vi.]  ELECTROLYTIC  DISSOCIATION  THEORY  71 

Cane  sugar,  a  non-conductor,  and  acetic  acid,  a  weak  acid  and 
feeble  electrolyte,  have  values  of  i  nearly  approaching  unity.  With 
KC1  and  LiCl,  strong  electrolytes  of  simple  constitution,  i  is  already 
nearly  two  in  j-J  molecular  normal  solutions.  With  MgS04,  another 
binary  electrolyte,  but  in  stronger  solution  and  containing  a  weaker 
base,  i  is  less,  but  still  between  one  and  two.  With  the  two  ternary 
salts,  it  tends  towards  three,  whilst  with  K4FeCy6,  a  complex  salt  in  fairly 
strong  solution,  the  maximum  value  of  five  is  far  from  being  attained. 
The  figures  Arrhenius  himself  calculated  (from  freezing-point  and 
conductivity  measurements)  did  not  agree  so  well,  as  the  existing 
experimental  data  were  rather  unreliable.  Yet  they  were  sometimes 
very  striking.  Thus,  for  BaCl2,  i  calculated  from  Raoult's  freezing- 
point  measurements  is  2  '63.  A  solution  of  the  same  concentration  gives 
a  —  0'77,  calculated  from  conductivity  measurements.  The  possible 
number  of  ions,  n,  is  three.  Hence 

t  =  1  +  0-77(3  -  1)  =  2-54. 

Another  important  consequence  of  the  electrolytic  dissociation 
theory  which  is  capable  of  quantitative  verification  is  the  following. 
The  equivalent  conductivity  of  an  electrolyte  at  infinite  dilution  is,  as 
we  have  seen,  composed  of  the  sum  of  two  constants  which  are  character- 
istic of  the  cation  and  anion  present.  Further,  as  the  conductivity  of  a 
solution  depends  on  the  rates  at  which  the  ions  travel  to  the  electrodes 
through  the  electrolyte,  it  is  obvious  that  these  constants,  already 
termed  equivalent  ionic  conductivities,  must  represent  the  relative 
velocities  x  with  which  the  ions  move  under  the  same  conditions  in  an 
electric  field.  But  these  relative  velocities  are  also  the  cause  of  the 
different  transport  numbers  of  different  salts  ;  and  hence,  knowing  the 
necessary  ionic  conductivities,  we  can  calculate  these  transport 
numbers.  If  l±  and  lc  represent  the  ionic  conductivities  for  anion  and 
cation  respectively,  then  the  fraction  of  the  current  carried  by  the 
anion  will  be 


1  The  actual  velocities  with  which  ions  will  move  under  given  conditions  are 
directly  proportional  to  their  ionic  conductivities.  The  methods  by  which  these 
ionic  mobilities  are  measured  cannot  be  treated  here.  The  following  figures  will 
give  some  idea  of  their  magnitudes.  They  hold  good  for  a  potential  fall  of  1  volt 
per  cm. 

UK      0-000669  cm.  /sec. 

UXa    0-000450  cm.  /sec. 

UH      0-003415  cm.  /sec. 

UN-OS  0-000640  cm.  /sec. 

UC1     0-000677  cm.  /sec. 

UQH    0-001802  cm.  /sec. 
Their  velocity  is  proportional  to  the  voltage  gradient. 


72    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 
Similarly 


will  be  equal  to  nc.  Table  XII  contains  values  of  n^  directly 
determined  by  migration  experiments  and  values  calculated 1  using  the 
figures  for  ionic  conductivities  on  p.  67. 


TABLE  XII 

Salt 


KI  0-505  0-506 

LiCl  0-65  0-63 

AgNO3  0-522  0-528 

HC1  0-172  0-172 

NaOH  0-80  0-81 

The  agreement  is  excellent. 

Dilution  Law. — The  application  of  the  equilibrium  laws  to 
solutions  of  electrolytes  gives  interesting  results.  First  consider 
the  dissociation  equilibrium  of  an  electrolyte  of  formula  CH  Aw,  where 
C  and  A  represent  cation  and  anion  respectively.  Suppose  the  molecule 
to  dissociate  electrolytically  according  to  the  equation 

CnAm  •£--+  nC  +  mA. 
Then  we  can  write 2  (substituting  S  =  salt  for  CM  A J 

K  x  [<3J  =  [Cc]»  x  [CA]-. 

If  one  gram-molecule  of  the  original  salt  be  contained  in  v  litres, 
and  the  degree  of  dissociation  at  equilibrium  is  a,  we  have 


rp  -,  _  ma 
And  therefore 


K  = 


l-a 

To  test  this  equation,  we  will  take  the  simplest  case,  the  dissociation 
of  a  binary  electrolyte.     Here  n  =  m  =  1,  and  K  becomes 


(1  -  a)v 

ultimat 

2  Law  of  Mass  Action. 


1  These  values  depend  ultimately  on  an  accurate  migration  experiment  for 
one  salt  only. 


VI.] 


ELECTROLYTIC  DISSOCIATION  THEORY 


73 


a  formula  which  has  been  shown  to  hold  very  exactly  for  the  dissocia- 
tion of  weak  acids  or  bases,  such  as  CH3COOH  and  ammonia.  The 
two  following  Tables,  XIII  and  XIV,  contain  for  CH2C1  .  .COOH  and  for 
ammonia  constants  calculated  using  values  of  a  obtained  from 
conductivity  measurements. 


Dilution  v 

20 

205 

408 

2,060 

4,080 

10,100 

20,700 

oo 


TABLE  XIII 
Monochloracetic  Acid  at  14°. 


Equivalent 
conductivity 

51-6 
132 
170 
251 
274 
295 
300 
311 


a  from 
conductivity 

0-166 
0-423 
0-547 
0-806 
0-881 
0-948 
0-963 
1-000 


1-65  x 

1-52 

1-61 

1-62 

1-60 

1-71 

1-21 


10    : 


Dilution  v 

8 

16 

32 

64 

128 

256 

oo 


TABLE 

XIV 

Ammonia 

at  25C. 

Equivalent 
conductivity 

a  from 
conductivity 

3-20 

0-0135 

4-45 

0-0188 

6-28 

0-0265 

8-90 

0-0376 

12-63 

0-0533 

17-88 

0-0754 

237 

1-000 

K 

2-3  x  10-' 

2-3 

2-3 

2-3 

2-3 

2-4 


If  we  try  to  apply  this  relation,  discovered  by  Ostwald  and  Planck, 
and  known  as  Ostwald's  Dilution  Law,  to  the  dissociation  of  strong 
electrolytes  except  in  very  dilute  solutions,  it  breaks  down. 

Solubility  Product.  —  Another  interesting  application  of  the 
electrolytic  dissociation  theory  deals  with  the  solubility  of  sparingly 
soluble  salts.  A  solid  salt  brought  into  contact  with  a  solvent  will 
dissolve  until  the  solution  is  saturated.  When  this  happens,  we  have 
two  equilibria  to  consider.  Firstly  the  solid  salt  is  in  equilibrium 
with  the  undissociated  salt  molecules  in  the  solution.  Secondly, 
these  undissociated  molecules  are  in  equilibrium  with  their  ions.  We 
can  write 

^solid   X   KI  =  L^undissociatedJ 
L^undissociatedJ    ^   ^2  =  L^cationJ      ^    L^anionJ 

supposing  a  molecule  of  salt  to  contain  n  cations  and  ra  anions.     Now, 


74    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 


as  the  active  mass  of  a  solid  is  constant,1  we  can  put  Csolid  =  A^,  and 
obtain  the  result 

flUJ"  X  t<UJ"  =  *.  •   *•  •   k*  =  K. 
K  is  termed  the  solubility  product  of  the  salt  in  question. 

If  we  again  take  a  simple  case,  and  suppose  the  salt  to  be  a  binary 
one,  n  =  m  =  1,  and  the  equation  becomes 

PUJ-  [0«ta]  =  K. 

That  is  to  say,  when  a  solution  is  saturated  with  a  binary  salt,  the 
product  of  the  molecular  concentrations  of  the  two  ions  concerned  is  a 
constant. 

As  a  concrete  case,  imagine  that,  to  a  solution  already  saturated  with 
CaS04,  a  little  Na2S04  is  added.2  The  concentration  of  the  SO/  ion  is 
thereby  increased.  But  as 

[QoJ  •  [CSO]  =  K 

it  follows  that  CCa  must  decrease.  Calcium  sulphate  is  consequently 
precipitated  from  the  solution  until  the  condition  given  by  the  equation 
is  once  again  fulfilled.  This  is  the  explanation  of  the  well-known 
fact  that  the  solubility  of  a  salt  is  decreased  by  the  addition  to  the 
solution  of  another  salt  having  a  common  ion. 

The  above  equation  allows  us  to  calculate  the  magnitude  of  this 
influence  in  different  cases.  If  this  is  done,  good  agreement  is  obtained 
when  the  salt  concerned  is  only  slightly  soluble,  but  a  much  poorer 
one  with  moderately  or  easily  soluble  salts.  Thus  Noyes 3  studied  the 
effect  of  the  addition  of  small  quantities  of  AgN03  and  KBr03  on  the 
solubility  of  AgBr03.  Table  XV  contains  some  of  his  results.  All 
concentrations  are  in  gram-molecules  per  litre. 

TABLE  XV 


Quantity  of 
AgN03  or 
KBrO3  added 

Solubility  of  AgBr03 

On  addition 
of  AgN03 

On  addition 
of  KBr03 

Calculated  from 
solubility  product 

0-0 
0-0085 
0-0346 

0-00810 
0-00510 
0-00216 

0-00810 
0-00519 
0-00227 

(0-00810) 
0-00504 
0-00206 

The  agreement  is  very  satisfactory. 

The  conductivity  of  a  solution  containing  two  or  more  different 
salts  is  now  seen  to  be  determined  by  the  ionic  equilibria  in  the 
electrolyte.  The  quantities  of  the  different  ions  and  of  the  different 

1  P.  18. 

2  A  larger  quantity  may  introduce  other  effects. 

3  Zeitsch.  Phya.  Chem.  6,  241 


vi.]  ELECTROLYTIC  DISSOCIATION  THEORY  75 

undissociated  salts  will  adjust  themselves  in  accordance  with  the 
dissociation  constants  of  the  salts.  This  done,  each  single  ion  will 
contribute  independently  to  the  conductivity  of  the  electrolyte,  which 
will  then  be  determined  by  the  ionic  conductivities  and  the  quantities 
of  the  different  ions  present. 

Such  calculations  as  those  on  the  last  few  pages  afford  the  best 
justification  for  the  use  of  the  conception  of  electrolytic  dissociation  as 
a  working  theory  in  electrochemistry.  A  good  working  hypothesis 
should  fulfil  two  conditions.  It  should  be  capable  of  explaining 
existing  facts  and  figures  and  also  of  guiding  investigators  to  the 
discovery  of  new  phenomena  and  laws.  In  both  respects,  Arrhenius' 
hypothesis  has  proved  satisfying  and  fruitful.  The  enormous  strides 
made  during  the  last  twenty  years  in  electrochemistry,  both  pure  and 
applied  to  other  branches  of  chemistry,  are  due  directly  to  its  use. 
Certain  chemists  still  refuse  to  accept  it,  .imagining  it  to  involve,  for 
example,  the  belief  that  a  solution  of  common  salt  contains  free  particles 
of  sodium  metal  and  chlorine  gas  floating  about  unattacked  in  the 
liquid.  It  seems  superfluous  to  point  out  that  this  is  not  so  ;  what 
are  present  are  very  highly  charged  atoms  of  sodium  element  and  chlorine 
element — quite  a  different  matter.  As  Le  Blanc  has  pointed  out,  the 
quantity  of  electricity  (100  coulombs)  which  is  associated  with  one 
milligram  only  of  hydrogen  in  the  ionic  condition,  would  suffice  to 
charge  up  to  discharging  point  an  air  condenser  of  several  square 
kilometres  in  area.  And  there  are  no  grounds  for  assuming  that, 
when  sodium  chloride  dissociates,  giving  elementary  atomic  sodium  and 
chlorine,  these  substances  must  necessarily  possess  the  properties  of 
metallic  sodium  and  liquid  or  gaseous  chlorine. 

The  necessity  of  the  assumption  of  the  existence  of  charged  current- 
carriers  or  ions  in  an  electrolyte  under  ordinary  conditions  can  be  seen 
from  the  following  considerations.  If  the  molecules  of  the  solute  must 
be  decomposed  before  they  will  conduct  the  current,  a  certain  definite 
amount  of  energy  will  be  absorbed  before  any  current  can  pass.  But 
it  can  be  experimentally  shown  that,  if  absorption  of  energy  is  avoided 
at  the  electrodes,  the  current  passing  varies  directly  with  the  voltage 
applied.  Ohm's  Law  is  obeyed,  and  no  minimum  voltage  is  required 
for  the  passage  of  current.  Thus  with  copper  electrodes  in  copper 
sulphate  solution,  where  converse  reactions  take  place  at  anode  and 
cathode,  the  current  passing  varies  directly  as  the  potential  difference 
used,  however  small  the  latter.  Hence,  no  energy  is  consumed  before 
conduction  starts  in  decomposing  CuS04  into  Cu"  and  S04A'  ions,  and 
these  conducting  particles  must  exist  normally  in  the  solution. 

There  is  no  doubt,  of  course,  that  Arrhenius'  hypothesis  needs  modi- 
fication and  extension  in  order  to  account  fully  for  all  the  phenomena 
of  electrolytes.  We  have  already  pointed  out  that  for  strong  electro- 
lytes it  is  only  capable  of  representing  the  equilibria  in  very  dilute 


76        PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY 

solutions.  Abegg  considered  that  many  discrepancies  would  be 
explained  by  the  further  study  of  the  ionisation  of  ternary  and  other 
complex  salts.  Undoubtedly,  in  such  salts,  we  are  often  not  dealing 
with  simple  ions,  but  with  complex  ones  similar  to  those  present  in  the 
extreme  cases  investigated  by  Denham.1  But  it  is  unlikely  that  com- 
plicated ionisation  causes  all  the  lack  of  agreement  between  experiment 
and  theory. 

Noyes 2  has  put  forward  the  view  that  the  process  of  ionisation  is 
primarily  of  an  electrical  nature,  quite  unlike  ordinary  chemical  dissocia- 
tion, and  that  the  laws  of  mass  action  cannot  with  justification  be  applied 
to  it,  as  the  electrical  forces  which  govern  the  formation  of  ions  obey 
quite  different  laws.  He  supports  this  view  by  considerations  based 
on  the  slight  influence  of  temperature  on  ionisation,  the  practical 
independence  of  the  concentration  shown  by  the  optical  properties 
of  electrolytes,  etc.  It  is  probable  that  his  conception  is  to  a  great 
extent  correct.  In  the  dissociation  of  strong  electrolytes,  electrical 
phenomena  may  play  a  more  important  part  than  chemical  phenomena. 
The  ionisation  of  weak  electrolytes,  which  obeys  the  mass  action  law, 
is,  on  the  other  hand,  more  of  a  chemical  than  an  electrical  process. 


Literature. 

Le  Blanc.     Electrochemistry. 

Lorenz.     Elektrochemisches  Praktikum. 

1  P.  57. 

2  Jour.  Amer.  Chem.  Soc.  30,  351  (1908). 


CHAPTER  VII 

ENERGY  RELATIONS 

1.  Total  Energy  and  Maximum  External  Work 

IN  this  chapter  we  must  consider  the  energy  changes  which  take  place 
in  an  electrochemical  cell,  and  their  relations  to  the  energy  changes 
of  the  corresponding  chemical  reaction.  As  a  concrete  case  we  can 
take  the  Daniell  cell  working  at  constant  temperature.  This  cell 
consists  of  a  copper  electrode  dipping  into  a  solution  of  CuS04  which 
is  separated  by  a  porous  partition  from  a  solution  of  ZnS04  containing 
a  zinc  electrode.  Written  briefly,  it  is 

Zn  |  ZnS04     CuS04  |  Cu. 

When  the  zinc  and  copper  poles  are  joined  externally  by  a  wire,  a 
positive  current  flows  through  this  wire  from  copper  to  zinc,  zinc  goes 
into  solution  as  zinc  sulphate,  and  copper  is  deposited  on  the  copper 
electrode.  If,  on  the  contrary,  current  be  forced  through  the  cell  in 
the  opposite  direction,  zinc  will  be  deposited  on  the  zinc  electrode,  and 
copper  enter  solution  at  the  copper  electrode.  The  corresponding 
chemical  reaction  is 

Zn  +  CuS04  .  aq.  =  Cu  +  ZnS04  aq. 

to  be  read  from  left  to  right  when  the  cell  spontaneously  furnishes 
current,  and  from  right  to  left  when  current  is  forced  through  the 
cell  from  outside.  At  first  we  will  limit  our  considerations  to  the 
direct  action — left  to  right  in  the  above  equation. 

Change  of  Total  Energy.— When  copper  sulphate  solution 
and  zinc  are  brought  together,  the  formation  of  copper  and  zinc 
sulphate  solution  is  accompanied  by  an  evolution  of  heat.  Suppose 
this  reaction  carried  out  in  such  a  calorimeter  (e.g.  an  ice  calorimeter) 
that  the  temperature  of  the  system  is  the  same  before  and  after  the 
reaction,  and  let  the  heat  liberated  be  measured.  Then  we  shall  have 
changed  copper  sulphate  and  zinc  into  zinc  sulphate  and  copper  at  the 
same  temperature,  and,  as  no  other  changes  involving  energy  trans- 
formation have  taken  place,  it  follows  that  the  heat  measured  is  equal 

77 


78     PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY     [CHAP. 

to  the  decrease  of  total  energy  of  the  system.  This  value  is 
usually  denoted  by  U.  In  the  present  case,  it  is  the  difference  of  the 
heats  of  formation  of  zinc  sulphate  and  copper  sulphate  solutions,  and 
for  the  concentrations  CuS04,  100H,0  and  ZnS04,  100H,0  is  50110 
cals.  per  gram-mol.  at  0°. 

Maximum  External  Work.—  The  matter  which  immediately  con- 
cerns us  is  the  following.  When  the  reaction  is  carried  out,  not  as 
above,  but  in  such  a  way  that  useful  (i.e.  completely  controllable  and 
transformable)  external  work  results,1  what  is  the  relation  between  the 
maximum  amount  of  work  so  obtainable  and  U,  the  decrease  of  total 
energy  of  the  system  ?  If  they  are  exactly  equivalent,  then  it  is  clear 
that  the  electromotive  force  of  an  electrochemical  cell  can  be  directly 
calculated  from  thermo-chemical  data.  For  if  U  be  the  number  of 
calories  liberated  by  the  interaction  of  n  gram-equivalents,  we  have 

4-19  X  U  =  n  X  96,540  X  E 


96,540  n 

With  the  Daniell  cell,  n  =  2  and  U  =  50,110,  whence  E  =  1-087  volts. 
The  above  assumption  was  at  first  regarded  as  correct,  and  this 
method  of  calculating  the  E.M.F.  of  a  cell,  known  as  the  Helmholtz- 
Thomson  rule,2  is  still  widely  applied.  But  though  it  often  leads  to 
approximately  correct  results,  it  rests  on  an  erroneous  basis.  The 
decrease  of  total  energy  of  a  working  system  U,  and  the  decrease  of 
free  energy  or  the  maximum  useful  work  obtainable,  denoted 
by  A,  are  not  identical.  Instead  of  writing  U  =  A,  we  must  write 

U  A  +         , 

Decrease  of  total  Maximum  work  heat 

energy  obtainable  evolved 

q  can  be  small  or  great,  positive  or  negative.  If  positive,  the  maximum 
useful  work  obtainable  during  the  process  is  less  than  U,  and  the 
balance,  set  free  as  heat,  tends  to  warm  the  system  up.  If  q  is  negative, 
then  U  <  A.  The  working  system,  besides  doing  external  work 
equivalent  to  the  decrease  in  total  energy,  also  abstracts  heat  from  its 
surroundings,  and  converts  it  into  useful  work,  thus  tending  to  cool 
down  during  operation. 

2.  Reversible  Processes 

The  next  point  is,  Under  what  conditions  must  a  galvanic  cell  give 
current  in  order  that  the  electrical  energy  produced  may  be  equivalent 
to  A,  the  maximum  external  work  obtainable  from  the  corresponding 

1  In  the  present  case,  as  electrical  energy. 

2  Both  of  these  investigators  soon  recognised  the  incorrectness  of  their  assump- 
tion. 


VIL]  ENERGY  RELATIONS  79 

chemical  change  ?  Let  us  again  consider  a  Daniell  cell  working 
isfitformally  (at  constant  temperature),  and  with  the  current  capable 
of  regulation  by  means  of  an  external  resistance.  Let  the  cell  furnish 
a  moderate  current  until  one  gram-atom  of  copper  has  been  deposited 
and  one  gram-atom  of  zinc  dissolved.  A  certain  amount  of  electrical 
energy  will  be  liberated,  mostly  in  the  external  circuit,  but  partly  in  the 
cell,  owing  to  the  resistance  of  the  latter  not  being  negligible.  Let 
this  amount  of  electrical  energy  be  measured  and  equal  to  alm 
Then  we  can  represent  the  total  change  of  the  system  by 

Zn  +  Cu"  — >  Zn"  +  Cu  +  alf 

When  the  process  is  finished,  let  the  current  be  reversed,  and  forced 
in  the  opposite  direction  through  the  cell  by  means  of  an  external 
source  of  voltage,  until  the  original  chemical  conditions  have  been 
regenerated,  one  gram-atom  of  copper  in  this  case  dissolving,  and  one 
gram-atom  of  zinc  depositing.  Let  the  current  passing  be  the  same 
as  in  the  first  operation,  differing  in  direction  only.  Correcting  again 
for  the  resistance  of  the  cell,  the  energy  used  will  be  the  product  of  the 
voltage  between  the  electrodes  and  the  quantity  of  electricity  passed 
through  (two  faradays).  We  can  denote  this  quantity  of  energy,  which 
will  be  somewhat  greater  than  aiy  by  a2,  and  the  equation  representing 
the  process  becomes 

Cu  +  Zn"  -f  a2  — >  Cu"  -f  Zn. 

The  condition  of  the  Daniell  cell  is  now  in  all  respects  the  same  as 
before.  It  has  not  altered  chemically,  and  its  temperature  throughout 
has  been  kept  constant.  The  sum  result  of  the  whole  cycle  is  therefore 
given  by 

[Cu"  -f  Zn]  -  [Cu  +  Zn"  +  aj  -  [Cu"  +  Zn]  +  [Cu  +  Zn"  +  aj 

=  «1   -   «2 

and,  as  a2  >  al5  the  result  is  a  loss  of  available  or  useful  energy  (in  this 
case  electrical  energy,  which  has  been  converted  into  heat  in  the  cell). 
Suppose  the  cycle  to  be  repeated,  using  a  much  smaller  current, 
perhaps  one-tenth  as  great.  Let  the  quantities  of  electrical  energy 
given  out  and  absorbed  in  the  two  partial  processes  (decrease  and  in- 
crease of  free  energy  respectively)  be  denoted  by  a/  and  a2'.  If 
these  magnitudes  be  compared  with  ax  and  a2»  it  will  be  found  that 
«/  >  «i  and  a2'  <  a2.  It  follows  that  (a/  --  a/)  <  (a2  —  ax). 
As  before  (a./  —  a/)  represents  the  result  of  the  cycle,  a  decrease  in 
power  of  doing  useful  work.  If  further  experiments  were  made  with 
ever-diminishing  current  densities,  we  should  find  the  difference  between 
the  external  work  done  during  the  first  operation  and  the  external 
work  absorbed  during  the  second  operation  becoming  ever  smaller, 
and  finally  negligible.  At  an  infinitely  small  current  density,  «2  —  a^  is 
zero.  A  process  .carried  out  under  such  conditions  is  known  as  a 


80    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY     [CHAP. 

reversible  process,  because  it  is  possible  to  return  to  the  original  state 
of  the  system  by  a  path  which  is  in  all  respects  the  reverse  of  that 
previously  taken,  and,  when  again  at  the  starting-point,  the  system 
is  in  all  respects,  including  the  very  important  one  of  energy  relations, 
the  same  as  it  was  before  the  process  commenced. 

The  electrical  energy  furnished  by  a  primary  cell  when  working 
isothermally  and  reversibly  is  equal  to  A,  the  decrease  of  free  energy 
or  the  maximum  external  work  obtainable  from  the  corresponding 
chemical  change.  The  truth  of  this  statement  follows  from  the  Second 
Law  of  Thermodynamics,  and  cannot  be  proved  here.  Similarly  the 
increase  in  free  energy  of  an  electrochemical  system  when  current  is 
passed  through  it  from  outside  is  equal  to  the  energy  absorbed  by  the 
cell,  working  reversibly  and  isothermally. 

3.  Irreversible  Processes 

An  isothermal  reversible  process  is  characterised  by  the  fact  that 
the  power  the  system  has  of  doing  useful  work  suffers  no  actual  diminu- 
tion— only  the  form  in  which  this  work  can  manifest  itself  is  changed. 
Thus,  in  the  above  example,  the  only  change  is  the  transference  of 
available  chemical  energy  into  the  same  amount  of  available  electrical 
energy  and  vice  versa.  In  an  irreversible  process,  on  the  other 
hand,  the  capacity  of  the  system  for  useful  external  work  decreases, 
owing  to  irreversible  heat  effects.  As  the  above  consideration  of  the 
Daniell  cell  indicated,  the  extent  of  this  degradation  of  useful  energy 
to  heat  is  greater,  the  greater  the  velocity  of  the  process.  This  is 
true  generally,  and  not  only  of  electrochemical  systems.  In  mechanical 
processes,  the  amount  of  useful  energy  converted  into  heat  by  friction 
increases  rapidly  with  the  rate  of  working.  Generally  speaking,  no 
processes  of  any  kind  are  reversible  in  practice.  It  is  obviously  im- 
possible to  let  them  take  place  sufficiently  slowly,  and  friction  or 
similar  passive  resistances  must  always  be  overcome,  causing  a  certain 
amount  of  energy  to  be  lost  as  heat. 

Electrochemical  processes  often  take  place  nearly  reversibly,  and 
again  are  often  irreversible.  This  irreversibility  may  not  only  be 
quantitative,  but  can  extend  itself  to  the  qualitative  aspect  of  the 
process.  If,  in  the  above  case,  the  ZnSOt  be  replaced  by  Hj>S04,  zinc 
will  go  into  solution  as  before  when  the  cell  gives  current  spontaneously. 
Hut  wlii-ii  current  is  passed  through  it,  zinc  is  not  deposited,  as  in  the 
Daniell  cell,  but  hydrogen  is  evolved  ;  that  is,  instead  of  the  reaction 

Zn  -f  CuS04  -  — »  ZnS04  +  Cu 
Ix-ini;  reversed,  the  reaction 

Cu  -f-  H2S04  -  -  CuS04  -f  H, 
is  brought  about.  Such  ;i  <•<•!]  is  irreversible. 


vii.]  ENERGY  RELATIONS  81 

Quantitative  irreversible  effects  are  very  general,1  though  often  only 
of  slight  magnitude.  The  cathodic  discharge  of  hydrogen  needs  widely 
different  potentials  at  different  electrodes ;  when  metals  dissolve 
anodically,  there  are  often  considerable  irreversible  losses.  The  cathodic 
deposition  of  metals  absorbs  more  energy  when  a  high  current  density 
is  employed — when  the  velocity  of  deposition  is  great — than  with  a  low 
one.  Just  as  with  any  other  change,  the  greater  the  working  speed, 
the  greater  the  irreversibility  and  the  energy  losses.  A  system  doing 
useful  work  gives  less  than  is  represented  by  A  ;  a  system  absorbing 
energy  needs  to  take  up  more  than  is  represented  by  A  to  bring  about 
a  certain  change. 


4.  Relations  in  Reversible  Galvanic  Cells 

But  in  many  cases,  the  electrical  energy  given  out  or  absorbed  by 
a  cell  working  at  a  low  current  density  corresponds  closely  to  A.  And 
if  the  E.M.F.  of  the  cell  be  measured  when  no  current  is  passing,2  and 
the  result  in  volts  be  multiplied  by  96,540,  the  value  of  A  per  gram- 
equivalent  of  the  substances  transformed  will  be  given  in  joules.  In 
cases  where  a  cell  behaves  practically  reversibly  at  a  convenient  working 
current  density,  the  relations  between  the  various  magnitudes  U,  A, 
and  q  may  be  demonstrated  as  follows.  Let  the  cell,  with  an  internal 
resistance  as  small  as  possible,  be  placed  in  a  calorimeter  (preferably 
an  ice-calorimeter,  so  that  it  will  work  isothermally)  and  allow  it  to 
discharge  through  an  external  circuit  consisting  of  a  suitable  coil  of 
wire  in  a  second  calorimeter.  Then,  as  the  electrical  energy  developed 
at  constant  temperature  represents,  in  the  case  of  a  reversible  galvanic 
cell,  the  maximum  work  obtainable  at  constant  temperature  from  the 
corresponding  chemical  reaction,  and  as  this  in  its  turn  is  transformed 
into  heat  energy  in  the  wire,  the  amount  of  heat  set  free  in  the  second 
calorimeter  will  be  a  measure  of  A,  the  maximum  external  work  obtain- 
able from  the  change.  In  the  first  calorimeter  (apart  from  the  small 
quantity  of  heat  produced  by  the  current)  the  heat  effect  is  equal  to  q. 
If  U  >  A,  then  q  is  positive  and  heat  is  liberated  in  the  calorimeter.  If, 
on  the  other  hand,  U  <  A,  then  q  is  negative,  and  heat  is  absorbed 
from  the  calorimeter  by  the  cell.  Further,  as  U  =  A  -f-  q,  the  algebraic 
sum  of  the  heat  effects  in  the  two  calorimeters  gives  U,  the  change  of 
total  energy,  and  this  must  be  equal  to  the  heat  liberated  when  the 
reaction  is  carried  out  thermochemically  in  a  single  calorimeter. 

We  will  now  see  by  examples  in  what  way  U  and  A  differ  in  typical 
cases.  The  values  of  U  are  determined  thermochemically  ;  the  values 
of  A  are  calculated  from  the  E.M.F.s  of  the  different  cells. 

1  Chap.  IX.  -  P.  90. 


82    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

1.  Daniell  Cell  at  0°. 

Zn  |  ZnS04100H20  |  CuSO4100H,0    |  Cu. 
Chemical  reaction  : 

Zn  +  CuS04,  aq.    —  >  Cu  +  ZnS04,  aq. 
U  =  50,110  cals.  E  =  1-096  volt. 

1-096  x  2  x  96,540 
Hence  A  =  — 

4'19 

=  50,510  cals. 
and  q  =  U  —  A  =  —  400  cals. 

2.  Lead  Accumulator  at  17°. 

Electrolyte  1  H2SO4  :  20  H20. 
Chemical  reaction: 

Pb02  +  Pb  +  2H2S04  —  ->  2PbS04  +  2H20 
U  =  87,200  cals.  E  =  2-01  volts. 

2-01  x  96,540  x  2 

4-19   ~ 
=  92,630  cals. 

and  q  =  U  —  A  =  —  5,430  cals. 

3.  Hydrogen-chlorine  Cell  at  30°. 

Electrolyte  4'98  N.  HC1. 
Chemical  reaction  : 

^H2  +  JC12  -  >  HC1 
U  =  22,000  cals.  E  =  M90  volt 

1-190  x  96,540 
A=        "409"" 
=  27,420  cals. 

and  q  =  U  —  A  =  —  5,420  cals. 

4.  Clark  Cell  at  18°. 

Zn  |  saturated  zinc  sulphate  solution  |  Hg2S04  |  Hg. 
Chemical  reaction: 


Zn  +  Hg2S04  +  7H20  —  >  ZnS04,  7H20  +  2Hg 
U  =  81,320  cals.  E  =  1-429  volt. 

1-429  x  2  x  96,540 
Hence  A-  -       ~^- 

=  65,850  cals. 
and  q  =  U  —  A  =  +  15,470  cals. 

5.  Cell  Ag  |  AgBr  |  ZnBr2,  25H20  |  Zn  at  0". 
Chemical  reaction  : 

2AgBr  +  Zn  +  aq.   —  >  2Ag  +  ZnBr.,  aq. 
U  =  39,764  cals.  E  =  0'828  volt. 

0-828  x  2  x  96,540 
Hence  A=  —  r^J- 

=  38,160  cals. 
and  q  =  U  —  A  =  +  1604  cals. 


VIL]  ENERGY  RELATIONS  83 

These  results  clearly  show  that,  as  has  been  already  stated,  U  and 
A  are  not  equivalent.  In  the  Daniell  cell  they  are  very  nearly  equal. 
But  in  the  other  cases  there  are  considerable  differences.  A  is  greater 
than  U  both  for  the  lead  accumulator  and  the  hydrogen-chlorine  cell. 
When  these  two  cells  are  in  action,  they  absorb  heat  from  their  sur- 
roundings, and  convert  it  into  useful  work  in  the  form  of  available 
electrical  energy,  tending  to  become  cooled  in  the  process.  On  the 
other  hand,  the  last  two  combinations  do  not  convert  all  the  energy 
set  free  during  the  process  into  useful  external  work,  but  a  certain 
amount  of  it,  given  by  q,  is  set  free  as  heat.  Such  cells  tend  to  warm 
up  when  working.  In  the  Clark  cell,  some  19  per  cent,  of  the  total 
energy  change  appears  as  heat.  It  follows  that  the  E.M.F.  of  a  cell 
cannot  be  directly  calculated  from  the  corresponding  change  of  total 
energy  U.  In  some  cases  a  very  close  agreement  would  be  got ;  e.g.  for 
the  Daniell  cell  the  value  is  correct  to  within  1  per  cent.  But  usually 
this  is  not  so.  A  calculation  for  the  lead  accumulator  would  give  a 
result  6  per  cent,  too  low,  for  the  hydrogen-chlorine  cell  discussed 
a  result  20  per  cent,  too  low,  for  the  Clark  cell  a  figure  19  per  cent, 
too  high,  and  for  the  last  combination  mentioned  an  E.M.F.  4  per  cent, 
too  great. 

By  applying  the  Second  Law  of  Thermodynamics  to  changes  of 
temperature  and  the  corresponding  changes  of  the  different  terras 
of  the  equation  U  =  A  +  2  we  arrive  at  the  celebrated  Gibbs-Helm- 
holtz  equation 

A  -  U  =  T  .  dX. 
dT 

This  result  (which  must  be  taken  for  granted  here)  differs  from  the 

dA. 
equation  already  used  in  that  q  is  replaced  by  —  T  .  -  -,  where  T  is 

dA. 

the  absolute  temperature,  and  ^  the  rate  of  change  with  tempera- 
ture of  the  maximum  work  obtainable  from  the  process  under  con- 
sideration. Applying  this  equation  to  the  particular  case  of  galvanic 
cells,  we  can  substitute  for  A  96,540nE,  where  n  is  the  number  of 
gram-equivalents  transformed  during  a  change  of  total  energy  of  U 

calories.     Then  -  -  becomes  96,540ft  — ,  and  we  finally  have 

OL  dl 

j-p 
w96,540E  -  419  U  =  Trc96,540  -==, 

all  terms  being  expressed  in  joules.      From  this  we  see  that,  knowing 

7T71 

E  and  U.        ,  the  coefficient  of  increase  of  electromotive  force  with 
eZT 

G  2 


84  PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY  [CHAP. 
temperature  can  be  calculated.  Moreover,  the  sign  of  —  depends 

(t  J_ 

on  the  relative  magnitudes  of  A  and  U.  If  the  maximum  work  which 
can  be  performed  by  the  cell  exceeds  the  corresponding  change  of  total 
energy  (in  which  case  we  remember  the  cell  tends  to  cool  when  working), 
then  the  E.M.F.  rises  with  increase  of  temperature.  If  on  the  other 

c^E 
hand  U  >  A,  and  the  cell  tends  to  heat  up  when  giving  current, 

is  negative,  and  the  E.M.F.  diminishes  with  rising  temperature. 

We  will  calculate  the  temperature  coefficient  of  the  Daniell  cell, 
taking  U  as  50,110  and  the  E.M.F.  at  0°  as  1*0962  volt. 

We  have 

dE 
(2  x  96,540  x  1-0962)  -  (4'19  x  50,110)  =  (273  x  2  x  96,540)  ^ 

dE      211,700  —  210,000  volt 

a"         5,271  X  10'          =  +(K)00032  degree" 

The  value  of  the  temperature  coefficient  determined  by  experiment  1  is 

+  0-000034  -—  —  ,  the  agreement  being  very  close. 
degree 

Similarly,  knowing  the  E.M.F.  and  its  temperature  coefficient  for 
a  given  cell,  we  can  calculate  U,  the  change  in  total  energy  or  heat 
of  reaction.  We  have 

_  96,54QttE     96,54Q/iT      dE 
4-19  4-19      '   (ZT 

For  example,  take  the  element 

Cu  |  Cu,O  NaOH  |  H2.2 

The  corresponding  chemical  reaction  is  Cu20  +  H2  ->  2Cu  -(-  H20,  and  the 
thermochemical  value  of  U  is  27,400  cals.  at  18°.  The  E.M.F.  at  18°  is  0'461 

volt 
volt,  and  its  temperature  coefficient  —  0'00066       ~-     We  get  then  :— 


QC    r»40      V      2 

U  =      '       9  —  [0-461  +  291  x  0-00066]  cals. 
=  27,530  cals. 

Table  XVI3  contains  in  the  first  column  the  combination  dealt 
with,  in  the  second  column  the  E.M.F.,  and  in  column  3  its  temperature 
coefficient;  in  the  fourth  column  U  (in  cals.)  calculated  by  the  above 

>  H.  Jahn,  Wied.  Ann.  28,  21  (1886). 

2  Allmand,  Trans.  Chem.  ,S'or.  99,  840  (HU1). 

3  H.  Jahn,  Wied.  Ann.  28,  491  (MM)  ;  50,  180  (MM).      Donnan  and  Allmand, 
Trans.  Chem.  Soc.  99,  845  (1911).      Bugarszky,  Zeitsch.   Anory.  Chem.   14,  145 
(1897). 


VII.] 


ENERGY  RELATIONS 


85 


equation  from  the  data  in  columns  2  and  3  ;  in  the  fifth  column  the 
thermochemical  value  of  U,  and  in  the  last  column  the  value  of  U  cal- 
culated from  E  by  the  incorrect  '  Helmholtz-Thomson  rule/ 

TABLE  XVI 


Cell 

E  in 
volts 

dE  /  volts  \ 

U  (calcu- 
lated) in 
cals. 

U  (experi- 
mental) in 

cals. 

/ 

17,533 

96,540wE 

dT  \degree/ 

4-19 

Cu 

1 
| 

Cu(aH3Oo).,  solution 
Pb(G,H3Oo),,  lOOHoO 
PbatO0 

0-4764 

+0-000385 

16,900 

21,684 

Ag 
1 

AgClZnCl2.100H.2G 
Zn  at  0° 

1-015 

—0-000402 

51,989 

52,046 

46,907 

Hg 

at 

|  HgO  n.NaOH  |  H2 
18° 

0-9243 

-0-00031 

46,750 

46,700 

42,590 

Hg  |  HgdO-OlN.KCl  | 
N.KN03  |  0-01N.KOH 
HgoO  !  Hg  at  18-5°. 

0-1656 

+0-000837 

-3,710 

-3,280 

7,566 

The  values  of  U  calculated  from  the  Gibbs-Helmholtz  equation  are 
in  all  cases  much  nearer  the  experimental  figures  than  are  the  values  in 
the  sixth  column.  The  last  case  is  particularly  remarkable  as  the 
calculation  of  the  heat  of  reaction  according  to  the  '  Helmholtz-Thomson 
rule  '  gives  a  value  which  actually  differs  in  sign  from  the  true  value. 


5.  Relations  during  Reversible  Electrolysis 

The  examples  given  above  all  refer  to  primary  cells.  The  same 
relations  hold  for  the  converse  phenomena  of  reversible  electrolysis, 
and  need  no  further  detailed  consideration.  The  minimum  quantity 
of  electrical  work  necessary  for  any  electrolysis  is  the  product  of  the 
quantity  of  electricity  passed  through,  and  the  minimum  reversible 
voltage  at  which  the  electrolysis  will  take  place.  This,  as  before,  is 
connected  with  the  change  of  total  energy  of  the  system  and  with  the 
heat  absorbed  or  given  out  in  the  cell  when  the  electrolysis  is 
isothermally  conducted  by  the  equation 

U  =  A  +  q. 

As  A  and  U  are  defined  respectively  as  decrease  of  capacity  of  doing 
useful  work  and  decrease  of  total  energy,  A  always  and  U  generally  will 
be  negative  in  such  cases,  for  the  capacity  of  the  system  to  perform 
useful  work  increases  when  electricity  is  passed  through  it.  Similarly, 
if  U  —  A  be  positive,  or  if  more  electrical  energy  be  passed  into  the  cell 


86        PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY 

than  corresponds  to  the  change  of  total  energy  in  the  chemical  sub- 
stance present,  q  will  be  positive  and  heat  will  be  liberated  in  the  cell. 
And  if  U  —  A  be  negative,  heat  will  be  absorbed  during  working. 

As  an  example  we  can  take  the  electrolysis  of  5n.  HC1  between  platinised 
platinum  electrodes,  which  takes  place  practically  reversibly  if  carried  out  at  a 
very  low  current  density.  1-190  volts  are  necessary  at  30°.  For  the  reaction 
HC1 >  iHo  -f-  £01o,  one  faraday  is  required.  Hence 

1-190  x  96,540 
A  =  -  4-ij)  ~  "    =  ~  27,420  cals. 

The  change  in  total  energy,  as  given  by  the  heat  of  reaction,  is  —  22,000  cals. 
Hence 

q  =  U  —  A  =    +  5,420  cals. 

Heat  will  be  given  out  in  the  cell  during  electrolysis,  and  the  voltage  required  to 
carry  out  the  electrolysis  will  increase  with  rising  temperature. 

We  shall  have  occasion  further  to  consider  the  conditions  necessary 
for  reversible  electrolysis  and  the  causes  bringing  about  irreversible 
electrolysis  in  succeeding  chapters. 


6.  Maximum  Work  and  Affinity 

It  should  be  pointed  out  in  conclusion  that  A,  the  maximum 
external  work  which  can  be  obtained  from  a  process,  is  also  a  measure 
of  the  driving  force  or  affinity  of  the  reaction.  The  tendency  for  a  re- 
action to  take  place  depends  on,  and  is  measured,  not  by  the  diminution 
in  total  energy  (U)  which  would  occur,  but  by  the  amount  of  useful 
external  work  (A)  which  could  be  done.  No  chemical  or  electrochemical 
reaction  will  set  in  spontaneously  unless  the  value  of  A  corresponding 
to  it  is  positive.  The  nearer  a  system  approaches  the  equilibrium 
point  the  smaller  A  becomes,  until  under  equilibrium  conditions  A 
reaches  the  value  zero.  Then  there  is  no  tendency  for  any  kind  of 
change  to  set  in.  A  cell  with  a  voltage  of  zero  will  furnish  no  current. 


CHAPTER  VIII 

ELECTROMOTIVE   FORCE 

1.  Necessary  Conditions  for  Electrochemical  Reactions 

IN  this  and  the  following  chapters  we  shall  consider  in  detail  the  free- 
energy  changes  which  take  place  in  electrochemical  systems — that  is, 
the  reciprocal  transformations  of  chemical  and  electrical  energy. 
We  have  seen  the  important  part  that  irreversible  effects  play  in  electro- 
chemical processes.  But  at  present,  on  account  of  simplicity,  they  will 
be  ignored,  and  subsequent  considerations  and  deductions  in  this 
chapter,  unless  otherwise  specifically  stated,  refer  to  reversible  processes 
only.  It  will  be  convenient  if  we  first  discuss  the  transformation  of 
chemical  into  electrical  energy,  before  considering  the  more  important 
reverse  case. 

We  are  at  once  faced  by  the  following  questions  :  Can  all  chemical 
changes  accompanied  by  a  decrease  of  free  energy  liberate  this  energy 
as  electrical  energy  ?  If  not,  what  types  of  chemical  change  are  capable 
of  so  doing  ?  And  what  conditions  must  be  fulfilled  in  order  that  the 
chemical  energy  shall  appear  as  electrical  energy,  and  not,  as  generally 
happens,  as  heat  ? 

The  first  question  we  can  at  once  answer  in  the  negative.  A  large 
number  of  reactions  which  proceed  very  easily  chemically  cannot  be 
carried  out  electrolytically.  The  combustion  of  carbon  and  sulphur 
to  their  oxides,  the  absorption  of  CO  by  NaOH  giving  H.COONa, 
and  many  organic  reactions  are  examples. 

The  next  two  points  can  be  taken  together.  As  electrolytic  reactions 
all  involve  the  combination  of  matter  with,  or  its  separation  from, 
electricity,  and  as  these  processes  generally  take  place  in  aqueous 
solution,  the  first  necessary  condition  is  that  the  substances  par- 
taking in  such  a  reaction  must  be  capable  of  ionising.  Copper 
can  be  electrolytically  refined,  because  it  can  combine  with 
electricity,  forming  the  ion  Cu",  which  in  its  turn  can  give  up  its 
electricity  and  reproduce  metallic  copper.  Chlorine  can  be  electro- 
lytically prepared  because  of  its  ion  Cl',  which  Qxists  in  aqueous  solution 
together  with  such  ions  as  Na"  and  Zn".  On  the  other  hand,  carbon 

87 


88    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY     [CHAP. 

cannot  ionise,  and  therefore  carbon  dioxide  cannot  be  formed  electro- 
chemically  at  a  low  temperature  from  carbon  and  oxygen.  Similarly 
nitrogen  does  not  ionise,  and  the  electrolytic  combination  of  nitrogen 
and  hydrogen  is  thus  rendered  impossible.  Possibility  of  ionisation  is 
consequently  necessary.  A  second  point  common  to  electrolytic 
reactions  is  that  they  all  involve  processes  of  an  oxidising  or  reducing 
character.  When  zinc  enters  solution  anodically  as  zinc  ions,1  it  has 
really  been  oxidised,  just  as  much  as  when  Fe"  ions  have  been  changed 
anodically  to  Fe'"  ions.  When  CF  ions  are  discharged  to  gaseous 
chlorine,  the  process  again  is  one  of  oxidation.  We  recognise  this  when 
speaking  of  the  oxidation  of  HC1,  in  which  the  chlorine  is  in  the  ionic 
condition,  to  chlorine  gas  by  Mn02.  On  the  other  hand,  when  H*  ions 
are  discharged  to  gaseous  hydrogen  or  Ag'  ions  to  metal,  the  process 
is  one  of  reduction.  Generally,  an  increase  in  the  number  of  positive 
charges  or  a  decrease  in  the  number  of  negative  charges  associated 
with  a  substance  means  oxidation  :  the  cuprous  ion  Cu'  is  oxidised 
to  the  cupric  ion  Cu",  the  ferrocyanide  ion  FeCy6""  is  oxidised  to  the 
ferricyanide  ion  FeCy6'".  And  a  decrease  in  the  number  of  positive 
charges  or  an  increase  in  the  number  of  negative  charges  means 
reduction  :  Au'"  ions  are  reduced  to  metallic  gold  Au,  and  perman- 
ganate ions  MnO/  reduced  to  manganate  ions  Mn04".  Now,  as  all 
electrode  reactions  consist  essentially  in  a  change  in  the  quantity  of 
electricity  associated  with  matter,  we  see  that  electrochemical  processes 
must  necessarily  all  be  of  an  oxidising  and  reducing  character.  In 
fact,  all  anodic  reactions  must  oxidise,  all  cathodic  reactions  reduce 
something  or  other. 

The  final  condition  necessary  for  an  electrolytic  process  is  that  the 
reacting  substances  must  be  kept  apart,  but  connected  by  two  conducting 
paths,  one  the  electrolyte,  the  other  an  external  circuit  of  a  metallic 
nature.  In  that  way  the  irreversible  effects  of  ordinary  chemical 
reactions,  where  the  free  energy  liberated  appears  as  heat,  are  avoided. 
Instead,  the  positive  electricity  set  free  at  the  cathode  (one  of  tin* 
electrolyte-metallic  circuit  junctions)  by  the  reduction  process  will 
proceed  to  the  other  electrode  through  the  external  circuit,  there 
recombine  with  matter,  thus  effecting  oxidation,  and  re-enter  the 
electrolyte.  On  its  way  through  the  outside  circuit,  it  can  be  use- 
fully employed  by  driving  a  suitable  motor,  thus  utilising  the  free 

1  The  chemical  solution  of  zinc  by  sulphuric  acid  is  expressed  by  tlio 
equation  Zn  4  H2S04  — >  ZnSO4  +  H« ;  or,  as  SO/'  ions  arc  present  both 

before     and     after     the     reaction,     by     Zn  -f  2H* >  Zn"  +  H,.       Metallic 

zinc  has  been  oxidised  to  zinc  ions,  hydrogen  ions  have  been  reduced  to 
gaseous  hydrogen.  Or  we  can  suppose  that  zinc  is  first  oxidised  by  the  water, 
giving  zinc  hydroxide  and  hydrogen,  and  that  the  acid  and  hydroxide  subse- 
quently combine. 


VIII.] 


ELECTROMOTIVE  FORCE 


89 


energy  liberated  in  the  chemical  reaction.     As  examples    take  the 
three  simple  reactions: — 

(a)  An  +  f  C12  — >  AuCl3 

(b)  Zn  +  CuS04  — >  Cu  +  ZhS04 

(c)  Fe  +  2FeCl3  — >  3FeCl2. 

The  arrangements  for  bringing  these  about  electrolytically  are 
shown  in  Fig.  18.  In  (a),  if  chlorine  water  be  added  to  the  vessel 
containing  the  gold,  the  latter  will  very 
slowly  dissolve,  and  no  current  will  pass. 
But  if  it  be  poured  over  the  platinum, 
the  gold  will  dissolve,  the  chlorine  water 
disappear,  and  a  current  will  flow  through 
the  wire.  Similarly  in  (6)  the  CuS04 
solution  must  be  added  to  the  right- 
hand  vessel,  when  copper  will  be  de- 
posited and  zinc  will  dissolve.  Whilst 
with  (c)  the  FeCl3  solution  must  be 
pipetted  on  to  the  platinum  electrode, 
when  it  will  be  reduced  to  ferrous 
chloride,  and  iron  from  the  iron  electrode 
will  enter  solution  as  Fe"  ions.  For 
a  chemical  reaction  to  be  carried  out  elec- 
trochemically ,  it  must' be  of  an  oxidation- 
reduction  nature,  the  substances  taking  part 
must  be  capable  of  ionisation,  and  must  be 
spatially  separated,  but  in  electrical 
connection. 

An  actual  chemical  change  is  not 
necessary  for  the  generation  of  an  E.M.F. 
Two  silver  nitrate  solutions  of  unequal 
concentrations,  if  mixed,  suffer  a  loss  of 
free  energy.  They  can  be  so  arranged  as 
to  produce  an  E.M.F.  depending  solely 
on  this  concentration  difference,  and 
not  on  any  kind  of  chemical  change. 

The  E.M.F.  is  determined  by  the  osmotic  pressure  difference  of  the 
two  solutions,  just  as  the  work  done  by  a  gas  expanding  into  a 
space  at  low  pressure  depends  on  the  pressure  difference  between 
the  two  spaces.1 

The  E.M.F.  of  any  cell  is  the  result  of  all  the  potential  differences 
existing  in  the  cell.     We  must  invariably  consider  at  least  two — the 


1  For  more  on  these  cells,  see  p.  103. 


90    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY     [CHAP. 

potential  difference  between  cathode  and  catholyte  and  that  between 
anode  and  anolyte.  When  catholyte  and  anolyte  differ,  we  may 
further  take  account  of  the  liquid  potential  difference  between  them. 
This  is,  however,  usually  very  small,  rarely  exceeding  0'02— 0'03  volt, 
and  can  be  neglected  for  most  purposes,  as  will  subsequently  be  done 
in  this  book. 

2.  Measurement  of  Electromotive  Force 

The  measurement  of  the  E.M.F.  of  a  cell  is  easily  carried  out  with 
the  arrangement  in  Fig.  19.  AB  is  a  fine  uniform  wire,  as  used  in 
conductivity  measurements,1  provided  with  a  sliding  contact  C.  A 


FIG.  19. 

voltage  furnished  by  D  is  applied  to  the  ends  of  AB.  It  must  be 
greater  than  the  E.M.F.  to  be  measured.  One  or  more  lead  accumu- 
l.i tors  (each  of  two  volts)  will  suffice.  The  cell  x,  of  which  the  E.M.F. 
is  to  be  determined,  is  connected  to  one  end,  A,  of  the  bridge.  In  series 
with  it  is  placed  a  galvanometer,  which  is  also  connected  with  C.  The 
poles  of  D  and  x  which  are  attached  to  the  same  end  of  the  bridge  must 
be  of  the  same  sign.  A  standard  cell 2  F  is  also  connected  to  A,  and  by 
means  of  the  key  H  either  a;  or  F  can  be  put  into  series  with  G.  Now 
the  cell  (or  cells)  D  produces  a  uniform  fall  of  potential  along  AB.  If 
the  standard  cell  F  be  put  into  series  with  G,  we  have  two  opposing 
E.M.F.s  acting  along  the  circuit  AFGC.  One  is  the  potential  difference 
produced  by  D  between  A  and  C.  This  tends  to  drive  positive  elec- 
tricity in  the  direction  AFC,  whilst  the  E.M.F.  of  the  standard  cell 
tends  to  send  a  current  in  the  reverse  direction  CFA.  As  long  as  these 
two  potential  differences  are  unequal,  a  current  will  flow  through  G. 
When,  by  moving  C  up  and  down,  the  potential  difference  between  A 


P.fiO. 


2  Sec  p.  <i. 


VIII.] 


ELECTROMOTIVE  FORCE 


91 


and  C  has  become  equal  to  the  E.M.F.  of  the  standard  cell,  no  current 
will  flow  through  G.  The  position  of  C  is  therefore  adjusted  in  this 
way.  and  when  the  correct  point  has  been  found  we  know  that  the 
potential  fall  between  A  and  C  is  equal  to  the  E.M.F.  of  F,  from  which 
result  we  can  calculate  the  potential  drop  per  mm.  of  AB.  By  means 
of  H,  the  unknown  cell  x  is  now  thrown  in,  and  C  again  moved  until 
no  current  passes  through  G.  Reading  the  new  length  of  AC,  and 
knowing  the  potential  drop  per  mm.,  we  arrive  directly  at  the  value 
of  the  E.M.F.  of  x.  This  method  is  exact,  easily  carried  out,  and  capable 
of  great  flexibility. 

The  only  standard  cell  for  E.M.F.  measurements  needing  considera- 
tion is  the  cadmium  cdl.  The  Clark  cell,  which  was  formerly  much 
used,  has  a  far  too  high  temperature  coefficient.  The  cadmium  cell  is 


Sealing  wax 
CorJc 

•Paraffin  wajc 


FIG  20. — Standard  Cadmium  Cell 

generally  constructed  as  in  Fig.  20.  One  limb  of  the  H-vessel  con- 
tains mercury,  and  this  is  covered  with  a  paste  of  mercurous  sulphate 
and  hydrated  cadmium  sulphate.  The  other  limb  contains  a  12*5 
per  cent.  Cd  amalgam,  which  is  readily  made  by  warming  the  right 
proportions  of  the  ingredients  in  a  test-tube.  The  electrolyte  consists 
of  saturated  cadmium  sulphate  solution,  filled  with  3  CdS04,  8  H20 
crystals.  A  small  air-space  is  left  above  the  liquid  level,  and  the  vessel 
is  closed  by  successive  layers  of  paraffin  wax,  cork,  and  sealing  wax. 
All  materials  used  must  be  very  carefully  purified.  The  E.M.F.  of  a 
cell  is  given  by  the  expression 

1-0184:  -  0-00004  (0  -  20)  volt. 

The  temperature  coefficient  is  seen  to  be  exceedingly  small,  ^5  milli- 
volt per  degree,  and  the  E.M.F.  remains  constant  if  none  but  very 
small  currents  are  allowed  to  go  through  it.1 


1  For  measurement  of  single  potential  difference,  see  p.  104. 


92    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY     [CHAP. 

3.  Electrolytic  Solution  Pressure 

The  most  important  type  of  electrode  process,  anodic  or  cathodic,  is 
that  in  which  a  metal  and  its  corresponding  ions  in  the  electrolyte  take 
part,  and  a  conception  of  Nernst's  enables  us  clearly  to  picture  what 
happens  in  such  cases.  Every  metal  has  a  certain  tendency  to  oxidise, 
and,  as  we  have  seen,  this  is  equivalent  to  a  tendency  to  take  up 
positive  electricity  and  enter  solution  as  positively  charged  metallic 
cations.  With  the  noble  metals — gold,  platinum  etc. — this  tendency 
is  very  slight ;  with  metals  such  as  copper  and  lead  greater,  with 
iron  and  zinc  greater  still,  with  the  strongly  electro-positive  alkali 
metals  very  high  indeed.  An  analogy  is  furnished  by  the  different 
tendencies  of  different  liquids  to  vaporise  or  give  out  gas  molecules- 
small  at  room  temperature  with  mercury,  higher  with  water,  very 
considerable  with  ether. 

We  express  these  facts  by  saying  that  the  liquids  have  different 
vapour  pressures,  and  similarly  we  can  suppose  that  each  metal  has 
a  definite  electrolytic  solution  pressure,  which  is  a  constant  for 
that  metal  at  a  given  temperature,  and  with  a  given  solvent.  A 
liquid's  vapour  pressure  measures  the  tendency  for  the  process 

liquid  ->   vapour 

to  take  place  ;  similarly  the  electrolytic  solution  pressure  of  a  metal 
measures  the  driving-force  of  the  process 

metal  ->  ion. 

This  electrolytic  solution  pressure  is  high  for  alkali  metals,  low  for 
noble  metals.  Now  the  presence  of  molecules  of  its  vapour  in  the 
space  above  it  counteracts  the  tendency  of  a  liquid  to  vaporise  ;  if 
the  space  is  supersaturated  with  vapour,  liquid  will  deposit  until  equili- 
brium is  reached  and  the  pressure  above  the  liquid  equals  the  pressure 
of  saturated  vapour.  Any  ions  of  the  metal  already  in  solution  will 
act  analogously.  The  tendency  to  deposit  metal  and  give  up  positive 
electricity  will  be  greater,  the  greater  the  concentration  of  those  ions, 
and  will  be  measured  by  their  osmotic  pressure.  If  that  is  very  high, 
or  if  the  electrolytic  solution  pressure  of  the  metal  is  low,  the  latter  will 
be  overcome,  and  the  net  tendency  will  not  be  for  metal  to  ionise,  but 
for  ions  to  discharge.  In  this  case  the  metal  will  become  positively, 
and  the  solution  negatively,  charged,  whilst  if  the  electrolytic  solution 
pressure  of  the  metal  overpowers  the  ionic  osmotic  pressure,  the  metal 
will  become  negatively,  the  solution  positively,  charged. 

The  two  cases  are  simply  illustrated  by  copper  in  normal  copper 
sulphate  and  zinc  in  normal  zinc  sulphate.  In  the  former  case  the 
electrode,  in  the  latter  case  the  solution,  is  positively  charged  (Fig.  21). 
As  the  ionic  concentrations  in  the  two  solutions  are  practically  equal, 
this  difference  is  entirely  due  to  differences  in  the  electrolytic  solution 


VIII.] 


ELECTROMOTIVE  FORCE 


93 


Cu, 


pressures  of  the  two  metals.  That  of  copper  is  low,  and  is  overpowered 
by  the  osmotic  pressure  of  the  Cu"  ions,  whilst  the  tendency  of  the 
strongly  electropositive  zinc  to  ionise  overcomes  the  opposing  action 
of  the  zinc  ions  already  in  solution. 

Electrical  Double  Layer.  —  One  could  object  that,  if  an  equi- 
librium of  the  kind  indicated,  analogous  to  the  equilibrium  between 
a  liquid  and  its  vapour,  does 
really  exist  between  a  metal 
and  its  dissolved  ions,  then, 
when  a  metal  is  dipped  into 
solutions  of  its  salts  of  differ- 
ent strengths,  it  should  either 
dissolve  or  ions  should  de- 
posit, until  in  every  case  the 
metal  ion  concentration  in 
the  solution  has  reached  the  FIG.  21. 

value  corresponding  to  equi- 
librium with  the  electrolytic  solution  pressure  at  that  temperature. 
This,  of  course,  does  not  occur.  The  electrolytic  solution  pressure  of 
silver  is  low,  but  strong  solution  of  silver  salts  do  not  deposit  silver 
when  in  contact  with  'a  silver  electrode,  nor  conversely  does  zinc 
enter  solution  as  zinc  ions  in  detectable  amounts  when  immersed  in  a 
solution  poor  in  zinc. 

The  reason  for  this  apparent  discrepancy  is  that,  whereas  with  a 
liquid  and  its  vapour  we  are  dealing  with  electrically  neutral  molecules, 
in  the  present  case  we  are  concerned  with  charged  ions.  If  we  consider 
zinc  in  zinc  sulphate,  the  first  action  is  certainly  the  passage  of  positively 
charged  zinc  ions  into  solution  leaving  behind  an  equal  number  of 
negative  charges  on  the  metal.  But  this  action  ceases  practically 
instantaneously,  owing  to  the  very  powerful  electrostatic  action  set  up 
between  the  separated  positive  and  negative  charges,  and  preventing 
any  further  separation.  When  we  remember  that  96,540  coulombs  are 
associated  with  one  gram-equivalent  of  ionic  matter,  this  fact  is  not 
surprising.  The  electrical  double  layer,  such  as  is  shown  in 
Fig.  21,  hinders  any  further  formation  or  discharge  of  ions,  which  can 
only  proceed  continuously  if  the  opposite  charges  are  somehow 
constantly  withdrawn. 

We  will  now  see,  for  a  concrete  case,  how  Nernst's  theory  explains 
(a)  the  chemical  replacement  of  one  metal  by  another,  (6)  the  production 
of  current  in  a  galvanic  cell.  If  we  place  a  piece  of  zinc  in  dilute  copper 
sulphate,  zinc  sulphate  will  be  formed  and  copper  deposited,  whilst  if 
copper  be  dipped  into  a  zinc  sulphate  solution  nothing  happens.  The 
explanation  is  simple.  Zinc  strongly  tends  to  ionise,  and,  when  placed 
in  an  electrolyte,  assumes  a  high  electrostatic  negative  charge,  the 
corresponding  positive  portion  of  the  electrical  double  layer  being  in  the 


94    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY     [CHAP. 

liquid.  Copper  ions  tend  powerfully  to  give  up  their  positive  charge. 
The  strong  negative  electrostatic  charge  on  the  zinc  enables  them 
to  do  so.  The  negative  charge  on  the  zinc  and  the  positive  charge 
of  the  Cu"  ions  neutralise  one  another  and  metallic  copper  results. 
The  electrical  double  layer  at  the  zinc  surface  is  renewed  by  more  zinc 
ions  passing  into  solution,  and  the  process  thus  continues.  With 
copper  in  zinc  sulphate,  the  electrostatic  negative  charge  on  the  metal 
is  very  low,  as  is  also  the  tendency  of  zinc  ions  to  lose  their  positive 
charges.  Hence  no  neutralisation  or  chemical  replacement. 

Let  us  now  consider  the  electrochemical  arrangement  for  bringing 
about  the  same  reaction  (Zn  +  CuS04 >  Cu  +  ZnS04),  the  well- 
known     Daniell    cell.     This    is    shown 
—                -1_               diagrammatically   in    Fig.  22. 

^  The  containing  vessel  is  divided  into 

two    parts    by   the    porous    diaphragm. 
One    compartment    contains     a    ZnS04 


solution  in  which  is  suspended  a  strip  of 
zinc,  and  the  other  copper  in  CuS04  solu- 
tion. We  have  already  seen  that  zinc  in 
FIG.  22. — Daniell  Cell.  ZnS04  is  negatively  charged  with  respect 
to  the  solution,  and  that  copper  is  posi- 
tively charged  with  respect  to  CuSO-j.  The  potential  difference 
lu'twuen  the  two  liquids  is  negligible,  and  it  therefore  follows  that 
the  copper  is  at  a  positive  potential  with  respect  to  the  zinc.  If 
these  two  electrodes  are  joined  externally  by  a  wire,  positive  elec- 
tricity will  therefore  flow  along  it  from  copper  to  zinc.  The  elec- 
trical double  layers  are  thus  destroyed,  and  will  reform  through  more 
Cu"  ions  discharging  and  more  zinc  ions  dissolving.  The  result 
will  be  a  continuous  current  flowing  through  the  cell  and  around 
the  circuit,  whilst  the  copper  sulphate  solution  will  become  weaker, 
depositing  copper,  and  the  zinc  sulphate  solution  stronger  at  the 
expense  of  the  zinc  anode.  The  E.M.F.  of  the  cell  is  given  by  the 
potential  difference  between  the  copper  and  zinc  strips  with  open 
external  circuit,  i.e.  equal  to  the  difference  of  the  single  electrode 
potentials.  In  this  case  it  is  (copper  potential — electrolyte  potential) 
minus  (zinc  potential — electrolyte  potential).  As  both  terms  increase 
with  an  increase  in  the  concentrations  of  the  respective  electrolytes,  the 
E.M.F.  of  the  cell  should  increase  with  increase  of  copper  sulphate 
concentration  and  with  decrease  of  zinc  sulphate  concentration.  This 
deduction  is  borne  out  by  facts. 

4.  Quantitative  Relations  at  Ionising  Electrodes 

Electrolytic     Potential.  —  As  the  E.M.F.     of     such     a     cell     is 
determined   essentially  by  the  two  electrode   potential   differences, 


viii.]  ELECTROMOTIVE  FORCE  95 

these  in  their  turn  depending  on  the  concentrations  of  the  electrolytes 
t  which  are  variable,  and  the  electrolytic  solution  pressures  of  anode 
and  cathode,  which  are  constants  for  the  substance  concerned  (at 
constant  temperature  and  with  the  same  solvent),  it  is  important 
that  these  latter  values,  or  measures  of  them,  should  be  known  and 
tabulated.  A  suitable  measure  of  the  electrolytic  solution  pressure 
is  the  potential  difference  between  electrode  and  electrolyte,  rendered 
comparable  with  values  for  other  substances  by  eliminating  effects 
due  to  concentration  differences  between  the  different  solutions.  As 
standard  electrolyte,  one  containing  always  one  gram-ion  per  litre  of  the 
ion  concerned  is  taken  :  thus  a  concentration  of  35*5  grams  Cl'ion, 
56  grams  Fe"  ion,  96  grams  SO/  ion  per  litre.  The  potential  difference 
in  that  case  is  known  as  the  electrolytic  potential  (E.P.)  of  the  electrode 
reaction  concerned.1 

Before  giving  any  numerical  figures  we  must  decide  on  the  con- 
ventions of  sign  and  potential  zero  to  be  used.  The  potential  of  an 
electrode  will  always  be  regarded  as  (potential  of  electrode  minus  potential 
of  solution),  never  as  (potential  of  solution  minus  potential  of  elec- 
trode), as  is  done  by  some  writers.  This  will  hold  both  for  electrodes 
which  give  off  positive  ions,  such  as  those  we  have  discussed,  and  for 
electrodes  furnishing  negative  ions. 

There  are  two  scales  for  potential  measurement  in  common  use. 
The  first  is  termed  the  absolute  scale,  and  potentials  expressed  on 
this  scale,  written  (5a,  are  certainly  near  the  actual  absolute  values. 
Unfortunately  it  is  not  quite  certain  how  close  they  are,  although  most 
of  the  evidence  available  indicates  that  the  difference  is  small.  This 
fact  has  led  to  the  introduction  of  another  scale  of  potential  measure- 
ment, in  which  the  zero  is  quite  arbitrarily  fixed,  and  reproducible 
with  accuracy.2  This  scale  is  the  hydrogen  scale  (potentials  written 
(5h),  the  electrode  potential  of  a  platinised-platinum  electrode,  half 
dipping  in  2n.  H2S04,  and  half  surrounded  by  an  atmosphere  of  pure 
hydrogen,  which  bubbles  at  atmospheric  pressure  through  the  sulphuric 
acid,  being  taken  as  zero-point.  This  electrode  is  termed  the  normal 
hydrogen  electrode,  the  2n.  H2S04  being  very  nearly  In.  with  respect  to 
H'  ion,  and  the  hydrogen  dissolved  in  the  platinised  platinum  behaving 
as  if  it  had  a  definite  electrolytic  solution  pressure. 

The  potential  of  the  same  electrode  on  the  absolute  scale  is 
+  0*277  volt.  This  difference,  of  course,  holds  for  all  potential  values 
expressed  on  the  two  scales.  Hence  we  have  the  relation 

&  =  &  +  °-277- 

The  advantage  of  using  the  absolute  scale  is,  of  course,  that  the  single 

1  Wilsmore,  Zeitsch.  Phys.  Chem.  35,  291  (1900). 

5  Similar  considerations  led  to  the  choice  of  O  =  16  as  the  basis  of  atomic 
weights,  instead  of  H  =  1. 


96    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY     [CHAP. 

potential  value  at  once  tells  us  whether  a  certain  electrode  process 
takes  place  with  decrease  or  increase  of  free  energy.  But,  on  the  other 
hand,  the  uncertainty  mentioned  above  has  led  to  the  more  general 
adoption  of  the  hydrogen  scale,  which  is  the  one  used  throughout  this 
book,  unless  otherwise  mentioned. 

Voltage  Series. — Table  XVII  contains  the  more  important  of 
these  electrolytic  potentials,  and  the  equations  expressing  the  electro- 
chemical processes  of  which  they  measure  the  driving  force. 

TABLE  XVII 


Electrolytic  Potential  =  potential  of  elec- 

trode minus   potential  of  solution  con- 

Electrochemical Process. 

taining  1  grain-ion  dissolved  per  litre. 

£h  in  volts. 

£a  in  volts 

Na'  >  Na  +  © 

—  2-72 

—  2-44 

Mg"  ^Mg  +  20 

-1-55 

-1-27 

Zn"  >  Zn  +  2  © 

-0-76 

-0-48 

Ye"  >  Fe  +  2  © 

-0-43 

—  0-15 

Cd"  >Cd  +  2© 

-0-40 

—  0-12 

Ni"  >  Ni  +  2  © 

-0-22 

+  0-06 

Pb"  >  Pb  +  2  © 

-0-12 

+  0-16 

Sn"  >  Sn  +  2  © 

-o-io 

-f-0-18 

H-  >  £  H2  +  © 

±0-00 

+  0-277 

Cu"  >  Cu  +  2  © 

+  0-33 

+  0-61 

}  02  +  H.,0  >  20H'  +  2  © 

+  0-41 

+  0-69 

*i2  —  *i'  +  © 

+  0-54 

+  0-82 

Hg2"  >  2Hg  +  2  © 

+  0-78 

+  1-06 

Ag*  >  Ag  +  © 

+  0-80 

+  1-08 

^Br,  *Br/  +  © 

+  1-08 

+  1-36 

\  Cl.2  >  Q'  +  © 

-f  1-36 

+  1-64 

Au'  >.Au  +  © 

+  1-5 

+  1-78 

Two  kinds  of  equilibria  are  represented  in  this  table  : — 

(a)  between  metal  and  ion, 

(6)  between  non-metal  and  ion. 

For  the  fornter  class  the  electrolytic  potential  measures  the  ten- 
dency for  ions  to  discharge  their  electricity  and  assume  the  metallic 
state.  To  get  a  measure  of  the  electrolytic  solution  pressure,  or  the 
tendency  for  metals  to  ionise,  we  must  simply  reverse  the  signs  of 
the  above  figures.  Then  we  see  that  for  easily  oxidisable  metals, 
such  as  Mg,  Zn  and  Fe,  we  have  high  positive  values,  and  for  the  noble x 
metals  silver  and  mercury  large  negative  values,  indicating  a  very 
small  ionising  tendency. 

1  A  positive  potential  is  very  often  spoken  of  as  a  noble  potential. 


viii.]  ELECTROMOTIVE  FORCE  97 

The  examples  given  of  equilibria  between  non-metals  and  ions  all 
deal  (except  oxygen)  with  the  halogens  and  the  anions  of  the  halogen 
acids.  Like  the  metals,  the  halogens  tend  to  send  into  solution  ions, 
which  are  negatively  charged,  corresponding  to  the  electronegative 
character  of  the  neutral  substance.  We  can  ascribe  to  each  halogen 
an  electrolytic  solution  pressure,  just  as  we  did  to  the  metals,  and  the 
explanation  of  the  potential  difference  at  a  halogen  electrode,1  the 
formation  of  an  electrical  double  layer,  etc.,  will  be  the  same  as  before. 
The  greater  the  ionising  tendency,  the  more  negative  the  solution 
becomes,  and  the  more  positive  the  electrode,  differing  in  this  way 
from  the  relations  at  a  metal  electrode.  As  the  electrolytic  potential 
always  measures  the  driving-force  of  that  reaction  which  tends  to 
charge  the  electrode  positively,  in  the  case  of  anions  it  will  be  equivalent 
to  the  electrolytic  solution  pressure,  and  will  give  a  measure  of  the 
tendency  to  ionise,  not  of  the  tendency  to  discharge.  The  more 
reactive  the  halogen,  i.e.  the  more  easily  it  enters  the  ionic  condition, 
the  higher  will  be  its  E.P.  Thus,  we  find  that  the  values  of  E.P.  for 
iodine,  bromine,  and  chlorine  successively  increase. 

The  above  table  sheds  light  on  certain  chemical  reactions,  more 
particularly  on  the  action  of  metals  in  displacing  hydrogen  from  acids, 
or  other  metals  from  salt  solutions.  We  can  make  the  general  state- 
ment that  a  metal  will  replace  another  of  lower  electrolytic  solution 
pressure  from  solutions  of  its  salts.2  As  the  metals  stand  in  the  table 
in  the  order  of  decreasing  electrolytic  solution  pressures,  each  metal 
should  be  able  to  displace  from  solution  all  those  below  it.  Conse- 
quently we  find  that  zinc  will  displace  lead,  iron  will  displace  copper, 
and  copper  turn  out  silver.  Further  tin  and  all  metals  above  it  should 
be  able  to  liberate  hydrogen  from  dilute  acids.  Copper,  silver,  and 
mercury,  on  the  other  hand,  should  be  precipitated  by  hydrogen. 

The  facts  are  generally  in  agreement,  and  apparent  exceptions  can 
be  readily  explained.  A  piece  of  very  smooth  pure  zinc  does  not 
dissolve  in  pure  dilute  H2S04,  very  probably  because  of  a  thin  film  of 
hydrogen,  which  is  unable  to  form  bubbles  and  so  escape.  If  the 
same  pure  zinc  be  roughened,  it  readily  dissolves.  Lead  does  not 
dissolve  in  dilute  H2S04  or  HC1  because  of  insoluble  coatings  of  the 
respective  salts  which  prevent  further  action.3  Hydrogen  gas  will  not 
precipitate  copper  from  solution,  because  it  cannot  ionise.  If,  however, 
a  piece  of  palladium  be  placed  in  the  solution,  the  hydrogen  dissolves 
in  it,  is  able  to  ionise,  and  readily  precipitates  copper. 

Effect  of  Ionic  Concentration.  —  We  have  seen  that  an  elec- 
trode potential  partly  depends  on  the  concentration  in  the  electrolyte 

i  See  p.  99. 

-  This  is  not  always  true  with  metals  of  nearly  equal  electrolytic  solution 
pressures,  as  the  relative  concentrations  of  the  different  metallic  ions  in  the  electro* 
lytc  will  then  play  a  part. 

3  Overvoltage  (p.  118)  also  plays  a  part. 

n 


98    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY     [CHAP. 

of  the  ion  concerned.  If  the  ion  is  positive,  the  potential  difference 
becomes  more  positive  with  increase  of  concentration.  If  the  ion 
is  negative,  as  Cl',  then  the  potential  becomes  more  negative 
or  less  positive  with  increase  of  concentration.  In  dealing  with  the 
quantitative  aspect  of  these  relations  we  can  attempt  no  proof 
of  the  simple  equations  finally  reached,  but  will  merely  state  the 
results.  For  a  metallic  electrode  in  a  solution  containing  the  corre- 
sponding cation  we  have 


an  equation  connecting  single  electrode  potential,  electrolytic  poten- 
tial, and  ionic  concentration.  [C]  is  the  gram-ionic  concentration  and 
n  the  valency  of  the  ion.  If  we  further  simplify  matters  by  assign- 
ing a  definite  value  to  T  —  say  290°,  which  is  room  temperature  —  we 
obtain 


From  this  simple  formula  can  be  calculated  the  single  potential  of 
an  electrode  for  different  concentrations  of  electrolyte. 

For  example,  what  will  be  the  potential  of  a  zinc  electrode  immersed  in 
n.  ZnSO,  at  17°  ?  We  have  for  this  solution  a  =  0'2.  Hence  [C]  is  0-5  x  0'2 
=  O'l.  We  can  put  E.P.  for  Zn"  ->  Zn  +  2  ©  as  —  0-765  volt.  Also  n  is  2. 
Hence 

0-058 
6  =  -  0-765  +  -y-  log  0-1 

=  _0-765-0-029 
=  —  0-794  volt. 
(The  experimentally  determined  figure  is  —  0-795  volt.) 

If  the  electrode  reaction  involves  an  anion,  instead  of  a  cation,  we 
have  a  similar  formula,  only  with  an  altered  sign.  The  electrode 
potential — i.e.  potential  of  electrode  minus  potential  of  solution — will 
be  higher  the  lower  the  ionic  concentration.  That  is,  we  have 

<S  =  E.P.- 
And  at  17°, 


n 


Thus,  for  example,  what  will  be  the  electrode  potential  at  18°  of  a  chlorine 
electrode  in  T'n  n  .  KC1  ?  We  have  n  for  T'o  n  .  KC1  =  0-85,  and  therefore 
[C]  =  0-085.  Also  E.P.  is  +  1-363  volt,  and  n  =  1.  Hence 


=  l-363  -  log  0-085 

=  1-363  —0-058  (-  1-07) 
=  1-425  volt. 


vin.]  ELECTROMOTIVE  FORCE  99 

A  general  formula  for  the  E.M.F.  of  cells  ill  which  both  electrodes 
consist  of  substances  in  equilibrium  with  their  corresponding  ions 
follows  at  once,  being  simply  given  by  the  difference  of  the  two  single 
electrode  potentials.1  The  formula  will  naturally  vary  somewhat, 
depending  on  whether  the  electrodes  give  out  anions  or  cations.  We 
will  again  take  the  Daniell  cell  as  example.  Our  formula  (at  17°)  will  be 

E  =  &  -  &  =  (E.P.K  +  —  -  log  [CJ 


-  (B.P.),  -  log  [CJ. 

nz 

The  cathode  is  the  copper  electrode,  which  is  more  positively  charged 
than  the  zinc.     We  therefore  write 

(E.P.)x  =  B.P.CU--H.CU  -  +  °'33  volt- 
(E.P.)2  =  E.P.z,.^Zn=-  0-76  volt. 

n1  =  n2  =  2. 
And 

E  =  0-33*+  0-76  +  0-029  log  ^c^ 


=  1-09  +  0-029  log 


As  we  have  already  seen,  E  increases  with  [CCu..],  but  is  lessened  by 
an  increase  in  [CZn..].  It  can  also  be  decreased  by  lowering  [CCu..]. 
If  KCN  be  added  to  the  CuS04  solution,  the  Cu"  ions  are  almost  com- 
pletely removed  and  converted  into  Cu(Cy)2'  anions,  cyanogen  gas 
being  evolved.  [CCu..]  thus  assumes  an  extraordinarily  low  value,  so 
much  so  that  E  not  only  falls  to  zero,  but  actually  becomes  negative. 
That  is  to  say,  copper  dissolves  and  zinc  ions  are  discharged. 

5.  Gas  Electrodes 

Hitherto  we  have  chiefly  dealt  with  the  electrode  reactions  between 
metal  electrodes  and  metal  ions.  There  are  other  kinds  of  electrode 
reactions  which  also  need  discussion.  Gas  electrodes  have  already 
been  mentioned.  Besides  hydrogen  and  the  halogens,  oxygen  can 
also  ionise,  the  corresponding  equation  being  |02  -j-  H20  — >  20H'. 
Nitrogen  does  not  ionise.  To  render  these  gases  electronic tively 
active,  they  are  best  bubbled  through  an  electrolyte  containing  the 
ion  concerned  in  which  dips  a  piece  of  platinised  platinum,  half 
immersed  in  the  solution.  The  gas  will  dissolve  to  some  extent  in  the 
platinum  and  can  then  ionise.  Bromine  and  iodine  ionise  just  like 
hydrogen  and  chlorine,  but  in  the  case  of  oxygen  the  matter  is  not  so 

1  Neglecting  the  small  liquid  potential  difference. 

H  2 


100    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

simple.  An  oxide  of  platinum  plays  an  intermediate  role,  the  process 
being  partly  irreversible.  Smooth  platinum  can  be  used  in  all  cases, 
but  is  far  less  effective.  The  electrolytic  solution  pressure  of  a  gas 
dissolved  in  platinum  is  greater  the  higher  the  content  of  gas  in  the 
metal,  which  in  its  turn  is  greater  the  greater  the  gas  pressure  in  the 
surrounding  space.  The  potential  of  a  gas  electrode  depends  therefore 
on  the  pressure  of  the  gas,  but,  as  moderate  variations  of  pressure  only 
produce  very  slight  effects,  we  need  not  further  consider  the  matter. 


6.  Oxidation— Reduction  Electrodes 

We  have  seen  that  all  reactions  which  can  take  place  electromotively 
are  divisible  into  two  parts — an  oxidation  process  and  a  reduction 
process,  which  take  place  electrochemically  at  anode  and  cathode 
respectively.  The  oxidation  process  is  characterised  by  an  increase  in 
the  number  of  positive  charges  associated  with  the  substance  oxidised, 
and  the  reverse  statement  holds  of  the  reduction  process.  Ag  — >  Ag' 
is  an  oxidation,  and  Zn" — >  Zn  or  -|C12 — >  Cl'  a  reduction.  In  all 
cases  so  far  considered,  these  single  electrode  reactions  involve  the 
transference  of  electricity  between  an  ion  and  an  electrically  neutral 
substance,  as  in  the  above  examples.  But  many  reactions,  when 
resolved  into  their  constituent  oxidation  and  reduction  processes,  give 
electrode  reactions  which  involve  transference  of  electricity  from  ion 
to  ion,  no  electrically  neutral  substance  taking  part.  For  example, 
let  us  consider  the  reactions 

Cu  +  Fe2(S04)3  — ->  CuS04  +  2FeS04 

or  Cu  +  2Fe'"  — >  Cu"  -f  2Fc" 

and 

2  KMnO   +  10FeS04  +  8H2S04  — »  K2S04  +  2MnS04 

+  5Fe2(S04)3  +  8H20, 
also  expressed  by 

MnO/  +  5Fe"  +  8H*  -- >Mn"  +  5Fe'"  +  4H20. 
The  former  can  be  regarded  as  composed  of  the  reactions 

Cu+2©  — *Cu" 

and  2Fe"'  — *  2Fe"  -f  2 

and  the  latter  of 

5Fe"  +  5  ©  — >  5Fe'"  ) 

and  MnO/  +  8JT *  Mn"  +  4H20  +  5  ©j ' 

In  three  out  of  these  four  electrode  processes  we  have  ions  taking 
part  on  both  sides  of  the  equation. 


VIII.] 


ELECTROMOTIVE 


Now  such  chemical  reactions  can  also  generally  proceed  electro- 
motively.     In  the  first  case,  the  following  cell  would  be  set  up  : 


Pt 


Fe2(S04)3 
FeS04 


CuS04 


Cu 


The  platinum  acts  as  a  cathode,  by  means  of  which  positive  elec- 
tricity can  leave  the  solution ;  otherwise  it  is  unchanged.  The 
copper  is  the  anode,  and  the  electrode  reactions  will  be  as  given.  At 
the  platinum  Fe2(S04)3  will  be  reduced  to  FeSOi,  whilst  the  copper 
anode  will  dissolve  to  CuS04.  The  arrangement  for  the  second  case 
would  be 


Pt 


KMn04 
MnS04 
H2S04 


Fe2(S04)£ 
FeS04 


Pt 


The  electrode  on  the  left  acts  as  cathode,  and  at  it  KMn04  is 
reduced  to  MnS04,  and  H2S04  neutralised.  At  the  other  electrode, 
FeS04  is  oxidised  to  Fe2(S04)3.  In  both  these  arrangements  the 
diminution  of  free  energy  of  the  chemical  reaction,  usually  dissipated 
as  heat,  is  utilised  as  electrical  energy. 

Such  cells  are  termed  oxidation-reduction  cells,  and  the  corresponding 
electrode  systems  oxidation-reduction  electrodes.  As  in  the  types  of 
electrode  already  discussed,  the  essential  reaction  is  an  increase  or 
decrease  in  the  amount  of  electricity  associated  with  a  substance.  We 
can  imagine  potential  differences  to  be  produced  at  the  electrodes  in 
the  same  way  as  before.  Thus,  with  nickel  in  NiS04,  the  two  opposing 
tendencies  are  the  electrolytic  solution  pressure  of  the  nickel,  tending 
to  send  Ni"  ions  into  solution,  and  the  osmotic  pressure  ol  the  Ni" 
ions  tending  to  discharge  metallic  nickel.  If  the  former  tendency 
predominates  the  electrode  becomes  charged  negatively,  and  positively 
if  the  contrary  holds.  Similarly  with  a  platinum  electrode  in  a  mixture 
of  FeS04  and  Fe2(S04)3,  the  opposing  tendencies  are  that  of  Fe'"  ions 
to  give  up  positive  charges  to  the  electrode,  and  the  tendency  of  Fe" 
ions  to  take  up  positive  electricity  from  the  electrode  and  become 
oxidised  to  ferric  ions.  If  the  driving  force  of  the  reaction 


exceeds  that  of 


Fe'"  — >  Fe"  +  © 


Fe" 


the  electrode  will  become  positively  charged.  Increase  of  the  Fe"" 
ion  concentration  will  increase  this  positive  charge,  an  increase  of 
Fe"  ions  will  diminish  it.  The  formula  expressing  the  variation  of 


102    PKINGIfLElS  fl?  APPLIED  ELECTROCHEMISTRY    [CHAP. 

potential  difference  with  ionic  concentration  is  of  the  same  type  as 
before.     At  18°  it  is 


Here  [CJ  is  the  concentration  of  the  more  positively  charged  ion, 
[C2]  that  of  the  other  ion,  and  n  is  again  the  number  of  f  aradays  involved 
in  the  electrode  reaction.  In  the  case  of  Fe'"  —  >Fe"  -f  ©,  n  is  1, 
and  we  have 


E.P.,  the  electrolytic  potential,  is  the  potential  of  the  electrode  when 
[Ci]  =  [C2]  =  1,  i.e.  when  the  second  term  on  the  right  hand  of  the 
equation  is  zero.  In  the  case  of  Fe'"  —  *  Fe"  -f-  0,  the  electro- 
lytic potential  is  -j-  0*71  volt,  indicating  that  a  ferrous-ferric  sulphate 
mixture  has  a  strong  tendency  to  give  up  positive  charges,  and  hence 
to  oxidise  other  systems. 

With  more  complicated  electrode  reactions,  involving  several  ions, 
the  formula  is  less  simple.  The  equation  expressing  the  electrode 
reaction  must  first  be  written  in  such  a  way  that 

left-hand  side  -f-  positive  electricity  —  >  right-hand  side. 
Thus,  with  the  permanganate  electrode 

Mn"  +  4H20  +  5  ©  —  +  MnO/  +  8H', 
and  not  as 

MnO/  +  8H'  —  »  Mn"  +  4H20  +50, 
or  as 

MnO/  -f  8IT  +  5  0  —  >  Mn"  +  4H20. 

If  then  the  mass  action  chemical  equilibrium  constant  of  this  reaction  * 
be  taken  as  K,  the  single  electrode  potential  at  18°  is  given  by 


E.P.  being  the  electrode  potential  in  a  solution  of  gram-ionic  con- 
centration unity  for  all  the  ions  taking  part.  In  the  present  case  n 
is  five  and 


I  <',„,] 
Thus  we  have 


The  tendency  to  give  up  positive  charges—  i.e.  to  oxidise  —  increases  with 

1  P.  14. 


vm.]  ELECTROMOTIVE  FORCE  103 

the  acid  and  permanganate  concentrations,  and  decreases  with  the 
manganous  salt  concentration.1 

A  great  advantage  of  considering  oxidising  and  reducing  agents 
(and  reactions)  in  this  way  is  the  fact  that  we  can  plainly  see  that  there 
is  no  sharp  line  of  demarcation  between  them.  The  oxidising  power 
of  a  solution  is  directly  measured  by  the  potential  difference  between 
a  platinum  electrode  placed  in  it  and  the  solution  itself.  The  more 
positive  the  electrode,  the  more  strongly  oxidising  the  solution  ;  if  the 
electrode  assumes  a  high  negative  charge,  the  solution  is  a  strong 
reducing  agent.  Alkaline  SnCl2,  or  alkaline  pyrogallol,  gives  a  negative 
charge  to  the  electrode,  and  NaCIO  and  KMn04  will  charge  it  positively. 
Between  these  extremes  are  electrolytes  giving  every  conceivable  value 
of  single  potential  difference.  Generally  speaking,  a  solution  will 
oxidise  any  other  solution  which  charges  an  indifferent  electrode  to  a  lower 
positive  value,  exactly  as  a  metal  will  displace  from  solution  other 
metals  of  lower  electrolytic  solution  pressure. 


7.  Concentration  Cells 

We  have  now  discussed  the  different  types  of  reversible  electrode 
reactions,  and  can  understand  how  any  two  electrode  systems,  provided 
they  are  at  different  potentials,  when  put  together  will  give  a  primary 
cell.  If  joined  by  an  external  wire,  positive  electricity  will  flow 
through  this  circuit  from  the  more  positively  charged  electrode  (cathode) 
to  the  less  positively  charged  one  (anode),  and  back  again  through  the 
cell,  whose  E.M.F.  will  be  given  by  the  difference  of  the  two  single 
electrode  potentials. 

One  type  of  cell,  rather  different  from  those  hitherto  considered, 
should  be  briefly  mentioned  —  the  concentration  cell.2  Consider  the 
general  equation  for  the  E.M.F.  of  an  element 


=  (B.P.),  -  (B.P.),  +  -%[C1]  -  log  [CJ 

^1  n2 

(this  form  when  both  electrodes  give  cations).     If  the  two  electrodes 

1  It  might  be  pointed  out  that  the  mechanism  of  these  apparently  complex 
processes  is  really  simple.     Thus  the  reduction  of  MnO/  to  Mn"  ions  can  be  split 
into  two  stages 

MnO/  +  4H20  -  >  Mn  .......  +  80H' 

Mn  .......  -  >  Mn"  +  5  © 

The  first  reaction  takes  place  without  ionic  transference  —  so  that  the  process 
is  essentially  the  reduction  of  hepta-valent  to  di-valent  manganese,  and  falls  into 
line  with  the  reduction  of  tri-valent  to  bi-valent  iron.  Similarly  the  oxidation  of 
Cr*"  to  CrO/  (chromate)  can  be  regarded  as  essentially  the  oxidation  of  tri-valent 
to  hexa-valent  chromium. 

-  See  also  pp.  89,  115,  163,  207. 


104    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 


(and  the  two  ions)  are  identical,  then  (E.P.)!  =  (E.P.)2  and  nt  —  nz,  and 
we  get 


that  is,  the  E.M.F.  of  the  cell  is  determined  by  the  ratio  of  the  ionic 
concentrations  at  the  two  electrodes.1  It  increases  as  this  ratio  increases, 
and  is  in  fact  a  measure  of  the  diminution  of  free  energy  occurring 
when  the  two  unequally  concentrated  solutions  are  mixed.  Except 
when  the  concentration  ratio  is  enormous  these  E.M.F.s  never  reach 
large  values. 

8.  Measurement  of  Single  Electrode  Potentials 

The  measurement  of  single  potential  differences  is  carried  out  very 
simply  with  the  apparatus  in  Fig.  19.  The  electrode  to  be  measured 
is  opposed  to  a  standard  electrode,  the  potential  of  which  is 

known,  thus  producing  a  galvanic  cell, 
and  the  E.M.F.  of  this  cell  is  measured 
exactly  as  described  on  page  90. 
Suppose  it  to  be  E.  Then  E  =  &  -  (52, 
the  difference  of  the  two  single  poten- 
tials. Knowing  the  standard  potential 
difference,  and  having  determined  E, 
the  unknown  single  potential  at  once 
follows.  Fig.  23  shows  a  convenient 
form  of  vessel  in  which  to  set  up 
electrodes,  whether  standard  or  ex- 
perimental. It  is  about  6  inches  high, 
and  provided  writh  a  siphon-shaped  side- 
tube.  The  electrode  itself  is  sealed 
into  a  glass  tube  passing  through  a 
rubber  stopper,  or  else  is  directly  in- 
serted through  the  stopper.  When 
making  up  a  cell,  the  siphon  side-tubes  of  the  two  electrode  vessels 
are  filled  by  blowing  at  A,  and  dipped  into  a  small  vessel  containing 
an  electrolyte  in  order  to  make  electrical  contact.  When  this  vessel 
contains  a  saturated  AmN03  or  a  saturated  KC1  solution,  the  potential 
difference  at  the  cell's  liquid  junction  is  practically  eliminated  if  the 
electrolytes  concerned  are  neutral  or  not  too  strongly  acid. 

The  standard  electrodes  most  commonly  used  are  the  normal  and 
deci-normal  calomel  electrodes.  Their  electrode  system  is  Hg  |  KC1 
solution  saturated  with  HgCl.  They  are  readily  made  up.  Into  a 
vessel  of  the  type  described,  half  an  inch  of  pure  mercury  is  poured. 

1  The  liquid  potential  difference  in  the  cell  is  not  considered. 


FIG.  23. — Electrode  Vessel. 


VIIL]  ELECTROMOTIVE  FORCE  105 

This  is  covered  with  a  fine  paste  made  by  shaking  up  calomel  and 
mercury  several  times  with  some  of  the  standard  KC1  afterwards 
employed,  throwing  away  the  liquid  every  time.  The  vessel  is  then 
filled  up  with  KC1  which  has  already  been  saturated  with  HgCl,  and 
the  electrode  is  at  once  ready  for  use.  The  potential  of  the  normal 
calomel  electrode  (Hg'|  HgCl  n .  KC1)  is  £h  =  +  0-283  volt  at  18°  ; 
that  of  the  deci-normal  electrode  (Hg  I'HgCl  -fo  n .  KC1),  £h  =  +  0-336 
volt  at  18°. 

For  use  with  alkaline  solutions,  the  electrodes  Hg  |  HgO  n  .  NaOH  and 
Hg  |  HgO  j-L-  n  .  NaOH  are  the  best.  The  HgO  must  be  made  by  the 
thorough  and  careful  ignition  of  pure  mercuric  nitrate.  The  same 
type  of  vessel  is  used  as  for  the  calomel  electrode.  Pure  mercury  is 
put  in  first,  a  layer  of  oxide  added,  and  the  NaOH  solution  poured  on. 
The  potentials  (constant  after  three  days)  are  for  the  normal  electrode 
<^h  =  -f-  0-114  volt,  and  for  the  deci-normal  electrode  (5h  =  +  0*169 
volt  at  18°. 

For  use  with  acid  solutions,  the  hydrogen  standard  electrode  of 
potential  difference  +  0*0  volt,1  works  very  well.  But  more  con- 
venient is  the  use  of  the  system  Hg  |  Hg2S04  n  .  H2S04.  This  mercurous 
sulphate  electrode  is  made  up  similarly  to  the  others  quoted,  and  has 
a  potential  £h  =  -f  0-689  volt  at  18°. 

If,  as  sometimes  happens,  it  is  necessary  to  measure  the  potential 
difference  at  an  electrode  in  an  electrolysis  tank  or  vessel  whilst  current 
is  passing,  the  siphon  side -tube  of  the  vessel  containing  the  standard 
electrode  is  very  much  lengthened,  bent  horizontally  at  the  end,  and 
drawn  out  to  a  fine  point,  which  is  gently  pressed  up  immediately 
against  the  electrode  in  question.2  Working  thus,  the  error  introduced 
owing  to  the  potential  fall  produced  by  the  current  is  very  low,  seldom 
exceeding  0*002  volt  in  extreme  instances. 


Literature 

Le  Blanc.     Electrochemistry. 


1  See  p.  95. 

-  Haber,  Zeitsch.  Phys.   Chem.  32,  207    (WOO).      Also    Foerster   and   Miiller, 
ZcitscJi.  Elektrochem.  9,  200  (1C03). 


CHAPTER    IX 


Co, 


r Of 

-A 
a 


ELECTROLYSIS   AND    POLARISATION-ENERGY    EFFICIENCY 
1.  Polarisation 

IN  the  last  chapter  we  discussed  the  electrochemical  processes  occurring 
in  different  types  of  primary  cells,  the  result  of  spontaneous  chemical 
reactions  accompanied  by  a  decrease  in  free  energy.  We  must  now 
consider  those  cells  in  which  the  chemical  energy  of  the  system  increases 
during  action,  owing  to  electrical  energy  led  in  from  outside.  Such 
cells  are  technically  the  most  important. 

The  relations  here  existing  are  best  understood  by  first  considering 
the  behaviour  of  the  primary  Daniell  cell  under  varying  conditions. 

We  know  that  normally  the  copper 
electrode  is  about  O3  volt  positive 
to  the  solution,  and  the  zinc  elec- 
trode O8  volt  negative  to  the  ZnS04. 
The  cell  E.M.F.  is  consequently  1-1 
volt,  and  we  express  the  potential 
Sototiori,  difference  relations  as  in  Fig.  24. 
Suppose  now  the  cell  is  short-cir- 
cuited by  connecting  the  electrodes 
A  and  C  with  a  wire.  Positive 
i  electricity  will  flow  along  the  wire 

from  A  to  C.  A  becomes  less  posi- 
tive, and  the  potential  difference 
AB  becomes  less  and  falls  below 
its  equilibrium  value.  C  becomes 

c 1  more  positively  charged,  and   the 

potential    difference    BC    becomes 
less   negative  and  changes  from  its 
FIG.  24.  equilibrium   value.      In    order    to 

restore    the     electrode    equilibria, 

copper  ions  deposit  on  A,  increasing  its  positive  charge,  and  zinc  ions 
pass  into  solution  at  C,  thus  tending  to  regenerate  the  original  poten- 
tial difference  BC.  But  the  supply  of  Cu"  ions  in  the  electrolyte  is 

100 


:• 


'_ J    electrode 


ELECTROLYSIS  AND  POLARISATION 


107 


Cn 
electrode 


SdtaUon, 


I     Zn, 

i  electrode 


only  a  limited  one,  whilst  the  Zn"  ions  accumulate ;  hence  the 
potential  differences  AB  and  BC  must  both  gradually  diminish.  The 
E.M.F.  of  the  cell  given  by  AC  will  thus  decrease.  The  process  will 
continue  until  A  and  C  are  at  the  same  potential,  when  electricity 
can  no  longer  flow  from  one  to  the  other,  and  the  cell  ceases  to  act. 

This  final  state  is  represented  in  Fig.  25.  A'B  is  the  potential 
difference  between  copper  and  copper  sulphate,  C'B  between  zinc  and 
zinc  sulphate.  As  zinc  ions  have  been  con- 
tinually dissolving,  the  zinc  sulphate  con- 
centration has  increased  until  the  electrolyte 
has  become  saturated.  As  the  Zn"  ion 
concentration  cannot  further  increase  be- 
yond this  point,  the  potential  difference 
BC'  cannot  decrease  any  further,  which  is 
the  reason  of  the  far  greater  change  of 
potential  at  the  copper  electrode.  The 
copper  sulphate  concentration  after  the 
discharge  of  the  cell  is  practically  zero. 
The  fundamental  point  to  be  noticed  is  that 
when  an  electrode  potential  becomes  less 
positive  than  its  equilibrium  value,  positive 
ions  will  be  discharged  (or  in  other  cases 
negative  ions  will  pass  into  solution). 
When  on  the  contrary  it  becomes  more 
positive  than  the  equilibrium  value,  positive 

ions  will  enter  solution  (or  negative  ions  will  discharge),  the  tendency 
being  in  every  case  to  neutralise  the  equilibrium  shift. 

To  return  to  the  Daniell  cell.  Suppose  that  instead  of  short  cir- 
cuiting, and  thus  allowing  the  cell  to  work  with  a  loss  of  free  energy, 
corresponding  to  the  chemical  reaction 

Zn  +  CuS04  — >  Cu  +  ZnSQ, 

— suppose  electrical  energy  to  be  led  into  the  cell  from  outside,  reversing 
the  above  reaction,  and  causing  an  increase  in  the  power  of  the  system  to 
perform  useful  work.  This  is  done  by  making  the  potential  of  the 
copper  electrode  more  positive  than  its  equilibrium  value  by  connecting 
it  with  some  outside  source  of  positive  current,  and  at  the  same  time 
similarly  rendering  the  potential  of  the  zinc  electrode  more  negative 
than  its  equilibrium  value.  Reactions  tending  to  restore  the  electrode 
equilibria  will  set  in.  They  will  be,  at  the  zinc  electrode  discharge  of 
positive  Zn"  ions,  at  the  copper  electrode  formation  of  positive  Cu" 
ions.  With  the  external  circuit  closed  so  that  the  applied  source  of 
current  (and  electrical  energy)  can  steadily  act,  these  processes  will 
continue  uninterruptedly.  The  copper  electrode  will  dissolve,  and 


FIG.  25. 


108    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

zinc   will  be  deposited  on  the  zinc   electrode.       The   corresponding 
chemical  reaction  is,  as  we  have  seen, 

Cu  +  ZnS04  — >  CuS04  +  Zn, 

and  the  necessary  increase  of  free  energy  is  supplied  as  electrical  energy. 

When  current  is  passed  in  this  way  into  a  cell  from  outside,  raising 
the  voltage  of  the  cell  terminals  above  its  static  value  and  increasing 
the  free  energy  of  the  system,  the  cell  is  said  to  be  polarised,  and  the 
phenomena  observed  are  those  of  polarisation.  Similarly,  when 
a  primary  cell  discharges  in  such  a  way  that  the  voltage  falls  below 
its  static  value  (the  E.M.F.)  the  cell  is  polarised.  It  is  evident  that 
the  value  of  the  reversible  electrode  potential  represents  a  true  equili- 
brium point  for  the  electrode  concerned.  If  the  potential  is  raised  above 
this  figure,  current  will  tend  to  flow  one  way  ;  if  lowered  beneath  the 
equilibrium  value,  the  tendency  will  be  in  the  other  direction.  Gene- 
rally, when  an  electrode  potential  is  more  positive  than  its  equilibrium 
value,  the  electrode  will  attract  negative  charges,  and  function  as  an 
anode  ;  when  more  negative  than  the  equilibrium  value  the  electrode  will 
behave  as  a  cathode.  Just  as  a  cell  is  said  to  be  polarised  when  electri- 
cal energy  is  impressed  into  it  from  outside,  so  an  electrode  is  said  to 
be  polarised  when  its  potential  is  altered  from  the  equilibrium  value 
by  an  external  agency  or  some  other  cause. 

From  what  has  just  been  said,  it  follows  that  an  electrode  is  anodi- 
cally  polarised  when  its  potential  is  made  more  positive  than  the  equili- 
brium value,  and  cathodically  polarised  when  made  more  negative  than 
that  figure.  We  speak  correspondingly  of  anodic  and  cathodic  polarisa- 
tion. Strictly  speaking,  the  polarisation  of  an  electrode  should  be 
measured  by  the  difference  between  the  actual  value  of  the  electrode 
potential  and  the  equilibrium  value,  but  the  term  is  often  Io9sely 
applied  to  the  actual  total  value  of  the  electrode  potential.  The  con- 
ception of  anodically  and  cathodically  polarised  electrodes  will  be 
used  continually  in  this  book,  and  it  will  be  well  to  thoroughly  grasp 
its  significance  before  proceeding  further. 


2.  Energy  Efficiency 

Decomposition  Voltage. — When  the  single  potentials  of  .the  Danicll 
cell  have  the  values  corresponding  to  a  and  c  (Fig.  24),  and  the 
electrodes  are  joined  externally  by  a  conductor,  current  will  flow 
from  copper  to  zinc  through  this  external  circuit.  If,  on  the  contrary, 
the  Aectrode  potentials  be  kept  at  a'  and  c'  and  the  external  circuit 
be  closed,  current  will  flow  from  copper  to  zinc  through  the  cell,  and 
the  chemical  reaction  ZnS04  +  Cu  -  ->  CuS04  -f-  Zn  takes  place.  The 
potential  diffe^Jice  AC  (!•!  volt)  does  not  therefore  only  represent 
the  E.M.F.  of  tli^Daniell  cell,  but  also  represents  the  minimum  voltage 


ix.]  ENERGY  EFFICIENCY  109 

which  must  be  impressed  on  the  cell  terminals  from  outside  in  order 
that  the  current  may  flow  in  such  a  way  that  electrical  is  trans- 
formed into  chemical  energy.  This  minimum  voltage  is  known  as  the 
reversible  decomposition  voltage  of  the  cell,  and  is  equal  to  the 
reversible  E.M.F.  of  the  corresponding  primary  cell. 

It  must  be  noted  that  whereas,  when  a  cell  is  furnishing  current, 
its  E.M.F.  continually  tends  to  fall,  on  the  other  hand,  when  current  is 
being  forced  through,  the  decomposition  voltage  tends  to  rise.  This 
difference  is  due  to  the  opposite  chemical  effects  which  result.  In  the 
Daniell  cell,  the  Zn"  ion  concentration  increases,  and  the  Cu"  ion 
concentration  decreases  when  giving  current,  whilst  when  current  is 
being  sent  through  the  reverse  is  true.  In  the  one  case  there  is  a 
decrease,  in  the  other  case  an  increase,  of  free  energy. 

We  now  see  what  are  the  conditions  determining  preferential 
reversible  ionic  discharge  in  a  solution  containing  several  different 
anions  or  cations.  The  various  single  potential  differences  correspond- 
ing to  several  possible  cathodic  reactions  are  (at  18°)  :  — 


etc. 


Now,  a  cathodic  discharge  will  set  in  at  an  electrode  as  soon  as  its  single 
potential  difference  has  been  reduced  below  the  corresponding  equili- 
brium value.  Hence  that  cathodic  reaction  will  set  in  first  to  which 
corresponds  the  highest  equilibrium  potential. 

Suppose,  for  example,  a  solution  to  contain  copper  and  nickel  salts,  together 
vi  ith  free  acid.  Let  the  respective  gram-ionic  concentrations  be  [Cu"J  =  O015, 
[Xi"J  =  0-23,  [H']  =  0-72.  Then  the  possible  cathodic  reactions  (excluding 
Cu"  -  >  Cu'  +  ©)  are  Cu"  -  >  Cu  +  2  ©,  Ni"  -  >  Ni  +  2  ©,  and  H'  -  >  £H._> 
+  ©  and  the  corresponding  single  potentials 

<5fu"  —  *  Cu  =   +   °'33  +  °'029  %  °'015  =   +  °'277   volt» 

6xi   —  ->  xi  =  -  °'22  +  °'029  l°9  °'23  =  —  °'239  volt> 
£H.  _  j.  iHj  =  0-0  +  0-058  log  0'72  =  —  O'OOS  volt. 

The  highest  single  potential  difference  corresponds  to  the  reaction  Cu"  -  >  Cu, 
and  (assuming  the  reactions  to  take  place  reversibly)  discharge  of  Cu"  ions  will 
be  the  first  to  occur. 

The  case  is  very  similar  with  various  possible  anodic  reactions. 
At  this  point  we  will  consider  two  only,  the  solution  of  a  metallic 
electrode  and  the  discharge  of  an  anion.  As  before,  the  equilibrium 
potential  of  the  metal  is  given  (at  18°)  by 


110    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

whilst  the  potential  for  the  equilibrium  between  the  anion  in  solution 
and  its  corresponding  neutral  substance  is 


Current  in  either  case  will  pass  as  soon  as  the  potential  of  the  electrode 
has  become  more  positive  than  the  corresponding  equilibrium  potential. 
That  reaction  therefore  will  set  in  first  to  which  corresponds  the 
lowest  (most  negative)  equilibrium  potential. 

Suppose  we  have  a  nickel  anode  in  Nal  solution  at  18°.  The  possible  anode 
reactions  are  Ni  +  2  0  -  >  Ni"  and  I'  +  ©  --  >  £L>.  In  the  one  case  nickel 
ionises  directly  ;  in  the  second  case  I'  ions  are  discharged  and  the  liberated  iodine 
acts  on  the  nickel,  forming  nickel  iodide.  Whatever  happens  NiL>  is  formed,  but 
in  the  second  case  as  the  result  of  a  secondary  reaction.  The  two  electrode 
potentials  are 

dxr—  *xi  -  -  0-22  +  0-029  log  [Cxr], 
(5ji,  —  >  i  •  =  +  0-54  —  0-058  log  [Cr]. 

If  the  Ni"  ion  concentration  at  the  start  be  [Cxi-]  =  O'OOl,  log  [C\j--]  =  —  3. 
We  can  suppose  the  concentration  of  the  I'  ions  to  be  1  n.     Then  we  get 
6Xi   __>  Ni  =  -  0-307  volt, 
(5iT,  _^  \,  =  +  0-54  volt. 

The  direct  ionisation  of  nickel  occurs  much  more  easily  than  the  discharge  of  I'  ions 
(reversibility  assumed). 

Energy  Efficiency.  —  If  an  electrolysis  could  be  carried  out 
reversibly,  and  with  a  100  per  cent,  current  efficiency,  the  amount  of 
electrical  energy  needed  to  produce  a  certain  quantity  of  substance 
electrolytically  would  be  given  by  the  product  of  the  corresponding 
quantity  of  electricity  and  the  theoretical  decomposition  voltage. 
But  in  practice  this  is  never  so.  We  have  discussed  the  causes  which 
lower  the  current  efficiency.  And,  owing  to  irreversibility  of  electrode 
processes,  and  the  voltage  necessary  to  overcome  the  resistance  of  the 
electrolyte,  the  working  voltage  of  an  electrolytic  cell  always  exceeds  the 
theoretical  decomposition  voltage.  The  theoretical  quantity  of  energy 
necessary  to  form  one  gram-equivalent  of  product  is  96540  E  joules. 
The  necessary  quantity  in  practice  is  the  product  of  the  working 
voltage  into  the  quantity  of  electricity  required.  The  percentage  ratio 

theoretical  quantity  of  energy      .  , 

«    _  is  termed  the  energy  efficiency  of 
quantity  of  energy  actually  used 

the  process,  naturally  a  more  important  magnitude  than  the 
current  efficiency,  which  deals  with  one  factor  of  electrical  energy 
only.  Its  calculation  is  very  simple. 

If,  for  example,  the  reversible  decomposition  voltage  corresponding  to  a 
certain  transformation  is  1*7  volts  and  the  working  voltage  2  '8  volts,  whilst  a  90 
per  cent,  current  efficiency  is  obtained,  the  energy  efficiency  is 

1-7       90 

8*1  x  100  X  10°  =  55  P61  cent- 


IX.] 


ENERGY   EFFICIENCY 


111 


Measurement  of  Voltage.  —  Voltage  is  measured  by  voltmeters, 
which  need  no  description  here.  .They  are  simply  high-resistance 
ammeters,  which  are  shunt  connected  across  the  points  between 
which  the  voltage  drop  is  to  be  measured.  The  current  passing 
through  is  proportional  to  the  voltage,  and  the  scale  is  graduated 
directly  in  volts.  It  is  essential  that  the  resistance  of  the  instrument 
be  high,  compared  with  the  resistance  of 
the  main  circuit.  A  single  voltmeter  can 
be  adapted  for  a  wide  range  of  measure- 
ment by  introducing  a  series  resistance  C 
(Fig.  26)  into  the  voltmeter  circuit,  and 
thus  decreasing  the  current  going  through 
the  circuit  for  a  given  difference  of  poten- 
tial between  A  and  B.  If,  for  example,  C 
has  nine  times  the  resistance  of  the  volt- 
meter, then  the  resistance  of  the  circuit  is 
ten  times  what  it  would  be  in  absence  of  C, 
the  current  is  one-tenth  as  great,  and  the  Voltage 
value  of  each  scale-division  in  volts  is 
increased  tenfold. 

The  use  of  these  ballast  resistances  ('  multipliers  ')  enables  much 
higher  voltages  to  be  measured  than  would  otherwise  be  possible  with 
the  same  instrument.1  When  the  voltmeter  is  designed  to  measure 


with 


Ballast  Resistance. 


FIG.  27. — Voltmeter  Calibration. 

very  high  voltages,  the  multipliers  are  kept  outside,  but  for  low  range 
instruments  are  placed  inside  the  voltmeter  and  put  into  circuit  by 
means  of  separate  terminals.2 

Voltmeters  are  most  simply  calibrated  by  direct  comparison  with 

1  Cf.  use  of  shunts  for  current  measurement,  p.  31. 

~  See  p.  31  for  the  measurement  of  current  with  the  aid  of  a  high-resistance 
voltmeter. 


112    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

a  standard  instrument.  The  two  instruments  are  connected  in  parallel 
with  the  same  two  points  in  the  main  circuit,  which  contains  a  source 
of  E.M.F.  of  greater  magnitude  than  the  maximum  reading  of  the 
voltmeter,  and  an  adjustable  resistance.  By  varying  this  resistance, 


FIG.  28. — Measurement  of  Decomposition  Voltage. 

the  potential  drop  between  the  points  on  which  the  voltmeters  are 
laid  is  also  altered.  Simultaneous  readings  are  taken.  The  apparatus 
is  shown  in  Fig.  27. 

Measurement  of  Decomposition  Voltage. — A  knowledge  of  the 
reversible  decomposition  voltage  is  important  for  purposes  of  calcu- 
lation of  energy  efficiency.  When,  as  in  the  Daniell  cell,  the  two 
single  potential  differences  are  known,  no  measurement  is  necessary, 
but  that  is  very  often  not  the  case.  The  direct  determination  is 
carried  out  as  follows  (Fig.  28).  A  uniform  potential  drop  is  produced 
along  a  slide- wire  AB  by  a  battery  C.  In  the  shunt  circuit  ADE 

are  placed  the  electrolytic  cell  and  a 
sensitive  milliamperemeter  or  galvano- 
meter F,  making-  connection  with  the 
slide-wire  by  the  contact  E.  A  voltmeter 
V  is  placed  across  the  terminals  of  the 
cell.  At  the  commencement  E  is  brought 
down  close  to  A.  It  is  then  gradually 
moved  up  the  wire,  thereby  increasing 
the  voltage  applied  to  the  ends  of  the 
shunt  circuit.  Readings  of  voltmeter 
and  ammeter  are  simultaneously  taken. 
At  first  only  a  very  small  current 
goes  through  the  circuit,  but  when  the 

decomposition  voltage  has  been  reached,  the  current  passing 
will  suddenly  increase,  owing  to  decomposition  commencing.  The 
voltmeter  KMdJno  at  this  point  indicates  the  decomposition  voltage. 


Voltage 

FIG.  29. — Decomposition 
Voltage  Curve. 


ix.]  ENERGY    EFFICIENCY  113 

It  should  strictly  be  corrected  by  an  amount  equal  to  the  product  of  the 
resistance  of  the  cell  and  the  small  current  passing  through.  This 
correction  is  generally  small.  The  type  of  curve  obtained  is  shown 
in  Fig.  29.  An  exactly  similar  one  is  got  by  plotting  current  against 
the  electrode  potential  of  a  single  electrode.  When  the  potential 
difference  passes  the  equilibrium  value,  the  current  will  suddenly  rise. 
Such  a  curve  is  a  current  electrode-potential  curve. 

Back  Electromotive  Force. — We  have  seen  that  the  E.M.F.  of 
a  primary  cell  equals  the  reversible  decomposition  voltage  of  the 
same  system,  and  the  second  method  used  for  determining  that  magni- 
tude depends  on  this  fact.  It  consists  in  taking  the  polarisation 
discharge  curve  or  in  observing  what  is  termed  the  back  E.M.F. 
of  the  cell.  Current  is  passed  through  the  electrolysis  vessel,  and 
certain  amounts  of  the  products  collect  on  the  electrodes.  The  main 
circuit  is  then  broken,  and  the  cell  simultaneously  short-circuited 
through  a  high-resistance  voltmeter.  It  now  behaves  as  a  primary 
cell,  and  will  furnish  current  and  give  the  corresponding  voltage  as 
long  as  traces  of  the  precipitated  products  remain  on  the  electrodes. 
The  voltmeter  reading  under  these  conditions  is  equal  to  the  decom- 
position voltage  of  the  electrolyte.  The  voltmeter  used  must  be  of 
high  resistance,  otherwise  the 
products  will  disappear  too 
quickly  to  allow  of  satisfactory 
observations. 

The  voltmeter  readings  are 
generally  plotted  on  a  curve 
against  time.  A  horizontal 


portion,  indicating  constancy  of  Time 

voltage,  corresponds  to  the  back  ^G-  30.— Polarisation  Discharge  Curve. 
E.M.F.  and  to  the  decomposi- 
tion voltage  of  the  electrolyte.  Such  a  curve  is  shown  in  Fig.  30. 
The  experiment  is  most  conveniently  made  by  putting  the  voltmeter 
in  shunt  across  the  terminals  of  the  cell  whilst  the  main  current  is 
passing.  When  the  main  circuit  is  opened,  voltmeter  and  cell  are 
then  short-circuited. 

3.  Factors  affecting  Electrolysis 

So  far  we  have  only  considered  the  polarisation  of  an  electrolytic 
cell  as  due  to  the  reversible  decomposition  voltage  of  the  process 
taking  place.  But  there  are  other  effects  which  help  to  determine  the 
working  voltage.  A  certain  amount  of  energy  is  lost  in  overcoming 
the  electrolyte  resistance.  The  current  always  causes  concentration 
changes,  due  partly  to  the  different  rates  of  migration  of  the  different 
ions,  and  partly  to  chemical  changes  at  the  electrodes.  And  these 


114    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

concentration  changes  always  tend  to  act  as  a  concentration  cell,  oppos- 
ing the  main  impressed  voltage  and  causing  concentration  polarisa- 
tion. Still  more  important  is  the  fact  that  the  voltage  required  for 
passing  current  through  an  electrolyte  is  generally  augmented  by  the 
incomplete  reversibility  of  the  electrode  processes. 

Reaction  Velocity  Effects. — This  irreversibility  is  usually  due 
to  insufficient  reaction  velocity  of  some  part  of  the  electrode  pro- 
cess. Suppose,  for  example,  the  process  is  the  discharge  of  an  ion 
which  deposits  its  material  part  on  the  electrode.  There  are  two 
successive  stages  involved.  The  ions  give  up  their  charge,  liberating 
the  neutral  residues.  Then  the  equilibrium  in  the  electrolyte,  disturbed 
by  the  ionic  discharge,  is  adjusted  through  diffusion  to  the  electrode  of 
fresh  ions  or  by  dissociation  of  neutral  molecules  or  complex  ions. 
All  our  evidence  shows  that  the  ionic  discharge  takes  place  very  quickly 
indeed. 

But,  on  the  other  hand,  the  subsequent  diffusion  or  dissociation  may 
be  far  less  rapid,  and  ions  will  be  discharged  from  the  electrolyte  more 
quickly  than  they  can  be  regenerated  in  their  equilibrium  concentra- 
tion. Let  the  process  be  the  discharge  of  a  cation.  If  by  reason  of  an 
insufficient  reaction  velocity  the  concentration  of  the  ion  at  the  electrode 
keeps  permanently  below  its  static  equilibrium  value  (the  concentration 
it  would  have  if  no  current  were  passing),  then  the  working  value  of  £ 
will  fall,  and  consequently  a  greater  cathodic  polarisation  will  be  necessary 
to  discharge  ions  at  a  given  rate  (to  work,  that  is,  with  a  given  current 
density)  than  if  the  velocity  of  resupply  of  the  ions  were  as  great  as 
their  velocity  of  discharge.  In  other  cases  an  accumulation  of  pre- 
cipitated product  at  the  electrode  (for  example,  a  gas)  may,  so  to  speak, 
*  clog '  the  working  of  the  electrode  process.  The  mechanism  of  such 
effects  we  shall  consider  in  Chapter  X.  Electrodes  at  which  these 
irreversible  processes  commence  at  low  current  densities  are  said  to  be 
easily  polarisable.  Oxidation-reduction  electrodes  particularly  often 
behave  thus.  We  shall  see  later  how  sometimes  the  nature  of  the 
electrode  process  can  be  actually  changed  owing  to  some  irreversible 
effect  setting  in. 

The  nature  of  the  reaction  resistance  present  can  often  be  ascertained 
by  marking  the  effect  of  stirring  the  electrolyte  or  of  a  temperature  rise. 
When  the  slow  reaction  is  of  the  nature  of  a  diffusion,  stirring  will  tend 
to  decrease  the  necessary  polarisation ;  if  a  chemical  reaction  or  a 
dissociation,  an  increase  of  temperature  may  be  expected  to  produce 
that  effect.  Increased  temperature  should  also  act  favourably,  only 
less  so,  if  it  is  a  diffusion  phenomenon.  This  effect  of  temperature  is 
of  course  precisely  the  same  as  its  effect  on  reaction  resistances  met 
with  in  purely  chemical  reactions,  such  as  the  combination  of  hydrogen 
and  oxygen  to  water  at  low  temperatures.  Polarisation  must  naturally 
always  be  avoided  as  far  as  possible.  In  a  primary  cell,  it  lowers  the 


ix.]  ENERGY  EFFICIENCY  115 

E.M.F.  that  the  cell  would  otherwise  give.     In  electrolysis,  it  increases 
the  voltage  required. 

Depolarisation. — Any  agent  through  which  the  polarisation  in  a 
cell  can  be  lessened  is  termed  a  depolariser.  A  depolariser  can  act 
in  two  ways.  It  can  catalyse  the  slow  reaction  which  is  causing 
irreversibility,  in  which  case  the  E.M.F.  or  voltage  will  more  nearly 
approach  the  theoretical  reversible  value.  Thus,  when  hydrogen  is 
liberated  electrochemically  at  a  metal  surface,  the  cathodic  polarisation 
necessary  is,  in  many  cases,  very  considerable.  But  with  a  platinised 
platinum  cathode,  the  electrode  potential  only  very  slightly  differs 
from  the  reversible  value.  Similarly  a  low  concentration  of  Cl'  ion  will 
catalyse  the  anodic  solution  of  nickel  in  a  nickel  sulphate  solution. 
With  no  chloride  present,  the  nickel  will  practically  cease  dissolving 
when  a  certain  low  current  density  is  exceeded. 

Then  a  depolariser  can  also  act  by  reducing  the  energy  consumption 
at  the  electrode  below  the  amount  corresponding  to  the  equilibrium 
electrode  potential.  This  simply  means  altering  the  electrode  process, 
and  substituting  for  it  one  that  can  take  .place  more  easily.  For 
example,  in  the  Grove  or  Bunsen  primary  cell,  the  cathode  system  is  a 
platinum  or  carbon  electrode  surrounded  by  strong  nitric  acid.  In 
the  absence  of  the  strong  acid,  the  cathode  reaction  consists  in  the 
discharge  of  H'  ions  (at  the  anode  zinc  is  dissolved)  and  the  E.M.F.  of 
the  cell  is  about  O7  volt.  With  the  strong  acid  present  the  cathode 
reaction  is  depolarised.  The  H*  ions  are  no  longer  discharged,  but  the 
nitric  acid  is  reduced  to  nitrous  gases,  and  water  is  formed.  The 
E.M.F.  of  this  depolarised  cell  is  about  1-0  volt. 

Concentration  Polarisation. — We  have  seen  that,  when  a  current 
passes  through  an  electrolyte,  it  causes    concentration  changes   at 
the  electrodes,  and  that  these  changes  always  act  in  such  a  way  as  to 
oppose  the  E.M.F.  driving  current  through  the  cell.     Let  us  consider 
a  simple  case,  two  similar  copper  electrodes  in 
a  copper  sulphate  solution.     Provided  that  no     Anode 
concentration    differences     are    produced     the  Cathode 

single  potentials  of  the  two  electrodes  should 
be  equal ;  in  which  case  no  polarisation  would 
occur,  and  the  voltage  necessary  to  pass  a  given 
current  through  the  cell  would  be  simply  that 
required  to  overcome  the  ohmic  resistance  of 
the  CuS04  solution.  But,  as  a  matter  of  fact, 
copper  is  dissolved  from  the  anode  and  deposited 
at  the  cathode,  and  the  CuS04  concentrations  in  FIG.  31. 

anolyte  and  catholyte  respectively  increase  and 

diminish.  This  causes  a  raising  of  the  anode  potential  and  a  lower- 
ing of  the  cathode  potential  as  shown  in  Fig.  31,  and  the  voltage 
absorbed  by  the  cell  is  increased  by  the  polarisation  represented  by  the 
difference  in  height  between  the  dotted  lines. 

T  2 


116    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY 

If  the  contents  of  the  cell  are  thoroughly  stirred  or  the  concentra- 
tions in  some  other  way  equalised,  this  polarisation  will  more  or  less 
disappear,  the  more  completely  the  more  effective  the  mixing.  On  the 
contrary,  if  the  current  density  be  increased,  the  rate  of  stirring  being 
maintained  constant,  the  concentration  differences  between  the  layers 
immediately  surrounding  the  electrodes  and  the  great  bulk  of  the  electro- 
lyte will  become  greater,  and  the  potential  differences  and  the  polarisa- 
tion of  the  cell  will  accordingly  increase.  We  learn  therefore  that 
concentration  polarisation  increases  with  the  current  density,  but  is 
decreased  by  stirring  or  by  circulating  the  electrolyte.  In  the  case 
discussed,  which  is  met  with  technically  in  copper  refining,  the  concen- 
tration polarisation  is  reduced  to  very  small  amounts  by  an  efficient 
circulation  of  the  liquors. 


Literature 

Foerster.     Elektrocliemie  wdsseriger  LosunyeM. 
Lorenz.     EleJctrochemisches  Praktikum. 


CHAPTER   X 

CATHODIC  AND  ANODIC  PROCESSES   IN  DETAIL 
A.  CATHODIC  PROCESSES 

FOR  our  purpose,  we  can  divide  cathodic  processes  into  four  classes  : 
(a)  hydrogen  evolution, 
(6)  metal  deposition, 
(c)  formation  of  anions  (unimportant), 
((/)  electrolytic  reduction  processes. 

1.  Evolution  of  Hydrogen 

The  equilibrium  potential  of  a  hydrogen  electrode  in  a  solution 
containing  H'  ions  is  expressed  by  the  formula 

(5  ==  E.P.H--MH,  +  0-0002  T  log  [CH.] 

We  have  taken  as  our  zero  of  potential  difference  the  single  potential  of 
the  electrode  H2  |  N/iH',  which  means  that  we  write  E.P.H._>JH>=  O'O 
volt.  Then  the  cathodic  potential  at  which  reversible  H'  ion  discharge 
to  hydrogen  at  atmospheric  pressure  will  commence  is  given  by  the 
formula  0*0002  T  log  [CH.].  The  greater  [C],  the  sooner  hydrogen  evolu- 
tion will  set  in.  For  an  aqueous  solution,  in  which  the  water  dissociates 
giving  H'  and  OH'  ions  according  to  the  equation  H20  ^±  H*  +  OH', 
the  concentrations  of  these  ions  are  connected  by  the  mass  action 
equation 

K  .  [CHs0]  =  [CH.]  .  [Con-] 
which  can  be  written 

K»  =  [CH-]  •  [Co*] 

since  [Cifa0]  may  be  regarded  as  a  constant.  K^,  the  dissociation 
constant  of  water,  has  a  value  at  18°  of  0'56  X  10  ~  u.  In  a  neutral 
solution, 


[CH-]  =  [Con']  =  \Kw  =  0-8  X  10~7 
117 


118    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

Substituting  this  value  in  the  expression  for  the  potential  difference 
of  the  hydrogen  electrode,  we  get 

(5  =  0-058  log  0-8  X  10  ~ 7  =  —  0412  volt. 

This  gives  the  potential  at  which  reversible  hydrogen  evolution  will 
commence  in  a  neutral  solution  at  18°.  If  the  solution  is  normal 
with  respect  to  OH7  ions,  [CH.]  =  0-56  X  10  ~ u  and  we  calculate 
(5  =  —  0-827  volt.  So  much  for  the  reversible  discharge  of  H'  ions. 

Overvoltage. — But,  as  has  already  been  mentioned,  when  hydrogen 
is  electrolytic-ally  produced,  a  cathodic  polarisation  beyond  the  calcu- 
lated reversible  figure  is  almost  invariably  necessary,  its  amount 
depending  on  the  nature  of  the  electrode  material.  Table  XVIII 
contains  a  number  of  these  overvoltages — the  overvoltage  being  the 
difference  between  the  required  cathodic  potential  and  the  equilibrium 
value.  In  column  I  are  figures  obtained  by  Caspari1  for  different 
electrodes  when  the  formation  of  hydrogen  gas  bubbles  could  be  first 
observed.  Column  II  contains  measurements  of  Coehn  and  Dannen- 
berg,2  who  determined  the  cathode-potential  current  curve  of  the 
electrolyte  by  the  method  on  page  112.  Except  with  iron,  N/,  H2S04 
was  used  in  all  cases. 

TABLE  XVIII 

Cathode                             I.  II. 

Platinised  Platinum  0 '005  volt  O'OOOvolt 

Iron  (in  NaOH)  0'08  0'03 

Smooth  Platinum  0'09 

Silver  0-15  0'07 

Nickel  0-21  0-14 

Copper  0-23  0-19 

Tin  0-53  — 

Lead  0'64  0'36 

Zinc  0-70 

Mercury  0'78  0'44 

Though  the  two  figures  for  the  same  cathode  material  may  differ  con- 
siderably, the  order  is  the  same  in  both  columns.  Caspari's  values 
are  always  higher  than  those  of  Coehn  and  Dannenberg.  This  is  due 
to  the  fact  that  the  overvoltage  rises  very  quickly  with  increase  of 
current  density,  and  that  while  the  numbers  in  column  II  were  obtained 
with  a  very  low  current  density,  at  a  stage  prior  to  visible  gas  evolution, 
this  is  not  so  with  the  figures  in  column  I.  We  have  already  encountered 
this  rapid  rise  in  the  magnitude  of  irreversible  effects,  caused  by 
increased  velocity  of  a  process. 

Of  course,  a  greater  current  density  means  a  greater  concentration 
polarisation,  but  the  effects  noticed  here  far  exceed  any  due  to 

1   /»//.«•//.  Phya.  Chem.  30,  89  (1899). 
-   //,/,/.  38,  liO'J  (1901). 


x.]  CATHODIC  PROCESSES  119 

tration  changes.  A  comparison  of  the  following  figures  with  those 
in  the  table  will  show  this.  They  were  obtained1  in  2n.  H2S04  at  a 
cathodic  current  density  of  Ol  amp./cm2,  and  at  12°.  (Iron  and 
smooth  platinum a  in  NaCl  +  N/100  NaOH  at  22°.) 

Hg  Pb  Pb        Sn        Cu        Ni        Pt 

(polished)  (rough)  (smooth) 

1-30  1-30          1-23       1-15     0-79      O74      0-65 

Fe  Pt 

(platinised) 
0-55          0-07 

Graphite  behaves  very  much  like  nickel,3  as  does  brass  also.4  The 
following  figures  for  Hg  show  the  continuous  effect  of  increasing  current 
density.5 

Current  density  Overvoltage. 

0-0004  amp./cm.-  1'04  volts 

0-001  1-08 

0-002  1-12 

0-01  1-19 

0-02  1-22 

0-04  1-25 

0-1  1-30 

It  is  only  with  platinised  platinum,  at  which  hydrogen  overvoltage 
scarcely  comes  into  play,  that  the  difference  between  the  above  figures 
and  those  in  Table  XVIII  is  of  the  order  corresponding  to  an  increase  in 
concentration  polarisation.  With  the  other  metals  it  is  far  greater. 

Overvoltage  does  not  usually  assume  its  full  value  immediately 
after  current  begins  passing.  It  may  easily  take  hours  before  the 
maximum  effect  is  reached.  There  are  some  exceptions — thus  mercury 
and  lead,  two  of  the  metals  with  the  greatest  hydrogen  overvoltage, 
exert  their  full  influence  almost  instantaneously.  The  overvoltage  given 
by  a  metal  depends  much  on  the  nature  of  its  surface.  The  rougher 
the  surface,  the  less  will  it  be.  The  difference  between  platinised  and 
polished  platinum  is  a  good  example,  whilst  0*28  volt  more  is  required 
at  smooth  than  at  spongy  lead.6  This  is  doubtless  largely  an  effect 
of  current  density,  which  is  much  less  (under  otherwise  equal  conditions) 
at  a  roughened  or  spongy  surface.  But  the  phenomena,  including 
the  variation  of  overvoltage  with  time,  are  often  very  complex, 
as  the  results  of  Nobis 7  on  graphite  and  carbon  cathodes  show. 

1  Tafel.     Zeitsch.  Phys.  Chem.  50,  641  (1905). 
-  Sacerdoti.     Zeitsch.  Elektrochem.  17,  473  (1911). 

3  Nobis.     Dissertation  (Dresden,   1909). 

4  Mott.     Trans.  Amer.  Electrochem.  Soc.  15,  569  (1909). 
3  Tafel.     Loc.  cit. 

6  Strasser  and  Gahl.     Zeitsch.  Elektrochem.  7,  11  (1900). 
~  Loc.  cit. 


120    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

A  temperature  increase,  as  with  other  irreversible  effects,  diminishes 
the  overvoltage.  Thus  in  alkaline  NaCl  solutions  at  0*5  amp. /cm.2, 
Sacerdoti1  found  the  overvoltage  at  a  smooth  platinum  anode 
to  be  1-4  volts  at  22°  and  0'95  volts  at  99°.  The  possible  causes  of 
overvoltage  are  discussed  on  p.  133. 

As  we  shall  see,  overvoltage  can  be  technically  very  important. 
It  does  not  merely  mean  an  increased  energy  expenditure  in  the 
separation  of  hydrogen  (or  other  gases,  overvoltage  not  being  peculiar 
to  hydrogen),  but,  owing  to  the  changes  in  the  electrode  potential 
which  it  causes,  may  actually  alter  the  nature  of  the  electrode  process. 

2.  Cathodic  Metal  Deposition 

Irreversible  Effects.  —  The  reversible  process  and  concentra- 
tion polarisation  effects  have  already  been  discussed.  Irreversible 
effects  analogous  to  overvoltage  do  not  often  occur,  but  Foerster  and 
his  pupils  have  investigated  a  few  cases.  If  copper  or  zinc  be  deposited 
from  solutions  of  their  potassium  double  cyanides,  the  cathodic  polarisa- 
tions needed  increase  very  rapidly  with  increase  in  current  density,  far 
more  rapidly  than  when  using  the  simple  salts.  The  accompanying 
Table  XIX  well  shows  this.2 

TABLE   XIX 


Cathode  potential  in  volts 
Current  density 


N.CuS04 


2  N.ZnS04|N/ioK2ZnCy4 

0-0  (equilibrium  potential)  +0-302        —0-610        —0-795         -1-033 

0-001  amp. /cm.2  +0-273        —0-77         -0-829         -1-12 

0-003  amp. /cm.2  ,+0-262       —1-12         —0-838         -1-25 


With  the  simple  salts  the  difference  in  polarisation  between  the 
first  and  third  lines  only  amounts  to  0'04  —  0*05  volt ;  with  the 
complex  copper  cyanide  it  is  O51  volt,  and  with  the  complex  zinc 
cyanide  O22  volt.  The  considerable  difference  is  due  to  the  fact 
that  in  the  cyanide  solutions,  the  free  metal  ion  concentration — i.e. 
Cu*  or  Zn"  ions — is  very  low.  Thus  practically  all  the  copper  is 
present  as  Cu(Cy)2'.  When  current  passes,  the  store  of  free  Cu'  ions 
is  immediately  exhausted,  and  must  be  regenerated  by  dissociation 
of  the  complex  as  follows  : 

Cu(Cy)2' — >2  Cy'  +  Cu*. 
If  this  dissociation  cannot  keep  pace  with  the  removal  of  the  Cu* 

1  Loc.  cit. 

2  Spitzer,  Zeitsch.  Elektrochem.  11,  :U5  (ino:,)  ;    Coffetti    and    Foerster,  Ber. 
83,  2934 


x.]  CATHODIC  PKOCESSES  121 

ions  from  the  electrolyte,  the  Cu'  concentration  will  fall,  and  the 
electrode  potential  become  more  negative.  That  this  happens  is 
evidently  the  case. 

With  certain  simple  salts,  notably  those  of  nickel  and  iron,  similar 
retarding  influences  act.  The  following  Table  XX x  contains  the 
cathodic  potentials  for  the  precipitation  of  iron  and  copper  from  slightly 
acid  FeS04  and  CuS04  solutions  at  20°. 

TABLE  XX 

Current  density  CuSO4  FeSO4 

0-00  x  10 ~4  amp.  /cm.-  +  0-303  volt  —  0-465  volt 

0-57  —  0-573 

1-13  +0-292  —0-594 

2-27  +  0-290  —  0-606 

4-5  +  0-289  —  0-616 

11-3  +0-287  -0-630 

22-7  +  0-281  —  0-644 

The  difference  between  the  behaviour  of  the  two  salts  is  very 
marked.  Nickel  salts  behave  similarly  to  iron  salts,2  and  Foerster 
believes  the  explanation  to  lie  in  the  slow  rates  at  which  the  reactions 

Fe" >  Fe  -f  2  ©  and  Ni" >  Ni  +  2  ©    proceed.      In    order   to 

increase  these  velocities,  the  respective  cathodic  polarisations  must 
be  increased.  The  fact  that  this  abnormal  behaviour  becomes  less 
marked  at  higher  temperatures  strongly  supports  some  such  reaction 
resistance  explanation. 

Depolarisation  of  Metal  Deposition. — This  takes  place  when 
the  metal  precipitates  in  the  form  of  an  alloy.  Its  electrolytic  solution 
pressure  will  then  be  less  than  in  the  pure  state,  and  the  cathodic 
potential  drop  necessary  for  ionic  discharge  will  be  lessened.  The 
extent  to  which  this  depolarisation  occurs  will  depend  essentially  on 
the  type  of  the  alloy.  If  the  two  metals  form  a  solid  solution  in  all 
proportions,  the  electrolytic  solution  pressure  of  the  metal  concerned 
can  be  taken  as  very  roughly  proportional  to  its  concentration  in  the 
alloy.  If  this  contains  a  high  percentage  of  the  second  metal,  the 
electrolytic  solution  pressure  of  the  first  metal  will  be  correspondingly 
lowered,  and  vice  versa.  If  the  two  metals  only  mix  to  a  limited  degree 
the  depolarisation  effected  by  the  second  metal  will  in  all  probability  be 
small,  as  the  saturation  limit  will  soon  be  reached  and  the  great  bulk 
of  the  first  metal  will  have  to  overcome  its  full  electrolytic  solution 
pressure  during  deposition.  If  the  two  metals  form  a  compound 
which  dissolves  in  excess  of  the  second,  we  have  the  most  favourable 
conditions.  The  electrolytic  solution  pressure  of  the  first  metal  is 
very  much  lowered,  and  consequently  also  the  potential  drop  required 
for  its  deposition. 

1  Foerster  and  Mustad,  Abhand.  Bunnen  Ges.  2,  44  (1909). 
-  Schweitzer,  Zeitsch.  Elektrochem.  15,  602  (1909). 


122    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAI>. 

The  second  metal  which  does  the  depolarising  can  cither  serve  as 
cathode,  or  else  can  be  deposited  simultaneously  with  the  first  during 
electrolysis.  A  good  example  of  the  first-mentioned  method  is  the 
electrochemical  formation  of  alkali-metal  amalgams.  When  a  sodium 
chloride  solution  is  electrolysed,  using  a  mercury  cathode,  Na'  ions 
are  discharged,  the  liberated  metal  dissolving  in  the  mercury.  This 
happens  in  spite  of  the  exceedingly  high  electrolytic  solution  pressures 
of  the  alkali  metals  (that  of  sodium  is  £h  =  —  2 '72  volts).  The  result 
is  due  to  two  factors. 

The  first  is  the  high  overvoltage  of  hydrogen  at  a  mercury  cathode, 
rendering  the  discharge  of  H*  ions  more  difficult.  The  second  is 
the  fact  that  sodium  can  combine  with  mercury,  giving  such  com- 
pounds as  Hg6Na,  etc.,  and  that  the  electrolytic  solution  pressure  of  a 
solution  of  sodium  in  mercury  is  consequently  far  below  that  of  sodium 
itself.  This  case  is  technically  very  important,  and  we  shall  meet  it 
again  later.1  The  results  of  Haber  and  Sack 2  show  that  direct  precipita- 
tion of  the  Na"  ion  can  also  take  place  under  certain  conditions  when 
using  cathodes  of  other  materials,  e.g.  lead,  tin,  platinum.  The  use 
of  an  alkaline  solution  with  its  very  low  H'  ion  concentration,  and  a 
high  current  density  (with  its  high  overvoltage)  are  favourable. 

The  depolarisation  of  the  discharge  of  a  metal  by  another  simul- 
taneously deposited  can  be  illustrated  by  the  electrolysis  of  an  aqueous 
solution  of  a  mixture  of  zinc  and  nickel  salts.3  Although  zinc  is  more 
strongly  electropositive  than  nickel  and  should  therefore  not  deposit 
as  easily,  and  although  the  nickel  salt  may  be  present  in  excess,  the 
cathode  deposit  will  nevertheless  principally  consist  of  zinc,  so  strongly 
has  the  nickel  depolarised  the  discharge  of  the  zinc  ions.  The  slow  rate 

of  the  process  Ni" >  Ni  -f  2  0  (previously  mentioned)  cannot  here 

alone  explain  the  facts.  Similar  causes  act  and  similar  results  are 
obtained  in  the  electrolysis  of  mixtures  of  zinc  and  iron  salts.  The 
first  fractions  of  the  cathodic  deposits  contain  a  large  excess  of  zinc. 

Metals  giving  more  than  one  Cation. — We  must  briefly  consider 
the  potential  relations  existing  during  the  cathodic  deposition  of 
a  metal  which  can  furnish  more  than  one  kind  of  ion.  Common 
examples  are  tin  (Sn""  and  Sn"),  iron  (Fe'"  and  Fe"),  and  copper 
(Cu"  and  Cu').  If  such  a  metal  be  put  in  a  solution  of  one  of  its  salts, 
chemical  action  will  take  place  to  a  greater  or  less  extent  until  equili- 
brium has  been  set  up  between  the  metal  and  the  two  kinds  of  ions. 
Thus,  if  iron  be  placed  in  a  solution  of  FeCl3,  the  FeCl3  will  be  almost 
entirely  reduced  to  FeCl2,  and  the  equilibrium  arrived  at  will  correspond 
to  the  equation 

2Fe"'  +  Fe  ^  3Fe". 

»  P.  347.  2  Zeitach.  Eleklrochem.  8,  24f,  (1902). 

:t  Schoch  and  Hirsch,  Jour.  Amer.  Chem.  Soc,  29,  31 4  ( HHff). 


x.]  CATHODIC  PROCESSES  123 

With  copper  the  equilibrium  lies  over  in  favour  of  the  Cu"  ions,  with 
iron  and  tin  in  favour  of  the  ions  of  lower  valency.  One  of  the  ions 
may  be  to  a  great  extent  under  the  prevailing  conditions  converted  into 
a  complex  ion.  In  that  case  the  equilibrium  moves  over  very  much  in 
favour  of  that  valency.  This  happens  with  the  copper  system  when 
the  solution  contains  many  Cl'  ions.  The  tendency  of  the  Cu'  ion  to  form 
complex  anions  such  as  CuCl3"  is  much  greater  than  the  corresponding 
tendency  of  the  Cu"  ions.  When  therefore  a  solution  of  CuCl2  in  HCl 
is  boiled  with  metallic  copper,  it  is  almost  completely  converted  into 
the  cuprous  state.  But  very  few  Cu*  ions  are  present ;  the  equilibrium 

Cu"  +  Cu  ^±  2Cu- 

is  still  satisfied,  the  cuprous  copper  being  present  as  HaCuCla. 

When  such  a  metal  is  deposited  from  solution,  the  tendency  is 
always  for  the  corresponding  equilibrium  to  set  itself  up,  and  for  the 
two  different  ions  to  deposit  in  such  proportions  that  the  concentra- 
tions left  in  the  solution  always  correspond  to  equilibrium  conditions. 
From  CuS04,  the  metal  practically  deposits  as  a  di-valent  metal,  the 
Cu'  ion  concentration  in  the  equilibrium  solution  being  very  low,  and 

the  ratio  ^— -  deposited  consequently  high.     From  iron  sulphate  the 
Cu 

metal  behaves  as  if  only  Fe"  ions  were  deposited.  But  if  copper 
be  precipitated  from  a  strong  chloride  solution,  it  behaves  nearly 
quantitatively  as  if  its  ions  were  mono-valent.  The  lower  the  Cl'  ion 
concentration,  and  hence  the  smaller  the  proportion  of  copper  present 
as  complex,  the  greater  grows  its  apparent  electro-valency,  the  larger 
becomes  the  proportion  of  Cu"  ions  deposited.  With  gold  from  gold 
chloride  solutions  also,  the  metal  behaves  as  if  its  valency  were  inter- 
mediate between  one  and  three.  We  shall  meet  several  of  these  cases 
later.  The  above  considerations  hold  for  equilibrium  conditions,  and 
therefore  strictly  for  very  low  current  densities  only.  With  higher 
current  densities  one  may  get  rather  different  results,  if  the  ionic 
discharge  proceeds  more  quickly  than  the  chemical  equilibrium  can 
adjust  itself. 

The  conditions  for  reversible  cathodic  precipitation  of  metals  and 
hydrogen  have  been  discussed  in  Chap.  VIII,  and  it  has  been  shown 
how  to  calculate  the  electrode  reaction  which  sets  in  most  easily  on 
cathodic  polarisation  of  a  given  solution.  It  is  now  seen  that,  in 
practice,  a  number  of  irreversible  effects  must  also  be  considered 
before  it  can  be  stated  whether  the  deposition  of  hydrogen  or  of  such 
and  such  a  metal  will  occur  most  easily.  The  overvoltage  of  the 
hydrogen  evolution,  the  excess  polarisation  needed  for  the  precipitation 
of  certain  metals,  the  dependence  of  both  these  factors  on  current 
density,  any  possible  depolarisation  of  the  discharge  of  an  ion,  must 
all  be  allowed  for.  Thus,  though  zinc  is  considerably  more  electro- 


124    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

positive  than  iron,  yet  it  can  be  cathodically  precipitated  from  a  solution 
containing  much  H2S04  where  iron,  under  otherwise  similar  con- 
ditions, does  not  deposit.  This  is  due  to  the  higher  hydrogen  over- 
voltage  at  a  zinc  surface,  rendering  H'  discharge  more  difficult,  and 
to  the  resistances  opposing  the  deposition  of  Fe"  ions.  As  these 
resistances  to  metal  deposition  only  seldom  occur,  whereas  hydrogen 
overvoltage  invariably  increases  rapidly  with  increase  of  current 
density,  it  follows  that  high  current  densities  favour  the  deposition 
of  electropositive  metals  in  presence  of  acid. 

It  is  generally  best  to  keep  electrolytes  from  which  metal  is  being 
deposited  at  least  faintly  acid  ;  otherwise  basic  salts  or  hydroxides 
are  liable  to  be  precipitated  in  the  solution.  A  high  metal  ion  concen- 
tration is  also  of  course  advantageous.  With  very  strongly  electro- 
positive metals,  such  as  magnesium,  even  when  using  very  concentrated 
faintly  acid  solutions,  hydrogen  is  preferentially  evolved,  and  precipi- 
tates of  basic  salts  or  hydroxides  are  produced  near  the  cathode.  With 
the  alkali  metals  the  same  holds  good,  but  the  basic  product  is  not 
insoluble.  To  get  alkali  metals,  and  not  hydrogen,  cathodically 
deposited  from  an  aqueous  solution,  the  conditions  must  be  very 
favourable — powerful  depolarisation  of  the  metal  precipitation  and 
a  big  overvoltage  for  the  H'  discharge.1 

Physical  Condition  of  Cathodic  Deposits.  —  The  physical 
condition  of  a  deposited  metal  is  of  prime  importance.  The  extractor 
or  refiner  wants  it  in  coherent  form,  the  plater  must  have  it  so  that 
it  can  be  burnished.  In  practice  different  metals  can  be  deposited  in 
very  different  conditions  of  aggregation,  and  at  present  one  cannot 
satisfactorily  co-ordinate  all  the  differences  observed  with  the  con- 
ditions of  electrolysis.  Iron  and  nickel  give  very  regular  finely  grained 
deposits  from  moderately  strong  sulphate  solutions  ;  copper  gives  a 
coherent  product  of  a  markedly  crystalline  character.2  Other  metals— 
e.g.  lead  and  tin — are  deposited  (from  Pb(N03)2  or  SnCl2)  as  loose 
crystals,  and  platinum  forms  a  loose  amorphous  powder.  Many  other 
metals  —  e.g.  copper — can  give  such  an  amorphous  precipitate, 
particularly  from  very  dilute  solutions. 

The  effect  of  concentration  changes  indeed  is  very  marked  and 
regular.  The  more  concentrated  the  solution,  the  better  and  more 
coherent  the  deposit.  The  more  dilute  the  solution,  the  smaller  the 
crystals  and  the  looser  the  deposit  formed.  Now,  a  high  current 
density  causes  a  low  ionuTconcentration  in  the  immediate  neighbour- 
hood of  the  electrode  and  therefore  should  give  a  worse  deposit,  and 
this  conclusion  is  amply  confirmed  in  practice.  To  get  good  cathodic 
deposits,  the  cathodic  current  density  must  generally  in  technical  work 

1  Pp.  318,  347. 

F«»r  the  cause  of  the  nodular  or  crystalline  growths  often  observed  ;i(  tin- 
edges  of  cathodes,  see  p.  150. 


x.]  CATHODIC  PKOCESSES  125 

not  exceed  O02  amp./cm.2  at  room  temperature.  At  higher  tempera- 
tures the  conditions  are  different.  We  have  seen  that  concentration 
polarisation  can  be  largely  avoided  by  working  at  higher  temperatures 
(whereby  the  rate  of  diffusion  of  ions  to  the  electrode  is  increased)  or 
by  agitating  the  electrolyte.  The  same  treatment  should  therefore 
improve  the  character  of  the  electrode  deposit,  and  does  so.  At  high 
temperatures,  and  with  a  rapid  circulation  of  the  electrolyte,  or  using 
a  revolving  cathode,  good  deposits  can  be  obtained  at  a  current  density 
of  O'l  amp./cm.2  or  more.  In  the  electrolytic  determination  of  metals 
by  cathodic  deposition,  it  is  essential  that  the  precipitate  be  perfectly 
coherent  as  well  as  pure.  With  a  stationary  electrolyte  only  a  very 
low  current  density  can  be  used  ;  but  if  the  cathode  is  rapidly  rotated, 
this  can  be  enormously  increased  and  the  time  taken  for  the  analysis 
correspondingly  reduced.  Revolving  cathodes  are  occasionally  used 
technically.1 

A  bad  electro-deposit  is  often  accompanied  by  hydrogen  evolution 2 
and  the  formation  of  a  basic  precipitate,  and  is  sometimes  stated  to  be 
due  to  these  phenomena.  It  is  better  to  regard  all  three  effects  as 
caused  by  the  low  metal  ion  concentration  in  the  electrolyte.  The 
formation  of  hydrogen  is  thereby  facilitated,  and  the  removal  of  H' 
ions  of  course  tends  to  precipitate  basic  substances.  The  evolution 
of  hydrogen,  some  of  the  bubbles  of  which  adhere  to  the  electrode,  is 
one  of  the  causes  of  growths  on  the  same.  The  whole  effect  of  concen- 
tration on  the  nature  of  the  cathodic  deposit  runs  parallel  to  its  effect 
on  the  nature  of  metal  precipitated  chemically  in  the  wet  way.  In  this 
case  again  a  more  finely  crystalline  or  an  amorphous  deposit  results 
from  dilute  solutions.3 

The  nature  of  the  salt  from  which  the  metal  is  deposited  has  often 
considerable  influence.  But  the  only  traceable  regularity  is  that 
metals  deposited  from  solutions  in  which  they  are  chiefly  present  as 
complex  aniou  generally  come  down  in  a  dense  smooth  finely  grained 
form.  Examples  are  copper  and  silver  from  their  alkaline  cyanide 
solutions  and  tin  from  ammonium  oxalate  solution.  Danneel 4  suggests 
that  this  characteristic  form  of  metal  is  produced  by  the  discharge  of 
the  complex  anion,  not  of  the  simple  cation.  Thus  from  a  KAg(Cy)2 
solution  the  process  of  cathodic  discharge  is 

AgCy,'— >Ag  +2C/  +  ©, 

and  not 

Ag" — >Ag  +  ©. 

Other  independent  evidence  makes  this  explanation  very  probable.5 

1  Pp.  270,  286,  287,  316,  317. 
For  the  effect  of  the  hydrogen  content  on  electrolytic  iron  deposits,  see  p.  300. 

3  Mylius  and  Fromm.     Ber.  27,  630  (1894). 

4  Zeitsch.  Elektrochem.  9,  763  (1903). 

5  See  Haber,  Zeitsch.  Elektrochem.  10,  433,  773  (1904). 


126    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

One  other  point  must  be  noticed,  and  that  is  the  immense  effect 
often  produced  on  a  metallic  deposit  by  the  addition  of  a  trace  of  some 
organic  substance,  sometimes  a  colloid,  sometimes  not,  to  the  electro- 
lyte.1 A  very  fine-grained  smooth  velvety  deposit  results.  Instances 
are  the  addition  of  gelatine  to  a  nickel-plating  or  copper  bath,  or  of 
resorcinol  to  a  zinc  tank.  The  action  of  the  addition  agent  is  selective. 
Thus  resorcinol  does  not  improve  the  nature  of  lead  or  copper  deposits. 
Further,  if  the  quantity  added  increases  beyond  an  exceedingly  low 
limit,  the  deposit  becomes  very  brittle,  and  is  found  to  contain  organic 
matter.  Miiller  and  Bahntje 2  suppose  that  the  first  product  of  ionic  dis- 
charge is  colloidal  metal,  and  that  the  addition  agent  acts  as  a  protective 
colloid,  determining  the  size  of  metallic  particle  ultimately  cathodically 
precipitated  and  affecting  the  direction  in  which  the  metal  colloid 
wanders  under  the  electric  field.  The  same  hypothesis  would  account 
for  the  marked  differences  brought  about  by  small  changes  in  acid 
and  alkali  concentration  near  the  neutral  point,  presence  of  traces  of 
colloidal  hydroxide,  etc. 

Cathodic  Production  of  Anions. — This  does  not  concern  us  much. 
Two  cases,  howeve^*,  present  technical  interest.  If  the  cathodic 
solution  of  chlorine  and  of  oxygen  according  to  the  equations 

C12 — >2Cl'  +  2© 
and 

02  +  2H20  — >  40H'  +  4© 

could  be  successfully  brought  about  under  technical  conditions,  it  would 
be  a  matter  of  the  first  importance  for  the  construction  of  technical 
primary  cells*  But  it  has  not  been  achieved  up  to  the  present. 


3.  Electrolytic  Reduction 

This  is  the  last  type  of  cathodic  process  to  be  discussed.  The 
employment  of  electrolytic  methods  for  oxidation  (see  later)  and  re- 
duction reactions  possesses  several  advantages.  Thus  as  the  reduction 
consists  either  of  a  change  in  the  ionic  charge,  or  of  an  alteration  in  the 
hydrogen  or  oxygen  content  of  the  substance  effected  by  means  of 
cathodically  formed  hydrogen,  the  final  product  contains  no  impurity 
needing  removal,  as  happens  with  chemical  reducing  agents.  If  the 
reduction  of  nitrobenzene  to  aniline  be  considered,  the  electrolytic 
method  furnishes  directly  a  solution  of  an  aniline  salt  with  nothing 
else  present  but  excess  of  acid  and  some  unchanged  nitrobenzene, 

1  For  many  qualitative  data,  BCC  Kern,  Trans.  Amer.  Electrochem.  Soc.  15,  441 
(1909). 

2  Zeitsch.  Elektrochem.  12,  317  (WOO). 
*  See  pp.  209-219. 


x.]  CATHODIC  PROCESSES  127 

whilst  the  chemical  method  means  using  large  quantities  of  iron  or  tin, 
the  salts  of  which  are  found  in  the  product. 

A  second  advantage  is  that,  whereas  the  formation  of  different 
oxidised  or  reduced  products  from  the  same  starting  material  needs  a 
different  chemical  oxidising  or  reducing  agent  every  time,  with  electro- 
lytic methods  a  whole  series  of  different  reactions  may  be  effected  by 
variation  of  electrode  potential  and  electrode  material.  The  series  of 
reduced  products  of  nitrobenzene,  studied  particularly  by  Haber  and 
by  Elbs,  are  a  striking  example.  Nitrobenzene  (C6H5 .  N02)  can  give 
under  different  reducing  conditions 

nitrosobenzene  C6H.  .  NO 

ft  phenyl-hydroxylamine  C6H5  .  NH  .  OH 

A 

a  zoxy  benzene  C6H5  .  N  —  N  .  C6H5 

azobenzene  C6H5  .  N  =  N  .  C6H5 

hydrazobenzene  C6H5  .  NH  .  NH  .  C6H5 

aniline  C6H5  .  NH2. 

Of  these  substances,  no  fewer  than  four  —  aniline,  azoxybenzene, 
azobenzene,  and  hydrazobenzene — can  be  prepared  electrolytically  with 
excellent  yields  by  suitable  variation  of  temperature,  cathode  potential, 
and  alkalinity  or  acidity  of  electrolyte. 

Electrolytic  reductions  can  be  divided  into  two  classes  : 

(a)  reductions  consisting  in  an  alteration  of  the  charge  on  an  ion — 

such  as  Fe"*  — >  Fe"  +  0  and  MnO/ >  MnO/  +  0  ; 

(6)  reductions  in  which  the  hydrogen  content  of  the  reduced  material 
is  increased  or  its  oxygen  content  diminished.1  Examples  are — 

NO/  +  10H*  — >  NH4*  +  3H20  +80 
C6H5  .  N02  +  6H*  — >  C6H5NH2  +  2H20  +60. 

Reversible  reactions  of  the  first  kind  have  been  fully  discussed  in 
Chap.  VIII.  Reduction — i.e.  discharge  of  positive  electricity — will 
commence  as  soon  as  the  electrode  potential  is  lowered  beneath  the 
equilibrium  value  for  the  solution.  But  in  practice,  at  all  but  the 
lowest  current  densities,  polarisation  effects  are  bound  to  enter.  This 
is  due  to  exhaustion  of  the  more  highly  oxidised  ion,  and  to  accumu- 
lation of  the  reduced  ion  in  the  immediate  neighbourhood  of  the 
electrode,  and  to  the  slow  rate  of  the  diffusion  processes  which  restore 
equilibrium.  In  the  case  of  Fe"*  — >  Fe"  +  0,  Fe'"  ions  must 
diffuse  to  the  electrode  and  Fe"  ions  diffuse  away.  As  these  diffusions 
cannot  keep  pace  with  the  electrolytic  process,  the  polarisation  at 
constant  current  increases. 

1  For  the  theory  of  this  type  of  electrolytic  reduction,  see  particularly  Haber, 
Zeitsch.  Phys.  Chem.  32,  193  (1900)  •  Haber  and  R.  Russ.  Zeitsch.  Phys.  Chem. 
47,  257  (1904). 


128    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

Nor  do  reductions  of  the  second  type  usually  take  place  reversibly. 
Here  we  must  conceive  of  cathodic  reduction  as  consisting  of  two  super- 
imposed processes,  (a)  discharge  of  H*  ions  to  atomic  hydrogen  ;  (b)  re- 
duction of  the  depolariser  by  the  atomic  hydrogen.  (Gaseous  mole- 
cular hydrogen  cannot  reduce.)  This  depolarisation  of  H'  ion  discharge 
by  the  material  undergoing  reduction  prevents  the  hydrogen  concentra- 
tion in  the  electrode  from  reaching  the  value  necessary  for  gas  evolution 
against  atmospheric  pressure.  To  the  low  concentration  of  hydrogen 
corresponds  a  low  electrolytic  solution, pressure,  and  the  cathodic 
potential  needed  for  the  whole  process  is  consequently  less  negative 
than  would  be  required  for  the  discharge  of  gaseous  hydrogen.  Now 
the  velocity  of  the  whole  reaction  depends  essentially  on  the  velocity 
of  the  second  process.  When  this  can  keep  pace  with  the  velocity  of 
the  first  process,  the  reduction  will  proceed  reversibly.  But  when  the 
depolarisation  by  the  substance  to  be  reduced  is  slow,  then  the  concen- 
tration of  hydrogen  in  the  electrode  rises,  its  electrolytic  solution 
pressure  increases,  and  the  cathodic  potential  necessary  for  the  reaction 
becomes  more  negative. 

These  differences  manifest  themselves  in  various  forms  of  the 
cathode-potential  current  curve.  In  Fig.  32  EGD  represents  the 

cathode-potential  current  curve 
for  reversible  H'  ion  discharge, 
which  begins  at  the  potential 
corresponding  to  G.  If  a  de- 
polariser be  added  which  can 
react  very  quickly,  the  cathodic- 
potential  current  curve  will  now 
be  EFA.  The  working  cathode 
potential  is  less  negative,  and 

Caffwde  Potential.  the  curve  still  bends  sharply  at 

FIG-  32-  th.£  point  where   decomposition 

sets  in.     If  the  depolariser  does 

not  act  quickly,  then  the  curves  are  of  the  type  EFB  or  EFC.  At 
higher  current  densities  particularly,  the  cathode  potential  needed 
is  more  negative  than  it  would  be  with  a  reversibly  acting  depolariser. 
In  some  cases  the  curves  B  or  C  may  cut  the  curve  D.  When  that 
happens,  the  potential  needed  for  H*  discharge  is  reached,  and  part 
of  the  current  will  produce  hydrogen  gas.  Not  only  will  the  voltage 
necessary  for  the  reduction  be  increased,  but  the  current  efficiency 
will  fall.  The  velocity  of  depolarisation  is  therefore  all-important  in 
electrolytic  reduction.  There  is  no  essential  difference  in  this  respect 
between  depolarising  electrolytes  (as  HN03)  and  non-electrolytes  (as 
nitrobenzene). 

The  effect  of  increase  of  current  density  on  an  electrolytic  reduction 
is  always  to  increase  the  cathodic  polarisation.     Not  only  are  more  H* 


X.] 


CATHODIC  PROCESSES 


129 


ions  discharged  in  a  given  time,  but  the  layer  of  depolariser  around  the 
electrode  becomes  largely  depleted.  Both  causes  increase  the  concen- 
tration of  gas  in  the  electrode,  and  hence  its  electrolytic  solution  pressure. 
Where  the  reduction  is  incomplete,  the  fraction  of  current  used  in 
evolving  hydrogen  will  also  rise.  An  increase  in  concentration  of  the 
depolariser  will  of  course  bring  about  more  efficient  depolarisation  and 
a  less  negative  cathode  potential.  A  rise  in  temperature,  by  increasing 
the  rates  of  diffusion  of  the  depolariser  and  of  its  reaction  with  hydrogen, 
will  act  similarly. 

Catalytic  Action  of  Cathode. — The  effect  of  the  cathode  material 
on  the  course  of  reduction  is  twofold.  Firstly,  different  cathodes 
catalyse  the  reaction  between  depolariser  and  hydrogen  at  very  different 
rates.  The  result  is  that  the  same  reduction  process  can  take  place 
with  a  far  less  negative  cathode  potential  at  some  cathodes  than  at 
others ;  or  conversely,  at  the  same  cathode  potential,  the  permissible 
current  density  can  vary  widely.  Nitrobenzene  in  acid  solution  is 
far  more  easily  reduced  to  aniline  at  a  zinc  than  at  a  platinum  cathode. 
In  alkaline  solution,  the  reaction  goes  better  at  platinum  or  copper 
than  at  iron,  zinc,  tin  or  lead.  Chlorates  are  generally  not  capable  of 
electrolytic  reduction,  but  with  an  iron  cathode  they  go  readily.  Then 
the  physical  condition  of  the  metal  surface  is  also  significant.  Nitrates 
give  ammonia  at  an  ordinary  smooth  copper  cathode,  whilst  at  a 
spongy  copper  electrode  nitrites  result.  The  influence  of  the  electrode 
material  is  well  shown  in  the  accompanying  diagram1  of  cathode- 
potential  current  curves  for  the  reduction  of  a  slightly  alkaline 
n.  KN03  solution  (Fig.  33).  There  is  a  great  difference  between  the 
potentials  required  in  using  platinised  platinum  and  mercury  cathodes, 
the  reaction  between  depolariser  and  hydrogen  being  catalysed  far 
more  powerfully  at  the  former  electrode. 


Cathode  Pot 
FIG.  33. 


Cathode  Potential. 
FIG.  34. 


Overvoltage    Action    of    Cathode. — The    nature    of    the    cathode 
can  also  affect  electrolytic  reduction  by  increasing  the  polarisation 
needed  for  H*  ion  discharge.     In  Fig.  34  let  AB  be  the  cathode-potential 
1  Muller.^Zetec*.  Anorg.  Chem.  26,  1  (1901). 


130    PKINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

current  curve  for  reversible  hydrogen  discharge,  and  the  curve  DE 
the  curve  for  the  reduction  process.  It  is  obvious  that  if  the  electrode 
used  allows  reversible  H*  ion  discharge,  then  no  reduced  product  will 
be  formed — all  the  current  will  give  hydrogen.  But  if  the  hydrogen 
overvoltage  be  at  all  considerable,  then  the  H'  ion  discharge  curve  will 
have  the  form  AC,  and  at  current  densities  above  that  corresponding 
to  F  reduction,  still  accompanied  by  hydrogen  evolution,  will  begin. 
Similarly  in  Fig.  32,  the  curve  C  is  represented  as  cutting  the  reversible 
H'  ion  discharge  curve  D.  If,  however,  hydrogen  overvoltage  comes 
into  consideration,  the  hydrogen  curve  will  be  represented  by  H,  and 
even  at  high  current  densities  the  yield  of  reduced  material  will  be 
nearly  100  per  cent. 

The  use  of  such  cathodes  enables  us  to  carry  out  reductions  requir- 
ing a  very  large  cathodic  polarisation.  Tafel  has  been  the  chief  worker 
on  the  reduction  of  these  very  difficultly  reducible  substances,  such 
as  pyridine  and  various  alkaloids  of  the  uric  acid  group.  The  best 
electrodes  are  of  course  those  at  which  the  hydrogen  overvoltage  is 
highest,  e.g.  Hg,  Pb  and  Zn.1  One  point  should  be  noticed.  Although, 
with  complete  reduction,  temperature  rise  always  acts  favourably,  with 
incomplete  reduction  of  difficultly  reducible  substances,  made  possible 
by  hydrogen  overvoltage,  the  reverse  may  hold.  A  temperature  rise 
diminishes  overvoltage  and  therefore  facilitates  hydrogen  discharge. 
If  this  effect  overbalances  the  effect  of  increased  rate  of  diffusion  of 
depolariser  to  electrode,  the  yield  of  reduced  material  will  fall. 

Catalysts  in  Electrolyte. — Finally,  the  velocity  of  reduction  can 
often  be  essentially  increased  by  the  addition  of  some  catalyst  to  the 
electrolyte.  Titanium  and  vanadium  salts  are  often  active.  Thus 
quinone  (CeH40.>)  can  be  readily  reduced  in  acid  solution  to  hydro- 
quinone  (C6H4(OH)2)  in  presence  of  titanium  chloride,  whereas 
in  absence  of  this  salt  the  reduction  stops  at  quinhydrone,  C6H402, 
C6H4(OH)2.  The  mechanism  of  catalysis  is  here  clear.  The  TiCl4  is 
readily  reduced  cathodically  to  TiCl3 ;  this  salt,  a  powerful  chemical 
reducing  agent,  reacts  rapidly  with  the  depolariser  present,  being 
reoxidised  to  TiCl4  in  the  process,  and  the  cycle  continues. 


B.  ANODIC  PROCESSES. 
4.  Discharge  of  Anions. 

Oxygen  Overvoltage. — The  first  anodic  process  to  be  considered  is 
the  discharge  of  OH'  ions  to  oxygen  gas — 

20H'  — >  H20  +  J02  +  2  0. 

1  In  such  cases  the  electrolyte  must  be  free  from  all  traces  of  salts  of  metals 
which  have  a  low  hydrogen  overvoltage  and  are  also  liable  to  be  cathodically 
precipitated.  Otherwise  no  reduction  may  take  place,  only  hydrogen  evolution. 


x.]  ANODIC  PROCESSES  131 

This  process  is  invariably  accompanied  by  irreversible  effects,  and  it 
has  not  been  found  possible  to  directly  measure  the  value  of  E.P. 
Indeed  the  mechanism  of  the  anodic  formation  of  oxygen  is  probably 
different  from  that  expressed  by  the  above  equation.  For  example, 
Grube  x  has  shown  that  the  evolution  of  oxygen  in  2n  .  H2S04  at  a 
platinised  platinum  electrode  undoubtedly  takes  place  through  the 
intermediate  formation  of  Pt03.  But  it  has  been  calculated  with 
certainty  that  the  reversible  potential  of  an  oxygen  electrode  in  equilir 
brium  with  oxygen  at  atmospheric  pressure  is  1*23  volts  more  positive 
than  the  potential  of  the  hydrogen  electrode  in  the  same  solution.  In 
a  normal  H*  solution,  the  electrode  potential  will  be  -f-  1'23  volt. 
Similarly,  knowing  the  OH'  concentration  in  such  a  solution2  to  be 
0'56  x  10  ~u,  the  potential  in  a  normal  OH'  solution,  or  E.P.  for 


can  be  calculated  to  be 

+  1-23  +  0-058  log  0'56  X  10  ~14  ==  +  0-404  volt 

(all  figures  for  room  temperature). 

In  practice,  the  calculated  equilibrium  oxygen  potential  must 
always  be  more  or  less  exceeded  before  oxygen  is  discharged.3  This 
question  of  oxygen  overvoltage  was  first  studied  by  Coehn  and  Osaka.4 
Working  by  the  anode  decomposition  potential  method,  and  using 
therefore  exceedingly  low  current  densities,  they  obtained  the  following 
values  :  — 

TABLE  XXI 

Electrode  Overvoltage 

Spongy  nickel  0'05  volt 

Nickel  0-12 

Cobalt  0-13 

Iron  0-24 

Platinised  platinum  0'24 

Lead  peroxide  0-28 

Smooth  platinum  0-44 

Nobis,5  using  a  rather  higher  current  density,  found  the  overvoltage 
at  graphite  to  be  about  0*4  volt.  Oxygen  overvoltage,  like  that  of 
hydrogen,  increases  rapidly  with  the  current  density.  It  also  does  not 
reach  its  maximum  value  at  once,  but  after  hours,  and  in  some  cases 
(e.g.  platinised  platinum)  days.  The  following  Table  XXII  contains 

1  Zeitsch.  ElektrocJiem.  16,  621  (1910). 

2  P.  117. 

3  A  very  high  overvoltage  favours  the  production  of  the  strongly  oxidising 
ozone  which  is  often  formed  under  those  conditions. 

4  Zeitsch.  Anorg.  Chem.  34,  86  (1903). 

5  Dissertation  (Dresden,  1909). 

K  2 


132    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

figures  calculated  from  the  results  of  Foerster  and  Piguet,1  working  with 
2n  .  KOH  at  15°  and  with  a  current  density  of  0*033  amp. /cm.2 

TABLE  XXII 

Elect  rode  At  start  After  two  hours. 

Nickel  0-38  volt  0-56  volt 

Iron  0-50  0-59 

Platinised  platinum  0-44  0-86 

Smooth  platinum  0-84  1-46 

Finally,  the  overvoltage  diminishes  very  considerably  with  rise  of 
temperature. 

It  is  clear  that,  when  hydrogen  and  oxygen  are  liberated  by  the 
electrolysis  of  dilute  acid  and  alkali,  very  different  voltages  may  be 
needed  under  different  circumstances,  depending  on  electrolyte,  tem- 
perature, electrode  materials,  time  after  commencement  of  electrolysis, 
rate  of  circulation  of  electrolyte,  etc.  With  a  low  current  density,  e.g.  0*01 
amp. /cm.2,  a  platinised  platinum  cathode,  a  nickel  anode,  and  good 
circulation,  the  voltage  needed  soon  after  the  commencement  of  elec- 
trolysis will  consist  of  the  reversible  decomposition  voltage  1*23  volts, 
the  cathode  overvoltage  (say  0*03  volt),  the  anode  overvoltage  (say  0*25 
volt),  and  the  voltage  for  overcoming  the  resistance  of  the  electro- 
lyte (perhaps  0*05  volt),  a  total  of  1*56  volts.  Whilst  with  a  mercury 
cathode,  a  polished  platinum  anode,  and  a  high  current  density  (0*1 
amp./cm.2)  the  voltage  after  some  hours  will  amount  to  1*23  volts,  plus 
cathode  overvoltage  1*30  volts,  plus  anode  overvoltage  about  2  volts, 
plus  the  electrolyte  voltage  about  0*5  volt,  plus  any  amount  due  to 
concentration  polarisation.  This  makes  a  minimum  of  5  volts.2 

Halogen  Ion  Discharge. — Besides  oxygen,  the  discharge  of  Cl'  and 
Br'  ions  to  the  free  halogens  usually  necessitates  a  certain  over- 
voltage.  This  question  has  been  studied  by  Miiller,3  by  Boericke,4  and 
by  Foerster  and  Yamasaki.5  They  found  that,  at  platinised  platinum 
electrodes,  all  the  three  halogens — C12,  Br2,  and  I2 — are  precipitated 
very  nearly  reversibly,  provided  that  the  electrodes  remain  free  from 
oxygen.  But  at  smooth  platinum  electrodes  this  was  only  true  with 
iodine.  Bromine  required  an  overvoltage  of  about  0*33  volt,  chlorine 
0'7  volt,  the  current  density  in  both  cases  being  0*017  amp./cm.2. 
These  were  the  final  values  reached  after  continued  electrolysis,  for, 
like  hydrogen  and  oxygen,  chlorine  overvoltage  is  at  first  low,  and 
gradually  rises  to  a  maximum  constant  value.  Increased  temperature 

Zeitsch.  Elektrochem.  10,  714  (1904). 

For  the  technical  production  of  hydrogen  and  oxygen  by  electrolysis,  see 
p.  386. 

Zeitsch.  Elektrochem.  6,  573  (1900);  8,  426  (1902), 
Zeitsch.  Elektrochem.  11,  r,7  (/.W-i). 
Zeitsch.  Elektrochem.  16,  321  (1910). 


x.]  ANODIC  PROCESSES  133 

effects  a  big  diminution.  Thus  Sacerdoti,1  working  with  platinum 
electrodes  at  0'06  amp. /cm.2,  found  the  overvoltage  to  decrease  from 
0'7  volt  to  0'2  volt  on  raising  the  temperature  from  21°  to  100°. 

Theories  of  Overvoltage. — The  cause  of  these  complex  overvoltage 
phenomena  has  long  been  the  subject  of  investigation  and  discussion, 
and  is  still  not  finally  settled.  Haber 2  postulates  the  adsorption  or 
occlusion  of  a  layer  of  gas  at  the  electrode — electrolyte  surface.  This 
film  of  poor  conductivity  increases  the  potential  gradient  to  be 
overcome  by  ions  passing  from  electrolyte  to  electrode,  and  as  its 
thickness  depends  on  the  nature  of  the  electrode  material,  the  differences 
observed  with  different  electrodes  are  explained. 

According  to  Nernst,3  overvoltage  is  essentially  due  to  the  low  rate 
at  which  an  electrode  saturated  with  gas  gets  into  equilibrium  with 
the  atmosphere.  For  bubble  formation,  a  certain  minimum  gas  con- 
centration in  or  at  the  surface  of  the  electrode  is  required.  If  the 
electrode  be  of  a  material  with  a  very  low  solvent  power  for  the  gas, 
before  the  minimum  concentration  is  reached  it  will  have  become 
saturated  with  gas  at  atmospheric  pressure.  The  further  quantity  of 
gas  needed  to  produce  bubbles  corresponds  to  a  supersaturation  of 
the  electrode  and  to  an  increase  in  the  electrolytic  solution  pressure. 
Overvoltage  is  then  an  increase  in  the  electrode  polarisation,  due  to  an 
increase  in  electrolytic  solution  pressure,  caused  in  turn  by  the  slow 
rate  at  which  the  electrode,  charged  with  gas,  gets  into  equilibrium  with 
the  atmosphere.  Lately  Moller  *  has  modified  these  conceptions  as  the 
result  of  a  research  in  which  he  showed  that  overvoltage  and  surface 
tension  at  the  electrode -electrolyte  surface  run  parallel.  He  takes  the 
view  that  the  immediate  cause  of  the  former  is  the  energy  necessary  to 
produce,  between  electrode  and  electrolyte,  a  gas  film  of  sufficient 
thickness  to  give  bubbles. 

Foerster5  has  experimented  in  a  different  direction.  He  showed 
that  oxygen  evolution  at  a  platinised  platinum  anode  undoubtedly  has, 
as  an  intermediate  stage,  the  formation  in  the  metal  of  a  solid  solution 
of  an  unstable  oxide6  of  platinum,  which  subsequently  decomposes. 
In  the  pure  state  this  oxide  would  give  a  very  high  anodic  potential. 
Its  solid  solutions  in  platinum  have  anodic  potentials  the  more  positive 
the  greater  the  oxide  concentration.  The  longer  electrolysis  con- 
tinues, the  greater  this  concentration  becomes,  and  therefore  the  higher 
the  anode  potential.  The  solid  solution  finally  decomposes  as  quickly 
as  it  is  formed  by  OH'  ion  discharge,  and  the  electrode  assumes  a 

Zeitsch,  Elektrochem.  17,  473  (1911). 

Zeitsch.  Elektrochem.  8,  539  (1902).  Also  Haber  and  R.  Russ.  Zeitsch. 
Phys.  Chem.  47,  257  (1904). 

Theoretische  Chemie  (4th  edition),  p.  714  (1903). 

Zeitsch.  Phys.  Chem.  65,  226  (1909). 

Zeitsch.  Phys.  Chem.  69,  236  (1909);  Zeit-sch.  Elektrochem.  16,  353  (1910). 

Identified  by  Grube  with  PtO:{. 


134    PKINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

constant  potential  value.  This  accounts  for  the  frequent  rise  of  over- 
voltage  after  the  commencement  of  electrolysis.  Its  dependence 
on  current  density  can  also  be  explained.  As  the  final  concentration 
is  determined  by  the  amounts  of  oxide  formed  and  decomposed  in 
the  same  time  being  equal,  it  is  clear  that  a  higher  current  density,  and 
therefore  a  greater  rate  of  formation  of  the  oxide,  must  mean  a  higher 
final  concentration,  and  hence  a  higher  anode  potential. 

Foerster  supposes  these  relations  to  hold  generally  for  oxygen  over- 
voltage,  an  intermediate  unstable  superoxide  being  always  formed.  If 
this  decomposes  very  quickly,  as  with  nickel,  where  the  production  of 
Ni02is  assumed,  then  the  final  concentration  of  the  oxide  solid  solution 
and  the  overvoltage  will  be  low.  Experimental  evidence  is  difficult 
to  obtain,  but  it  exists  with  nickel,  as  with  platinum.1  Further,  anodic 
oxygen,  discharged  at  graphite,  produces  a  certain  amount  of  an  insoluble 
substance  of  strong  oxidising  properties,  which  readily  evolves  oxygen 
and  is  probably  a  higher  oxide  of  carbon.2  Applied  to  H'  discharge, 
one  must  imagine  produced  a  solid  solution  of  hydrogen  in  the  metal, 
and  gaseous  hydrogen  indirectly  evolved  through  this.  In  the  case 
of  chlorine,  Luther  and  Brislee3  and  Pfleiderer4  had  before  suggested  that 
there  is  some  unknown  anodic  product  intermediate  between  Cl'  ion 
and  C12  gas,  whose  slow  rate  of  decomposition  determines  the  over- 
voltage. 

To  sum  up,  the  discharge  of  H',  OH',  or  halogen  ions  very  probably 
first  results  in  the  formation  of  some  compound  or  solution  between 
electrode  material  and  precipitated  substance,  and  the  concentration 
of  the  gas  thus  dissolved  in  some  form  in  the  electrode  material  deter- 
mines the  anode  potential.  This  gas  concentration  itself  depends  on  a 
number  of  factors — viz.  the  rate  of  decomposition  of  the  same  solid 
solution,  current  density,  time,  and  the  surface  tension  relations 
between  electrode,  electrolyte,  and  gas. 

Depolarisation  of  Halogens.  —  The  overvoltage  increases  the 
polarisation  necessary  for  the  discharge  of  the  halogen  ions.  But 
the  discharge  of  Br'  and  I'  ions  is  also  to  some  extent  depolarised  in 
strong  solutions,  owing  to  the  tendency  to  form  complexes  such  as 
Br3'  and  I3' ;  and  all  the  halogens  will  be  depolarised  to  a  greater  or 
less  extent  by  any  OH'  ions  present,  whether  furnished  by  water  or 
alkali.8 

Of  the  discharge  of  other  anions —  SO/,  N03',  etc. — little  or  nothing 
is  with  certainty  known,  not  even  whether  the  anodic  oxygen  evolved 
during  the  electrolysis  of  a  salt  containing  such  ions  must  always 

1  See  also  the  interesting  results  of  Joost,  Dissertation  (Dresden,  1910). 

2  Joost,  loc.  cit.     Also  Nobis,  Dissertation  (Dresden,  1909). 

3  Zeitsch.  Phys.  Chem.  45,  216  (V.m). 

4  Zeitsch.  Phys.  Chem.  68,  49  (1902). 

5  Pp.  319-320. 


x.]  ANODIC  PROCESSES  135 

be  ascribed  to  the  primary  discharge  of  OH'  ions,  and  not  perhaps  to 
the  discharge  of  the  anion  present,  followed  by  interaction  with  water 
and  secondary  oxygen  evolution.  For  example  : — 

(a)  S(V— >S04  +  20 

(6)  S04  +  H20  — >  H2S04  +  i02. 

The  chemical  result  is  the  same,  and  as  the  phenomena  observed 
can  be  explained  satisfactorily  on  the  assumption  of  OH'  ion  discharge, 
we  will  not  further  consider  pros  and  cons. 

5.  Solution  of  Metals 

The  next  type  of  anodic  process  is  the  behaviour  of  metallic  anodes. 
We  can,  conveniently  for  our  purpose,  divide  anodic  processes  in  which 
metals  take  part  into  three  classes  : — 

(a)  the  metal  completely  and  easily  enters  solution ; 

(6)  owing  to  reaction  resistances,  the  solution  of  the  metal  is  partly, 
sometimes  wholly,  stopped; 

(c)  the  metal  does  not  dissolve,  and  some  other  anodic  reaction 
takes  place. 

We  will  first  discuss  the  case  of  a  pure  metal  which  completely 
dissolves.  If  it  can  only  furnish  one  kind  of  ion  the  process  is  a  very 
simple  one.  Reversible  solution,  when  possible ,  will  commence  when  the 
anode  potential  exceeds  the  equilibrium  value  for  the  electrolyte.  But 
in  general  the  anode  potential  will  be  higher  than  the  reversible  value, 
owing  to  concentration  polarisation.  Even  with  a  well-circulated 
electrolyte  the  metallic  ion  concentration  in  the  layer  immediately 
surrounding  a  dissolving  anode  is  bound  to  be  greater  than  in  the  bulk 
of  the  electrolyte,  and  the  anodic  potential  necessary  for  solution  is 
thereby  raised. 

Influence  of  Physical  Condition  of  Metal.— The  electrolytic  poten- 
tial of  a  metal  depends  to  some  extent  on  its  mode  of  preparation 
and  its  previous  thermal  and  mechanical  treatment.  Generally 
speaking,  if  it  has  been  cast  and  rapidly  cooled,  it  is  not  in  a  stable  state 
at  room  temperature,  and  will  tend  more  strongly  to  enter  the  ionic 
condition  than  the  stable  form  does.  Mechanical  working  will  tend  to 
remove  this  instability,  and  an  anode  of  wrought  metal  will  have  a 
nobler  potential  than  one  of  cast  metal. 

Finally,  an  electro-deposited  metal  will  have  the  noblest  single- 
potential  value,  as  it  is  usually  deposited  in  the  stable  condition,  though 
there  are  certainly  marked  exceptions.  It  follows  that  a  cast  anode 
dissolves  most  easily,  and  least  so  an  electro  deposited  one.  On  the 
other  hand,  an  electro -deposited  one  dissolves  most  evenly.  A  cast  or 
wrought  anode,  particularly  the  latter,  has  local  differences  in  structure, 
with  corresponding  slight  differences  in  electrolytic  potential.  The 


136    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

more  easily  dissolved  parts  of  the  anode  will  enter  solution  first,  and  j 
often  the  undissolved  particles  will  fall  off  and  escape  solution  alto- 
gether.1    This  is  one  of  the  causes  which  produce  anode  '  slimes/ 

Metals  giving  more  than  one  Cation. — When  a  metal  can  furnish 
two  or  more  kinds  of  ions  in  aqueous  solution,  the  conditions  corre- 
spond to  the  cathodic  relations  discussed  on  pp.  122-123.  The  anode 
will  always  tend  to  dissolve  so  that  the  different  ions  are  produced 
in  the  electrolyte  in  equilibrium  amounts — thus,  with  copper,  a  great 
excess  of  Cu"  ions  ;  with  iron,  Fe"  ions.  With  a  very  low  current  density 
this  will  actually  happen;  but  if  it  be  high,  reaction  resistances  may  cause 
one  ion  to  be  furnished  in  unduly  preponderating  amount.  When, 
moreover,  the  formation  of  one  ion  is  powerfully  depolarised  by  some 
constituent  of  the  electrolyte  and  is  thus  continuously  removed  from  the 
solution,  the  metal  will  apparently  dissolve  as  if  its  own  valency  were 
that  of  the  ion  in  question.  The  tendency  for  Cu*  ions  to  form  com- 
plexes with  Cr  and  CN'  ions  being  much  greater  than  that  of  Cu"  ions,2 
a  copper  anode  in  a  cyanide  or  chloride  solution  dissolves  as  a  mono- 
valent  metal,  the  copper  practically  entirely  going  into  the  complex 
form. 

The  possibility  of  the  formation  of  more  than  one  kind  of  ion  is 
another  cause  of  the  production  of  anode  '  slimes/  The  concentrated 
layer  of  electrolyte  near  the  dissolving  anode  may  be  in  equilibrium 
with  the  electrode,  but  when  it  mixes  with  the  bulk  of  the  solution  of 
different  concentration,  one  ion  will  be  present  in  excess.  If  this  be 

the  one  of  lower  valency,  it  will  break  up  as  follows  :  2Cu* >  Cu"  -f-  Cu 

(taking  copper  as  an  example),  and  metallic  powder  will  be  precipitated 
near  the  anode.3 

Alloys. — Very  important  practically  is  the  question  of  the 
anodic  solution  of  alloys,  which  we  meet  in  electrolytic  metal  refining. 
The  anodic  behaviour  of  alloys  is  governed  by  the  same  principles  which 
determine  their  cathodic  deposition,  or  the  depolarisation  of  the  dis- 
charge of  a  metal  ion  by  the  material  of  the  cathode.4  The  equilibrium 
potential  of  an  electrode  determines  how  it  will  dissolve,  and  the 
potential,  in  turn,  depends  on  how  the  different  constituents  of  the 
electrode  are  combined  together.  Consider  the  alloy  (a  binary  one  for 
simplicity)  as  electrode  in  some  solution  which  does  not  react  on  it 
chemically.  Then,  if  the  alloy  contains  any  appreciable  quantity  of  its 
more  electropositive  constituent  as  a  pure  independent  phase,  uncom- 
bined  with  the  other  constituent  either  as  compound  or  as  solid  solution, 
the  potential  of  the  alloy  will  be  the  potential  of  this  metal.  If  the  alloy 
consists  of  a  solid  solution  of  the  two  metals,  it  will  have  a  potential 
somewhere  between  those  of  its  pure  constituents.  If  the  alloy  consists 

1  In  an  anode  which  has  been  worked  in  some  way,  the  surface  layer  dissolves 
less  readily  than  the  parts  underneath. 

•  See  p.  123.  :«  Cf.  pp.  248,  272.  4  See  p.  121. 


ANODIC  PEOCESSES 


137 


of  pure  compound  without  excess  of  either  constituent,  its  potential 
will  probably  he  between  those  of  the  two  metals,  but  can,  however, 
exceed  the  potential  of  either  of  them.  If  a  mixture  of  solid  solution 
and  compound,  then  the  potential  is  that  of  the  more  electropositive 
of  these. 

Figs.  35  and  36  illustrate  these  cases.     The  zero  of  potential  is  always 
taken  as  the  potential  of  the  more  electropositive  constituent  of  the 


Potential  la  Millivolts. 

=>  i  1  1  S  1  §  1  1 

1 

1 

/ 

J 

/ 

JL 

/ 

£ 

J 

/ 

_j- 

0      10     20     30     40     50     60     70     80     90    100 
Atomic  per  cent  of  more  electronegative  metal. 

FIG.  35.     I.— Sn-Cu  in  N.  H2S04.         II.— Cd-Cu  in  N.  H2S04. 

alloy  when  in  the  same  electrolyte.  In  curve  I  *  we  have  the  potentials 
of  tin-copper  mixtures  (bronzes)  of  different  compositions.  Up  to 
66  per  cent.  Cu  (atomic)  the  potential  is  practically  that  of  pure  tin. 


ouu 
HTOO 

^600 

,2 

|500 
|400 
•S300 
|200 

|ioo 

0 

180 
—UnN 

—  H70H 

/Ll 

/PI 

/  1^3 

/ 

^~\^ 

/30a 

ZF> 

f 

/Li 

^- 

S 

33 

HL 

-UlO£ 

0      10     20     30    40     50     60     70     80     90   100 
Atomic  per  cent  of  more  electronegative  metal. 

FIG.  30.     III.— Sn-Bi  in  N.  H2S04.         IV.— Cu-Ag  in  N.  CuS04. 

Then  there  is  a  sharp  rise  of  0'04  volt  to  the  potential  of  the  compound, 
whose  composition,  Cu2Sn,  is  given  by  the  value  of  the  corresponding 
abscissa. 

A  second  rise  up  to  a  potential  difference  of  nearly  0'5  volt  corresponds 
to  a  second  compound,  Cu3Sn,  which  has  practically  the  same  potential, 

1  Puschin,  Zeitsch.  An&rg.  Chem.  56,  1  (1W7). 


138    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

against  the  electrolyte  as  copper  itself.  In  the  cadmium-copper  curve l 
(II),  we  observe  a  horizontal  line  denoting  the  presence  of  a  compound, 
which,  as  the  line  commences  at  33  per  cent.  Cu  (atomic),  has  evidently 
the  composition  Cd2Cu.  The  two  sloping  curves  denote  the  potentials 
of  solid  solutions  of  Cd  and  Cd2Cu  and  of  Cd2Cu  and  Cu  ;  whilst  the 
horizontal  line  up  to  20  per  cent.  Cu  means  that  alloys  with  less  copper 
than  this  amount  will  give  the  potential  of  pure  Cd.  Curve  III 2  shows 
that  Sn-Bi  mixtures  contain  no  compounds,  and  only  form  one  set  of 
solid  solutions  within  very  narrow  concentration  limits.  Their  potential 
up  to  90  per  cent,  of  Bi  is  that  of  pure  tin.  Curve  IV 3  finally  indicates 
that  Cu  and  Ag  mix  in  all  proportions,  giving  one  series  of  solid  solu- 
tions, of  which  the  potentials  vary  continuously  between  those  of 
the  pure  metals. 

Let  us  now  consider  what  will  happen  on  reversible  anodic  solution 
of  some  of  these  alloys. 

(a)  If  the  alloy  be  50  per  cent.  Sn  :  50  per  cent.  Cu  (atomic  per  cent.), 
it  will  consist  of  a  mixture  of  pure  tin  and  of  the  compound  Cu2Sn  (with 
psrhaps  some  Cu3Sn).  If  anodically  polarised,  it  will  dissolve  when  the 
potential  has  been  raised  to  the  equilibrium  value  for  pure  tin  in  the 
same  solution.  Tin  will  dksolve,  leaving  the  compound  behind. 
When  all  the  free  tin  has  entered  solution,  the  anode  potential  must  be 
increased.  If  the  compound  left  behind  be  Cu2Sn  it  may  be  possible  to 
dissolve  out  the  tin,  leaving  behind  the  copper.  But  if  Cu3Sn  be  also 
present,  it  is  evident  from  the  diagram  that  solution  of  copper  will 
take  place  just  as  easily  and  the  compound  will  dissolve  as  a  whole. 

(6)  Alloy  is  60  per  cent.  Cu  :  40  per  cent.  Cd  (a  omic  per  cent.),  a 
mixture  of  the  compound  Cd2Cu  and  of  either  pure  copper  or  else  a 
solid  solution  of  Cd2Cu  and  Cu.  It  will  not  dissolve  until  the  anodic 
potential  is  raised  0'25  volt  above  that  of  a  cadmium  anode.  Then 
cadmium  will  dissolve  out  of  the  compound,  leaving  behind  copper. 
(If  the  current  density  be  too  high,  copper  will  also  to  some  extent  enter 
the  solution.)  When  the  compound  has  been  entirely  decomposed  the 
potential  must  be  further  increased,  when  copper  will  dissolve. 

(c)  Sn-Bi  alloy  with  3  per  cent.  Bi  (atomic).     It  will  consist  of  pure 
tin  with  a  small  quantity  of  a  Sn-Bi  solid  solution,  containing  a  large 
excess  of   Bi.     It  will  dissolve  readily,   pure  tin  entering  solution. 
When  all  this  has  disappeared,  the  tin-bismuth  alloy  will  dissolve  as  a 
whole,  necessitating  increased  polarisation. 

(d)  A  copper-silver  alloy  of  any  composition,  except  perhaps  very 
extreme  values.     It  will  completely  enter  solution,  needing  a  higher 
anode  potential  the  bigger  the  silver  content. 

We  see  that  complete  separation  of  the  constituents  of  an  alloy  in 


1  Puschin,  loc.  cit. 
Puschin,  loc.  rif. 
3  Herschkowitsch,  Zeitsch.  Phys.  Chem.  27,  123  (1898). 


x.]  ANODIC  PROCESSES  139 

this  way  is  not  often  possible.  Pure  material  may  dissolve,  but  a  solid 
solution  or  a  compound  will  generally  be  left  behind.  Further,  in 
technical  work  there  are  other  considerations.  Firstly,  the  necessity 
of  using  fairly  high  current  densities.  This  means  that  the  anode  poten- 
tial must  somewhat  exceed  its  equilibrium  value,  and  that  therefore 
constituents  may  dissolve  which  otherwise  would  not  do  so.  Still 
more  important  is  the  fact  that  a  coating,  of  undissolved  metal  may 
form  on  the  electrode,  hinder  the  solution  of  the  right  constituent, 
and  itself  dissolve.  In  this  connection  the  structure  of  the  alloy  will 
be  of  importance.  If  the  undissolved  crystals  readily  fall  away  from 
the  anode,  these  disturbances  will  be  eliminated.  Other  points  to  be 
noticed  are  that  the  crude  anodes  used  technically  have  generally  only 
a  few  per  cents,  of  impurities,  commonly  consisting  of  small  quantities 
of  a  large  number  of  metals,  not  of  one  metal  only.  Both  these  cir- 
cumstances favour  efficient  electrolytic  refining.  A  solid  solution 
composed  of  many  constituents  will  dissolve  less  readily  than  one 
containing  two  or  three  only. 

Passivity. — We  must  now  discuss  the  behaviour  of  metallic 
anodes  which  dissolve  partly  or  not  at  all  when  current  passes.  Some- 
times this  behaviour  is  caused  by  another  anode  reaction — e.g.  the 
discharge  of  an  anion,  setting  in  at  a  lower  reversible  anodic  potential. 
But  generally  this  is  not  so.  The  non-solution  or  partial  solution  of  a 
metallic  anode  is  commonly  bound  up  with  the  phenomena  of  passivity. 
If  a  piece  of  chromium  or  iron  with  a  fresh  active  metallic  surface  be 
dipped  into  strong  HN03,  no  action  takes  place.  When  subsequently 
washed  and  dipped  into  cold  dilute  H2S04  or  a  CuS04  solution,  no 
hydrogen  is  evolved  or  copper  precipitated,  as  would  have  happened 
in  the  absence  of  the  treatment  with  HN03.  In  a  solution  of  one  of  its 
salts  it  has  the  same  potential  as  an  indifferent  electrode,  such  as 
platinum.  No  change  in  weight  or  difference  in  appearance  is  per- 
ceptible. These  metals  behave  in  fact  like  noble  metals,  far  more 
electronegative  in  character  than  ordinary  iron  and  chromium.  They 
are  said  to  have  become  passive,  the  passivity  being  produced  by  the 
action  of  the  strong  nitric  acid. 

This  chemical  passivity  has  its  analogy  in  electrochemical  passivity, 
a  metal  being  termed  passive  when  it  behaves  electromotively  as  a 
nobler  metal  than  it  really  is — thus,  when  it  requires  an  anodic  polarisa- 
tion exceeding  the  equilibrium  value  before  it  will  enter  solution,  the 
excess  polarisation  often  sufficing  to  cause  another  process  to  com- 
mence. Examples  are  abundant.  Platinum  under  most  conditions, 
gold  in  KCy  or  in  AuCl3,  chromium  in  alkaline  solutions,  nickel  in 
Na2S04  solution,  iron  in  dilute  H2S04,  all  more  or  less  show  the 
phenomena. 

The  cause  in  every  case  is  the  low  velocity  of  the  corresponding 
electrode  reaction.  In  order  that  this  may  proceed  at  a  convenient, 


140    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

or  even  at  a  measurable,  rate,  the  anodic  polarisation  must  be  increased 
beyond  the  equilibrium  value.  Passivity  can  manifest  itself  in  several 
ways.  Sometimes  the  metal  dissolves,  not  as  it  would  at  the  equili- 
brium potential,  but  as  a  more  strongly  oxidising  ion,  corresponding 
to  the  higher  anodic  polarisation.  The  best  known  example  is 
chromium.1  Normally  it  should  dissolve  as  the  ion  Cr",  analogous  to 
I-'  .  But  if  anodically  polarised  in  an  alkaline  solution,  it  does 
not  dissolve  till  a  potential  (5h  =  +  0'62  volt  is  reached,  and  then 

as    hexa-valent    ions    Cr These    immediately  react    with    the 

alkali, 

Cr +  80H'  — >  Cr04"  +  4H20, 

the  final  result  being  an  alkaline  chromate.  But  generally,  as  the 
anodic  current  density  (and  anodic  potential)  is  raised,  the  fraction  of 
the  current  carried  by  metal  ions  entering  the  electrolyte  decreases, 
anions  are  discharged,  and  finally  the  metal  may  entirely  cease  to 
dissolve,  the  current  being  completely  employed  in  discharging  anions. 
Thus  at  a  gold  electrode  in  AuCl3,  chlorine  gas  is  evolved,  and  at  a  nickel 
anode  in  caustic  alkali,  or  at  an  iron  anode  in  an  alkaline  sulphate, 
oxygen.  The  higher  the  current  density,  the  greater  the  passivity,  as 
measured  by  the  fraction  of  the  current  carried  by  the  anions  leaving 
the  electrolyte. 

The  nature  of  the  anion  exerts  much  influence  on  passivity  pheno- 
mena. Generally,  the  ions  of  acids  which  are  also  strong  oxidising 
agents  favour  passivity — for  example  CIO/  and  NO/.  Cl'  and  Br'  ions 
tend  to  remove  it.2  The  action  of  other  ions  is  intermediate.  Of  cations, 
the  H'  ion  always  acts  against  passivity,  whilst  others  have  little  or  no 
effect.  An  increase  of  temperature  always  tends  to  destroy  passivity. 
As  its  cause  lies  in  slow  reaction  velocities,  this  fact  is  natural.  Cathodic 
polarisation  acts  similarly.  Passive  iron  which  has  been  cathodically 
polarised  is  rendered  active.  The  passive  state  of  a  metal  is  not  a 
permanent  one.  Iron  which  has  been  passivated  by  anodic  polarisation 
will,  after  some  time,  dissolve  in  dilute  acids.  The  single  potential  of 
chromium  which  has  been  similarly  treated  becomes  more  and  more 
negative  on  standing,  the  metal  gradually  losing  its  passivity  and 
acquired  noble  nature.  On  the  other  hand  a  fresh  (and  therefore  active) 
surface  of  iron  exposed  to  dry  air,  though  unaltered  in  appearance, 
becomes  to  a  certain  extent  passive,  as  shown  by  its  potential  against 
an  electrolyte. 

Theories  of  Passivity.  —  The  cause  of  passivity  is  not  yet 
certainly  known.  It  is  now  generally  accepted  that  a  reaction  resist- 
ance of  some  kind  to  the  reversible  anodic  process  is  concerned,  but 

1  Hittorf,  Zeitsch.  Elektrocliem.  4,  482  (7-S.W)  ;  6,  0  (1WJ)  ;  7,  108 

2  For  the  case  of  gold  in  AuCl:i,  .see  p.  271. 


x.]  ANODIC  PROCESSES  Ui 

its  precise  nature  remains  in  dispute.     It  may  vary  in  different  cases. 
The  following  explanations  have  been  suggested  :  — 

(1)  Slow  rate  of  ionisation  of  the  anode  metal  :  e.g.  of  the  reaction 

Ni  +  2©  -  >  Ni"  (Le  Blanc). 

(2)  Slow  rate  of  combination  of  discharged  anion  and  material  of  anode, 
with  action  consequently  taking  place  between  the  former  and  water. 

E.g.  SO/'—  *S04  +  20 

S04  4-  Ni  -  >  NiS04  (slow  reaction). 
S04  +  H20  —  ^HS04  +  J02. 

(3)  Discharge   of  anions  which  react  with  water  giving  oxygen. 
Then  a  slow  co)nbination  of  the  atomic  oxygen  and  the  anode  material  to 
an  oxide  (subsequently  dissolved  by  the  electrolyte)  and  consequent 
accumulation  of  an  oxygen  layer  on  the  electrode  surface. 

E.g.  SO/'  —  >  S04  +  20. 

S04+H20  —  >H2S04  +  0. 

Ni  +  0  -  >  NiO  (slow  reaction). 

NiO  +  H2S04  -  >  NiS04  +  H20  (Fredenhagen  :  Muthmann 
and  Frauenberger). 

(4)  Formation  of  an  oxide  or  other  insoluble  compound  on  the 
anode.     Its  removal  by  decomposition  or  by  diffusion  is  slow,  and  as  it 
coats  the  electrode  and  hinders  ions  entering  the  solution,  the  potential 
rises  and  anions  are  discharged.     (Faraday  ;   Haber  and  pupils).     The 
insoluble  coating  may  be  sometimes  formed  by  discharge  of  anions,  and 
sometimes  by  metal  ions  entering  the  solution,  reacting  there  with 
anions  present,  and  forming  a  precipitate  on  the  electrode. 

(5)  Sackur  assumes  every  metal  anode  to  contain  a  certain  amount 
of  dissolved  hydrogen  formed  by  chemical  reaction  with  the  H*  ions 
in  the  electrolyte,  its  concentration  (pressure)  corresponding  to  the 
equilibrium  expressed  by  the  equation  (for  a  bi-valent  metal) 


The  primary  anodic  process  consists  in  the  discharge  of  anions. 
The  resulting  halogen  or  oxygen  combines  with  the  hydrogen  dissolved 
in  the  electrode,  giving  water  or  a  halogen  acid.  To  maintain  equili- 
brium, the  above  reaction  must  then  proceed  from  left  to  right,  with 
formation  of  fresh  hydrogen  and  dissolving  of  fresh  metal.  The  anodic 
solution  of  metals  is  consequently  a  chemical  process.  Passivity  results 
from  a  slow  rate  of  combination  of  hydrogen  with  the  gas  discharged 
from  the  electrolyte,  a  layer  of  the  latter,  usually  oxygen,  accumulating 


142    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

on    the    electrode.      The    immediate    cause    is    therefore   identical 
with  (3). 

E.g.  Ni  +  2H-  ^=±  Ni"  +  H2 

SO/'  — >  S04  +  20. 
S04  +  H20  — >  H2S04  -f  0. 
H2  +  0 >  HaO  (slow  reaction). 

It  follows  that  those  metals  should  most  readily  become  passive 
which  do  not  catalyse  the  combination  of  hydrogen  and  oxygen  to 
water.  Sackur,  in  fact,  showed  that  there  is  some  kind  of  parallelism 
between  these  two  properties.  Thus  Ni,  Fe,  and  Cr  all  catalyse  the 
velocity  of  this  reaction  very  slightly  only. 

(6)  Foerster,  from  experiments  on  iron,  assumes  with  Le  Blanc  the 
slow  reaction  to  be  ionisation  of  the  metal :  e.g.  Ni  +  2  0  — >  Ni". 
He  has  shown1  the  converse  process  (metal  deposition)  to  be  retarded, 
and  it  follows  directly  that  the  ionisation  should  then  also  suffer 
retardation.  He  goes  further  and  assumes  that  the  pure  metal  in 
these  cases  is  passive,  and  only  becomes  active  when  charged  with 
hydrogen.  On  these  lines  he  accounts  satisfactorily  for  the  effects  of 
cathodic  polarisation,  content  of  hydrogen,  standing  in  air,  oxidising 
agents,  degree  of  fineness  of  division,  etc. 

We  have  no  space  here  to  consider  the  experimental  evidence  for 
and  against  these  different  theories.2  The  actual  mechanism  of 
passivity  possibly  varies  in  different  cases.  Where  an  insoluble  film  of 
salt  or  oxide  can  actually  be  seen  on  the  anode,  that  explanation  is 
probably  the  correct  one.  But  even  then  we  can  suppose  with  Foerster 
that  such  films  are  often  the  effect,  rather  than  the  cause,  of  passivity. 
The  views  of  Foerster 3  or  of  Sackur 4  perhaps  most  nearly  approach 
the  actual  truth.  Sackur  of  course  assumes  the  primary  anodic  process 
to  consist  of  anionic  discharge,  never  of  cations  entering  the  solution, 
the  latter  being  the  view  taken  throughout  this  book. 

Aluminium  Rectifier. — One  remarkable  case  of  passivity  un- 
doubtedly due  to  a  film  of  insoluble  solid  is  furnished  by  aluminium. 
If  an  aluminium  electrode  be  made  anode  in  certain  electrolytes, 
current  passes  for  an  instant,  but  immediately  drops  to  zero.  The 
passage  of  more  current  now  demands  a  very  high  anodic  polarisation, 
depending  on  the  temperature  and  nature  of  the  electrolyte.  With  a 
strongly  acid  solution  it  may  amount  to  27  volts.  With  cooled  elec- 
trodes in  very  dilute  AmHB204  it  may  reach  600  volts.  When  the 

1  Seep.  121. 

2  For    an    excellent    critical    review    of    the     subject,    see    (Jrave,    Jnlirl>. 
Radio.  8,  91  (1911). 

3  Abhand.  Bunsen  Oes.  8,  20  (1909). 

4  Zettech.  Elektrochem.  10,  841,  929  (1904)  ;  12,  637  (1906)  ;  14,  607  (1908). 


ANODIC  PROCESSES 


143 


current  does  pass  the  process  is  of  course  a  violent  one,  arcing  and 
boiling  taking  place.  The  cause  of  this  behaviour  is  the  formatio  i 
on  the  surface  of  the  electrode  of  a  thin  compact  film  of  basic  salt  or 
hydroxide,  impervious  to  AT"  or  S04*  ions,  and  to  many  other  anions. 
H'  ions  can  pass  through,  and  hence  such  a  coated  electrode  will  act  as 
a  cathode.  Cl',  Br',  and  N03'  ions  can  also  pass  through,  and  therefore 
the  electrode  can  function  as  anode  in  solutions  containing  these  ions. 
The  work  of  G.  Schultze x  renders  it  very  probable  that  a  gas-layer 
underneath  the  layer  of  basic  salt  or  hydroxide  plays  a  part  in  the 
phenomenon.  But  that  the  layer  of  solid  also  is  effective  was  shown 
by  Taylor  and  Inglis,2  who  found  that  a  platinum  plate  coated  with 


A,  Source  of  alternating  current, 
a,  Aluminium  electrodes. 
c,  carbon  electrodes. 
F,  external  circuit. 
FIG.  37. — Aluminium  Electrolytic  Rectifier. 

A1(OH)3  acted  similarly.  The  phenomenon  is  not  peculiar  to  Al ;  Bi, 
Ta,  and  Cb  in  particular  exhibit  it  in  very  dilute  alkaline  solutions.  It 
has  been  proposed  to  use  such  a  cell  as  a  high  voltage  safety  fuse,  and 
also  to  apply  it  to  the  problem  of  converting  alternating  into  direct 
current.  For  this  purpose  the  arrangement  is  shown  in  Fig.  37.  The 
four  cells,  containing  each  an  aluminium  and  a  carbon  electrode,  are  so 
connected  with  a  source  of  alternating  current  that  electricity  of  one 
sign  only  is  continually  fed  into  the  arm  B,  and  electricity  of  the 
opposite  sign  into  the  arm  C.  If  the  external  circuit  be  now  joined  to 
B  and  C,  direct  current  will  flow  through  it.  In  the  arrangement  as 
shown,  positive  electricity  only  can  pass  from  A  through  cells  2  and  3 
into  B,  negative  electricity  only  through  cells  1  and  4  into  C.  In  the 
external  circuit  F  there  will  be  a  positive  current  flowing  from  B  to  C. 
Th  energy  efficiency  of  the  arrangement,  however,  only  averages  60 
per  cent. 

One  other  important  case  of  anodic  passivity,  that  of  hydrogen,  will 
be  met  with  later.3 

1  Zeitsch.  Elektrochem.  14,  333  (1908). 
-  Philos.  Mag.  [6],  5,  301  (1903). 
3  Pp.  213,  217,  219. 


144    PEINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

6.  Electrolytic  Oxidation 

This  is  the  last  class  of  anodic  processes  to  be  considered.  Much 
already  said  about  electrolytic  reduction  applies  here,  and  we  need  go 
into  no  very  great  detail.  Anodic  oxidation  processes  can  be  divided 
into  three  classes  : — 

(a)  Processes  involving  a  decrease  in  the  number  of  negative  charges 
or  an  increase  in  the  number  of  positive  charges  on  an  ion. 

E.g.  Cu*  +  © *  Cu" 

Fe"  +  © *  Fe"' 

Mn04"  +  ©  — >  MnO/. 

(6)  Processes  resulting  in  the  polymerisation  of  anions. 

E.g.  2S04"  — *  S208"  +20 

(persulphate). 

2C03" >C206"+2© 

(percarbonate). 

(c)  Processes  during  which  the  oxygen  content  of  a  substance 
increases  or  its  hydrogen  content  diminishes. 

E.g.  NH3  +  70H'  — >  NO/  +  5H20  +  6  © 

H .  COOH  +  20H'  — »  C02  +  2H20  +  2  ©. 

We  know  that  reversible  oxidation,  if  possible,  commences  in  every 
case  when  the  anodic  potential  has  been  raised  above  the  equilibrium 
value  for  the  given  system.     Oxidations  of  the  first  type,  apart  from 
concentration  polarisation  due  to  depletion  of  the  less  oxidising  ion 
near  the  anode,  proceed  nearly  reversibly.    The  same  general  statement  1 
holds  of  processes  of  the  second  kind.     But  in  class  (c),  reaction  resist- 
ances are  very  often  met  with.     We  can  suppose  OH7  ions  to  be  first  j 
discharged  to  oxygen,  and  the  oxygen  to  oxidise  the  reducing  agent 
present.     It  is  this  depolarisation  which  is  subject  to  the  reaction  1 
resistances,  considerable  with  most  organic  depolarisers.     Their  presence 
necessitates  an  increased  driving  force  if  the  electrode  reaction  is  to 
proceed  at  anything  above  a  very  low  rate  (current  density).     Now, 
as  the  reversible  potentials  at  which  many  of  these  oxidations  begin  are    ; 
very  near  the  potential  required  for  the  discharge  of  free  oxygen  at    ; 
the  electrode — generally  far  nearer  than  the  corresponding  potentials    \ 
in  electrolytic  reduction  are  to  the  potential  of  hydrogen  discharge — 
it  often  happens  under  such  conditions  that  this  potential  is  reached, 
oxygen  is  evolved,  and  the  yield  of  oxidised  product  falls  below  100 
per  cent.      When  once  this  has  happened  the  anodic  polarisation 
slowly  increases,  owing  to  the  gradually  rising  oxygen  overvoltage.1 

1  P.  132. 


x.]  ANODIC  PROCESSES  145 

Finally,  such  high  oxidising  potentials  are  often  reached  that  the  depo- 
lariser, if  an  organic  substance,  is  oxidised  much  more  vigorously  than 
is  intended,  and  a  low  yield  of  the  product  sought  after  results.  For 
this  reason  electrolytic  oxidation  is  far  less  used  than  might  be 
supposed,  both  in  the  laboratory  and  technically. 

Changes  of  current  density,  temperature,  and  concentration  act 
as  in  cathodic  reduction.  A  high  concentration  of  depolariser  opposes 
concentration  polarisation  and  keeps  down  the  anode  potential,  whilst 
a  high  current  density  has  the  opposite  effect.  A  rise  in  temperature 
increases  the  velocity  of  diffusion  of  the  depolariser  to  the  electrode, 
and  is  therefore  favourable  (if  the  oxidation  is  complete).  The  influence 
of  the  OH'  concentration  is  twofold.  Firstly,  it  determines  the  potential 
at  which  oxygen  evolution  can  begin.  This  is  necessarily  much  higher 
in  acid  solution,  and  the  oxidation  of  difficultly  oxidisable  substances 
is  consequently  best  carried  out  in  an  acid  electrolyte.  Secondly,  it 
may  actually  alter  the  nature  of  the  substance  submitted  to  oxidation. 
If  this  be  an  acid,  and  the  anolyte  be  made  alkaline,  we  shall  be  dealing 
with  the  anion  of  the  acid  ;  but  if  the  anolyte  be  acidified  with  a  strong 
mineral  acid,  the  depolarising  acid,  if  not  a  strong  acid,  will  largely 
be  present  as  undissociated  molecules ;  and  the  results  of  oxidising  anion 
and  free  undissociated  acid  can  sometimes  be  quite  different. 

Overvoltage  Effect  of  Electrode.— The  nature  of  the  electrode 
material  is  very  important.  As  in  cathodic  reduction,  it  can  act  in 
two  ways,  one  in  virtue  of  its  oxygen  overvoltage,  the  other  depending 
on  its  catalytic  influence  on  the  reaction  between  oxygen  and  depolariser. 
We  have  seen  that  the  oxygen  overvoltage  at  nickel  and  iron  anodes  is 
somewhat  less  than  at  a  platinised  platinum  anode,  and  considerably 
less  than  that  necessary  at  smooth  platinum.  Dealing  therefore  with  a 
substance  that  is  only  oxidised  with  difficulty  at  a  very  high  anodic 
potential,  it  is  best  to  use  a  smooth  platinum  anode  and  a  high  current 
density  which  invariably  increases  the  overvoltage  of  oxygen  dis- 
charge. But  if,  on  the  contrary,  an  oxidation  takes  place  with  a  cur- 
rent efficiency  of  less  than  100  per  cent.  (i.e.  with  the  accompaniment  of 
oxygen  evolution),  and  if  the  materials  present  are  readily  still  further 
oxidised,  such  a  metal  as  nickel  or  iron  should  be  used  as  anode.  It 
must  be  remembered  that,  as  the  full  oxygen  overvoltage  does  not 
set  in  immediately,  a  reaction  may  progress  after  a  lapse  of  time 
differently  from  the  way  it  did  at  the  start. 

The  influence  of  an  increase  of  temperature  on  an  electrolytic 
oxidation  when  oxygen  is  being  simultaneously  evolved  can  vary.  It 
acts  favourably  by  increasing  the  velocity  of  diffusion  of  the 
depolariser.  But  it  also  lowers  the  oxygen  overvoltage,  and  thus 
facilitates  the  discharge  of  oxygen.  According  as  one  or  the  other 
of  these  two  effects  predominates,  it  will  be  better  to  work  at  a  high 
or  a  low' temperature. 


146    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [c  HAP. 

Catalytic  Effect  of  Electrode. — This  may  affect  the  working 
electrode  potential,  just  as  in  electrolytic  reduction.  A  good  example 
is  the  oxidation  of  iodic  acid  (HI03)  to  periodic  acid  (HIOJ.1  With  a 
current  density  of  0'0375  amp./ cm.2,  the  reaction  takes  place  with  a 
100  per  cent,  current  efficiency  at  an  anode  of  Pb02,  and  with  a  1  per 
cent,  current  efficiency  at  a  smooth  platinum  anode.  The  reason  is  that, 
using  Pb02,  the  depolarisation  is  so  rapid  that  the  anode  potential 
is  kept  some  0*2  volt  below  the  value  necessary  for  oxygen  evolution, 
whilst  with  platinum  the  reaction  is  catalysed  very  slightly,  and  the 
potential  consequently  reaches  a  figure  at  which  oxygen  is  liberated. 
Pb02  is  often  found  to  have  a  similar  action  in  other  cases.  For 
example,  the  electrolytic  oxidation  of  Cr'"  ions. to  Cr04"  ions  also 
takes  place  far  more  readily  at  Pb02  than  at  Pt  anodes.2  It  is  highly 
probable,  as  Haber  suggests,3  that  not  only  at  Pb02  but  also  at  electrodes 
of  platinum,  nickel,  etc.,  it  is  a  superficial  coating  of  oxide,  continually 
regenerated  by  the  current,  which  does  the  oxidising,  and  not  oxygen 
at  all.4 

Catalysts  in  Electrolyte. — Not  only  the  material  of  the  elec- 
trode, but  also  additions  to  the  electrolyte,  can  act  as  catalysts  in 
anodic  oxidations.  Some  of  these  oxygen  carriers  are  very  important. 
Cerous  sulphate,  Ce2(S04)3,  is  an  example.  This  salt  can  be  readily 
anodically  oxidised  to  eerie  sulphate,  Ce(S04)2,  a  powerful  oxidising 
agent  which  reacts  rapidly  with  many  organic  substances,  converting 
them  smoothly  and  quantitatively  into  single  oxidation  products.  If 
therefore  a  suitable  organic  substance  be  added  as  depolariser  to  the 
anolyte,  together  with  some  Ce2(S04)3,  on  passsing  a  current,  the  latter 
will  be  oxidised  to  Ce(S04)2,  which  will  then  react  with  the  organic 
depolariser,  regenerating  cerous  sulphate.  This  will  be  reoxidised, 
the  result  of  the  whole  operation  being  the  quantitative  oxidation  of 
the  organic  substance.5  If  carried  out  without  the  addition  of  the 
oxygen  carrier,  the  oxidation  would  probably  have  proceeded  much 
less  smoothly  in  every  way,  owing  to  reaction  resistances.6  The  mode 
of  action  of  the  catalyst  in  this  case  is  perfectly  clear.  But  sometimes 
this  is  not  so.  Thus  the  oxidation  of  Cr'"  ions  to  Cr04"  ions  is  strongly 
catalysed  by  small  quantities  of  various  salts7—  e.g.  Na2HP04,  KF,  etc.  ; 
the  presence  of  chlorides  or  fluorides  facilitates  the  formation  of  H2S208 


1  Mullor  and  Friedberger,  Ber.  35,  2655  (1902) ;   Mullcr,  Ztitwh.  Elcktrochcm. 
10,  <U  (1M1). 

•  Miiller  and  Sollcr,  Zeitsch.  Klektrochem.  11,  863  (/W,). 

:t  Die  Elektrolytischen  Prozesse  der  Organischen  Chemie  (Moser),  p.  59  (lit  10). 

4  Cf.  the  considerations  on  pp.  133-134. 

5  Cf.  p.  130. 

6  See  also  p.  145. 

7  Schmiedt,  Dissertation  (Charlottenburg,  1909). 


x.]  ANODIC  PROCESSES  147. 

from  H2S04 ; l  and  with  a  little  Cu(HO)2  present,  ammonia  can  be 
oxidised  at  an  iron  anode  almost  quantitatively  to  NH4NP2,2  nitrogen 
being  the  chief  product  without  this  addition. 


Literature. 

Foerster.     Elektrochemie  wdsseriger  Losungen. 

Haber-Moser.    Die  Elektrolytischen  Prozesse  der  Organischen  Chemie. 

1  Elbs  and   Schonherr,   Zeitsch.  Elektrochem.   2,  250  (1896) ;  Muller,   Zcitsch. 
EhktrocJicm.  10,  776  (1904). 

-  \\\  Traube  and  Biltz,  Ber.  37,  3130  (1904). 


t  2 


CHAPTER  XI 

THE  ELECTROLYSIS  BATH 

General. — A  technical  electrolysis  bath  may  take  up  to  six 
volts  between  anode  and  cathode  when  working,  seldom  more.1  As 
current  is  usually  supplied  by  direct  current  machines  at  least  150-200 
volts,  a  number  of  electrolysis  tanks  are  generally  connected  in  series, 
the  same  current  being  passed  through  each.  Thus,  a  1,000  K.W. 
500- volt  generator  furnishes  2000  amperes.  If  cells  could  be  con- 
veniently designed  to  carry  this  current,  each  with  its  leads  absorbing 
five  volts,  the  arrangement  would  simply  consist  of  one  hundred  baths 
in  series  with  one  another  and  with  the  dynamo.  The  disadvantage 
of  this  is  that  a  mishap  in  one  of  the  cells  means  a  stoppage  of  the  whole 
plant  unless  special  means  are  taken  to  avoid  it  (movable  shunt  leads, 
etc.). 

The  remedy  is  to  use  a  dynamo  of  lower  voltage,  but  higher  amperage, 
and  to  arrange  the  cells  in  two  or  more  shorter  series,  in  parallel  with  one 
another.  This,  however,  means  more  expensive  generating  plant.  The 
amount  of  current  a  single  unit  will  take  depends  on  the  nature  of  the 
electrolysis,  on  considerations  of  convenience  of  size,  on  the  efficiency 
of  circulation  required,  on  the  working  temperature,  on  the  first  cost, 
repairs,  labour  charges,  etc.  If  the  current  density  used  is  high,  the 
current  carried  by  a  cell  of  convenient  standard  size  will  of  course 
be  greater.  If  the  working  temperature  is  to  be  kept  low,  a  unit 
with  relatively  great  cooling  surface — that  is,  one  of  small  size- 
will  be  advantageous,  and  vice  versd.  If  exceptionally  good  circula- 
tion of  the  electrolyte  is  needed,  then  again  the  units  should  be 
small,  necessitating  frequent  changes  in  the  direction  of  flow  of  the 
solution. 

Arrangement  of  Electrodes.— With  a  bath  carrying  a  large 
current,  the  use  of  one  anode  and  one  cathode  only  would  mean  the 
employment  of  a  tank  and  electrodes  of  inconvenient  size  and  shape. 
Hence,  unless  the  conditions  of  electrolysis  preclude  it,  it  is  customary 
to  have  a  large  number  of  smaller  anodes  connected  in  parallel,  alter- 

1  A  perchlorate  bath  may  take  seven  volts,  etc.     See  p.  402. 
148 


THE  ELECTROLYSIS  BATH 


149 


nating  with  a  large  number  of  smaller  cathodes  similarly  connected. 
In  this  way  the  necessary  electrode  surface  is  obtained,  and  the  elec- 
trolysis vat  can  be  constructed  in  a  convenient  rectangular  form 
(Fig.  38).  It  sometimes1  happens  that,  instead  of  connecting  many 


FIG.  38.— Paralleled  Electrodes. 

low  voltage  tanks  in  series,  it  is  convenient  for  the  total  fall  of  potential 
to  occur  in  the  one  tank.  In  that  case  the  electrodes  are  so  arranged  that 
one  side  of  each  acts  as  anode  and  the  other  side  as  cathode  to  the 
cathode  and  anode  sides  respectively  of  its  two  neighbours  (Fig.  39). 


Anode  fl 


Ca&iode, 
Side 


FIG.  39.—  Bi-  Polar  Electrodes. 


A  complete  electrolysis  process,  absorbing  the  voltage  of  an  ordinary 
vat,  will  then  take  place  between  every  pair  of  plates.  Such  electrodes 
are  termed  bi-polar  electrodes. 

Bath  Voltage. — The  voltage  drop  across  an  electrolytic  bath  is 
determined  by — 

(a)  the  reversible  decomposition  potentials  at  the  two  electrodes, 

(6)  irreversible  effects  at  the  two  electrodes, 

(c)  the  product  I X  R,  where  R  is  the  liquid  resistance  of  the  bath, 

(d)  the  voltage  drop  at  the  diaphragms  (if  any)  used  to  separate 
anolyte  and  catholyte,  and 

(e)  the  voltage  drop  in  the  leads  and  terminals,  depending  on  the 
resistance  of  their  material,  and  on  the  efficiency  of  the  electrical 
contacts  made. 

Current  density  and  temperature  are  of  importance  in  determining 

1  See  pp.  258,  327-333,*340,  387,  392. 


150    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

these  magnitudes,  (a)  is  independent  of  current  density,  and  usually 
decreases  with  rise  of  temperature.  So  does  (6)  (the  result  of  concentra- 
tion polarisation,  passivity,  overvoltage,  etc.),  whilst  a  heavy  current 
density  increases  it.  (c)  and  (d)  both  vary  directly  as  the  current,  and 
are  less  at  high  temperatures,  (e)  will  increase  with  the  current  density, 
whilst  small  rises  of  temperature  will  have  but  little  effect  (larger  ones 
will  appreciably  increase  it).  Generally  then,  from  the  point  of  view 
of  voltage,  a  bath  is  most  favourably  worked  at  a  low  current  density 
and  a  high  temperature.  But  in  technical  work  the  necessity  of  rapidly 
obtaining  a  large  output  and  the  cost  of  heating  usually  outweigh 
these  considerations. 

Not  only  the  electrolyte  between  two  electrodes,  but  also  the 
electrolyte  above,  beneath,  and  at  the  sides,  takes  part  in  the  conducting 
of  a  current,  as  the  path  followed  by  the  latter  is  not  straight,  but 
curved.  Thus  the  resistance  of  a  cell  is  always  less  than  the  calculated 
value,  assuming  that  only  the  liquid  between  the  electrodes  conducts. 
Hence  a  current  does  not  enter  and  leave  an  electrode  with  a  uniform 
current  density  at  all  points.  On  the  contrary,  this  is  perceptibly 
greater  at  all  edges  and  corners,  and  causes  the  growths  often  observed 
at  such  edges  in  the  deposition  of  metals.  For  the  same  reason  it  is 
inadvisable  to  use  too  small  electrodes,  as  then  an  appreciable  fraction 
of  the  electrolysis  may  occur  at  higher  current  densities  than  the  calcu- 
lated one,  and  we  have  seen  how  markedly  an  increased  current  density 
can  affect  the  results. 

As  material  for  connecting  leads  and  busbars,  copper  is  generally 
used,  its  conductivity  (per  unit  volume)  being  the  highest  of  all 
commonly  available  metals.  Copper  that  has  been  electrolytic-ally 
refined  has  a  particularly  high  conductivity  (twelve  times  that  of  lead, 
seven  times  that  of  iron,  four  times  that  of  zinc).  Of  late  aluminium 
has  been  coming  into  use.  Although  per  unit  volume  its  conductivity 
is  not  much  more  than  half  that  of  copper,  yet,  on  account  of  its 
extreme  lightness,  its  conductivity  per  unit  weight  exceeds  that  of 
copper.  And  its  price  is  at  present  rather  the  lower  of  the  two.  It 
is  stated  that  in  an  atmosphere  containing  chlorine  it  is  considerably 
less  attacked  than  copper.1  Voltage  losses  can  also  occur  in  the  leads 
nwin.u  to  defective  contacts.  In  copper  refining  they  have  occasionally 
amount^!  to  20  per  cent,  of  the  total  voltage  across  the  tank.2  AVheiv 
electrodes  are  permanently  fixed  in  position  they  can  be  securely 
Attached  to  their  leads,  but  when  they  are  constantly  being  removed 
and  replaced,  as  in  copper  refining,  any  elaborate  system  of  con- 
nection is  impossible.  They  are  often  then  simply  suspended  from 
bu<bars.  In  that  case  a  clean  plane  metal  surface  is  of  prime 
importance. 


1  BUetrockem.  I,;L  7,  :n 
J'   LV.i,  .     Cuiupare  also  p.  424 


XL] 


THE  ELECTROLYSIS  BATH 


151 


Cathodes.— Of  cathodes  used  in  technical  operations  where 
hydrogen,  and  not  a  metal,  is  produced,  little  need  be  said.  The  most 
important  materials  are  iron  and  graphite.  Platinum  is  only  occasion- 
ally used,1  on  account  of  its  high  price  ;  copper  gauze  is  said  to  be 
tin1  cathode  in  the  Hargreaves-Bird  electrolytic  alkali  cell ; a  lead  is 
used  in  the  electrolysis  of  dilute  H2S04  for  obtaining  hydrogen  and 
oxygon  ;3  mercury  is  the  cathode  in  the  various  'mercury'  alkali- 
chlorine  cells.4  But  usually,  when  the  catholyte  is  alkaline,  iron  is  used, 
and  when  acid,  graphite.  The  most  important  property  of  a  cathode  is 
the  amount  of  its  hydrogen  overvoltage.  Figures  are  given  on  pp.  11&- 
119.  It  is  fairly  low  for  iron  and  platinum,  which  is  fortunate,  consider- 
ing the  extended  use  of  the  former  in  alkali-chlorine  cells.  Copper, 
nickel,  and  graphite  give  values  nearly  equal,  but  a  little  higher  than 
iron.  Lead  and  mercury  have  very  high  overvoltages,  in  the  latter  case 
advantageous  when  acting  as  cathode  in  '  mercury  '  alkali-chlorine 
cells.  Platinised  platinum  of  course  would  usually  be  an  ideal  material, 
but,  apart  from  its  cost,  it  has  a  surface  that  is  easily  mechanically 
destroyed. 

Anodes.— Technical  insoluble  anodes  should  satisfy  two  conditions. 
They  should  not  be  attacked  or  disintegrated  when  under  use,  and  the 
oxygen  or  chlorine  overvoltage  should  be  low.  They  should  also,  of 
course,  conduct  electricity  well.  Of  those  used  or  suggested  we  may 
mention  platinum,  smooth  and  platinised  ;  hard  artificial  carbon  ; 
graphite  ;  Fe304  ;  carborundum  ;  ferro-silicon  ;  iron  ;  Pb02 ;  Mn02. 
Except  the  last  four,  they  are  particularly  intended  for  use  with 
chlorine.  (Ferro-silicon  and  carborundum  are  of  minor  import- 
ance.) 

Platinum  Anodes. — Pure  platinum  is  by  no  means  always 
unattacked  by  anodic  chlorine.  Under  certain  conditions  it  can  to 
some  extent  lose  its  passivity,  as  was  shown  by  Haber  and  Grinberg 5 
and  by  Bran.6  But  alloyed  with  10  per  cent.  Ir  it  is  far  more  resistive.7 
The  accompanying  Tables  XXIII  and  XXIV  show  this. 


TABLE  XXIII.    Platinum— Iridium 


1  Pp.  328,  332. 
4  P.  347. 


Electrolyte 

Current 
Densit  v. 

Temp. 

Amp.-hours 

Loss  of  weight 
•    per  d.m.2  of 

amp.  /cm.  - 

electrode. 

Cone.  KC1 

o-i 

20° 

240 

O'O  mg. 

or  NaCl 

0-167 

80° 

(  (a)  200 
/,)  200 

(a)3-omg.  \ 
(b)  0-4  mg./ 

2  P.  376.  :'  P.  387. 

•'•  Zfitwh.  Anorg.  Chem.  16,  446 


f>  Zeitsch.  Elektroclicm.  8,  197  (1902). 

'    Dcnso.  Zi-.if.vlt.  Kli-ktrochi  in.  8,  14!)  (1'.'(>'2}. 


152    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 


TABLE  XXIV.   Platinum 


Electrol 

Temp. 

Current  density 
amp.  /c.m.2 

Per  cent,  current  used 
in  dissolving  Pt 

30  per  cent.  HC1 

17-4° 

2 

o-o 

36  per  cent. 

11° 

1 

0-3 

32-5  per  cent. 

25° 

* 

10-79 

25-8  per  cent. 

25° 

A 

7-48 

20  per  cent. 

23-4° 

* 

1-02 

16-2  per  cent. 

24-5° 

^r 

0-06 

11  -2  per  cent. 

24-2° 

A 

0-081 

32  per  cent. 

50° 

i 

4-16 

32  per  cent.                      65° 

i 

5-2 

Platinum  is  expensive,  and  when  used,  the  maximum  possible  surface 
must  be  employed — e.g.  a  gauze  construction.  Such  anodes  are  fragile 
mechanically,  and  must  be  carefully  mounted.1  In  order  to  decrease 
the  chlorine  overvdtag^  platinised  electrodes  would  be  preferable. 
Here,  again,  thexfmrc^Etture  of  the  surface  prevents  their  use.  The 
use  of  grey  platinum — i.e.  platinum  which  has  been  carefully  heated  up 
for  a  few  hundpdNdegrees — would  seem  to  avoid  this  drawback.2 

Carbon  and^rfaphite  Anodes.— At  one  time  hard  artificial 
carbon  anodes  vuj£  much  us^PKT1  chloride  electrolysis.  They  are 
prepared  by  compressing  together  lampblack  and  tar,  heating  to  a 
temperature  of  Wflbout  140Qj^tnd  slowly  cooling.  When  of  good 
quality,  they  jlrTmten&elyJ^HTand  give  a  sharp  metallic  ring  when 
struck.  Used  as  aj^Ges  they  behave  satisfactorily  as  long  as  oxygen 
evolution  is  isxahfffed.  But  when  this  commences  they  usually  rapidly 
burn  away  and  disintegrate,  fouling  the  electrolyte  with  carbonaceous 
matter  and  often  with  inorganic  constituents.  Lepsius  states3  that 
the  anodes  formerly  used  in  the  Griesheim-Elektron  alkali-chlorine 
cell  lasted  about  a  year,  an  exceptionally  long  life  for  the  conditions 
of  electrolysis  in  that  cell.  Graphite  anodes  *  are  now  generally  used 
in  the  electrolysis  of  chlorides.6  They  conduct  better  than  those  of 
artificial  carbon,  are  less  readily  attacked,  do  not  disintegrate  to  the 
same  extent,  and  are  very  easily  machined  on  account  of  their  softness. 
Their  resistance  indeed  to  the  action  of  anodic  chlorine  is  extraordinary. 
If  oxygen  evolution  be  excluded,  very  little  action  can  be  detected  after 
a  year  or  more.*  Lately7  the  technique  of  making  artificial  carbon 

1  See  p.  363.  2  See  p.  339.  trochtn.  /////.  7,  isn  (iwn). 

'  s<-«-  j>.  4 '.».->.  ErocAm,  /////.  1,  21;  (/ 

'/,  5.  209  ( / 

^  Joost,  Dissertation  (Dresden,  1910).  This  publication  also  contains  results  of 
experiments  which  «ho\v  that  the  evolution  of  o\v_'< -n  from  alkaline  solution  at 
carbon  electrodes  is  considerably  facilitated,  ami  the  oorrodoo  «.f  the  electrodes 
much  diminished,  if  the  latter  are  previously  soaked  in  a  strong  solution  of  a  cobalt 
salt.  The  oxygen  evolution  t.-ikr-,  pkvr  through  the  successive  formation  and 
decomposition  of  a  higher  oxide  of  <  I -I'  I  '•'>•>>  -1  •'*  I.  241-242, 


XL] 


THE  ELECTROLYSIS  BATH 


153 


a 


electrodes  appears  to  have  been  much,  improved,  and  anodes  can 
now  be  obtained  which  are  very  nearly  as  chemically  resistive  against 
oxygen  as  those  of  graphite.  These  anodes  are  much  softer  than  the 
older  ones,  and,  although  still  really  amorphous  carbon,  resemble 
graphite  in  several  respects. 

In  cases  where  carbon  or  graphite  anodes  are  actually  attacked, 
they  are  so  constructed  that,  instead  of  having  to  scrap  the  whole 
electrode,  that  part  only  which  is  attacked  need 
be  replaced.  With  graphite  this  is  very  simply 
effected  by  screwing  the  new  part  into  a  per- 
manent seat  in  the  main  portion  of  the  anode. 
Fig.  40  shows  a  more  complicated  construction, 
such  as  is  used  by  the  Electrolytic  Alkali  Co.,  of 
Middle wich.  The  carbon  blocks  are  cemented 
into  the  copper  frame  by  molten  lead.  The 
metallic  parts  are  subsequently  covered  with 
some  such  material  as  asbestos  or  tar,  and 
finally  with  Portland  cement.  The  porosity  of 
a  carbon  anode  is  of  importance  in  determining 
the  extent  to  which  it  will  be  attacked.1 

The  use  of  Siemens  carborundum  electrodes 
in  chloride  electrolysis  has  achieved  no  import- 
ance, though  comparative  experiments  carried 
out  with  graphite  anodes  gave  favourable 
results.2  Where  graphite  had  lost  17  per  cent, 
of  its  weight,  carborundum  was  unaffected  in 
twice  the  time.  A  considerable  drawback  is 
its  low  conductivity.  Fe304,  on  the  other 
hand,  is  now  used  for  the  anodes  in  the 

Griesheim  alkali-chlorine  cell.3  It  is  both  cheap  and  resistive,  and 
for  that  particular  type  of  cell  is  the  best  electrode  available. 

Chlorine  Overvoltage. — Billiter4  found  this  to  be  very  low  at 
graphite.  For  smooth  platinum  and  magnetite,  Sacerdoti5  found 
almost  the  same  figures — in  both  cases  (at  a  current  density  of 
4  amp./ cm.2)  an  overvoltage  of  about  0*7  volt  at  18°  and  0*35  volt 
at  100°.  Billiter  (loc.  cit.)  found  far  larger  values,  those  for  magnetite 
moreover  being  much  greater  than  those  for  platinum.  At  the  same 
current  density  and  at  17°  he  obtained,  for  platinum  TO  volt,  for 

1  See  p.  345.  Also  for  studies  of  various  kinds  of  carbon  and  graphite 
electrodes,  see  Foerster,  Zeitsch.  Angew.  Chem.  14,  647  (1901) ;  Zeitsch.  Elektro- 
chem.  8,  143  (1902);  Chem.  Ind.  26,  86  (1903);  Sprosser,  Zeitsch.  Elektrochem. 
7,  (1901) ;  Joost,  loc.  cit. 

-  Zeitsch.  Elektrochem.  13,  2,  3,  12  (1907). 

3  See  p.  363. 

4  Die  Elektrochemischen  Verfahren,  etc.,  vol.  ii.,  p.  140  (1911). 
3  Zeitech.  Elektrochem.  17,  473  (1911). 


a,  copper. 

b,  carbon. 

c,  Portland  cement. 

FIG.  40.— Electrolytic 
Alkali  Co.  Anode. 


154    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

magnetite  1*9  volts:  while  at  70°  he  found  O8  and  1'4  volts  respec- 
tively. 

Anodes  to  withstand  Oxygen.— For  anodes  at  which  oxygen 
is  being  evolved  hard  carbon  and  graphite  are  useless.  They  are 
vigorously  attacked,  more  rapidly  in  acid  than  in  alkali,  carbon  more 
than  graphite.  Platinum  is  usually  ruled  out  as  too  expensive,  though 
otherwise  suitable.  For  alkaline  solutions,  iron  and  nickel  are  suitable 
on  account  of  their  passivity.  Iron  is  used  in  the  electrolytic  prepara- 
tion of  hydrogen  and  oxygen,1  and  also  employed  in  the  precipitation 
of  gold  from  the  dilute  cyanide  liquors2  used  in  its  extraction  from 
"  tailings/  In  acid  or  neutral  solution  also,  iron  is  to  a  great  extent 
passive  (particularly  at  high  current  densities)  if  chlorides,  etc.,  are 
absent.  But  nevertheless  some  dissolves,  and  if  absolutely  necessary 
to  avoid  contamination  of  the  electrolyte  with  metallic  impurities  it 
cannot  be  used.  This  condition  must  be  fulfilled  in  the  electrolytic 
deposition  of  zinc  from  acid  ZnS04  liquors  obtained  by  leaching 
calcined  zinc  ores.3  Electrodes  of  graphite,  FeaO^  hard  lead,  etc.,  are 
useless.  But  success  has  been  obtained  with  Ferchland's  anodes  of 
••  >lytically-deposited  PbO.,.4  No  trace  of  lead  appears  to  enter 
solution,  though  the  surface  of  the  anode  is  slowly  broken  up. 
Chlorides  must  presumably  be  absent.  Electrodes  of  Mn02  are  stated 
to  be  even  better,  and  promise  to  last  several  years.  They  are  also 
mechanic-ally  stronji.-r.  and  can  be  prepared  in  more  convenient 
shapes. 

Oxygen  Overvoltage.— The  table  on  p.  132  shows  that  the 
amounts  of  oxygen  overvoltage  at  nickel  and  at  iron  are  low  and  nearly 
equal  to  one  another,  whereas  at  platinum  it  is  much  higher.  In 
olution  we  find  the  overvoltage  at  platinum  to  be  far  lower,  and 
that  of  Pb02  to  be  very  nearly  equal  to  it.  Thus  Miiller  and  Seller 5 
found  the  overvoltage  at  smooth  platinum  in  n  .  H2S04  at  20°  with  a 
e  urrent  density  of  O0228  amp./ cm.2  to  vary  between  O37  and  0*42 
volt,  whilst  Pb02  under  the  same  conditions  gave  0'42  volt ;  and 
Miiller  •  found  the  overvoltage  at  Pt  to  be  by  0'03-0'08  volt  the  greater. 
Mn02  apparently  has  a  still  lower  oxygen  overvoltage.  Working  in 
"•:>  n  .  H2S04,  when  Pb02  gave  an  overvoltage  of  0'35  volt,  Schmiedt7 
found  Mn02  to  give  0'23  volt. 

Diaphragms. "—  We  finally  come  to  the  question  of  diaphragms,  one 
that  frequently  determines  the  success  or  otherwise  of  electrolytic 
processes.  A  good  diaphragm  must  first  of  all  be  of  suitable  mechanical 
strength,  and  as  far  as  possible  unattacked  chemically  by  any  solu- 
tions or  gases  with  which  it  comes  into  contact,  so  that  renewal  ex- 
penses and  disturbances  in  working  are  reduced  to  a  minimum.  Then. 

1  See  pp.  its?   388.  '  P.  27&  P.  284,  •«  E.P.  24,800  (HOfi). 

Zeitsch.  EUHrochtm.  11,  8M  ( iw-.).  ••  £,  l(,,-h.  l-.l,  tirochem.  10,  til  ( !'..<) I). 

"'  DfmHatfem  (Charlottenburg,  1909).  •  S«T  also  pp.  :if,r,.  :{,;.)   ;{7(,. 


XT.]  THE  ELECTROLYSIS  BATH  155 

whilst  capable  of  effectually  stopping  diffusion  and  convection,  it 
must  not  for  obvious  reasons  have  too  high  an  electrical  resistance. 
Generally  this  last  condition  is  comparatively  easily  satisfied,  and  most 
diaphragm  troubles  are  concerned  with  chemical  attack  or  mechanical 
weakness. 

Alkali-resisting  Diaphragms.— For  diaphragms  to  withstand  alkaline 
liquors,  asbestos  is  generally  a  good  starting  material.  The  pores  of 
ordinary  asbestos  sheet  easily  become  choked  up,  but  asbestos  which 
has  been  treated  with  acid  and  baked  acts  very  well.  A  good  example 
is  the  diaphragm  prepared  by  Bernfeld  (Leipzig).  Asbestos  also 
-es  sufficient  mechanical  strength  to  enable  it  to  be  securely 
fastened  into  position,  if  necessary  under  pressure,  without  fear  of 
breakage.  Asbestos  diaphragms  have  been  used  extensively  in  alkaline 
chloride  electrolysis,  and  we  might  particularly  refer  to  those  employed 
in  the  Townsend  and  Billiter  cells.1  Another  material  from  which 
alkali- resisting  diaphragms  can  be  prepared  is  Portland  cement.  These 
again  are  used  in  the  electrolytic  alkali  industry.2 

Acid-resisting  Diaphragms.— These  are  made  with  more  difficulty. 
Most  clays  and  porcelains  contain  too  much  basic  material  to  be  satis- 
factory. For  weak  acid  solutions,  Betts  3  recommends  that  powdered 
sulphur  be  sifted  over  asbestos  millboard,  the  whole  heated  just  above 
the  melting-point  of  sulphur,  and  then  allowed  to  stand  in  an  acid 
solution  for  a  few  weeks  before  use.  For  electrolytes  containing  much 
sulphuric  acid  and  chromic  acid,  Le  Blanc,4  together  with  the  firm  Villeroy 
and  Bock  (Mettlach),  devised  diaphragms  consisting  (in  the  finished 
state)  of  25  per  cent.  A1203  and  75  per  cent.  Si02.  Such  a  material  is 
naturally  useless  in  alkaline  electrolytes,  but  it  was  unaffected  by  the 
chromic  acid  mixture  in  the  course  of  a  year.  Its  chief  disadvantage  is 
its  brittleness,  though  the  difficulties  of  preparing  pieces  of  large  size 
were  to  a  great  extent  overcome. 

For  diaphragms  in  acid  electrolytes  the  material  recommended 
by  Buchner5  would  also  appear  to  be  suitable.  It  is  composed  of  a 
mixture  of  kaolin  and  the  artificial  corundum  resulting  as  a  by-product 
in  the  Goldschmidt  Thermite  process.  Not  only  is  it  very  resistive 
to  acids  even  at  high  temperatures,  but  its  coefficient  of  expansion  with 
heat  is  very  low.  It  consequently  does  not  readily  crack  on  cooling, 
and  plates,  etc.,  of  large  size  can  be  made.  Many  other  materials  have 
been  suggested  and  tried  at  different  times  for  diaphragm  construction, 
but  it  is  hardly  necessary  to  enumerate  them,  much  less  discuss  them. 
They  include  paper,  leather,  silk,  felt  and  parchment,  treated  in  various 
ways ;  unglazed  porcelain  and  artificial  clays,  composed  of  different 
basic  and  acid  constituents  in  almost  every  possible  proportion ;  soap, 

1  Pp.  375,  380.  2  See  p.  3<>3.       :i  Ehctrocliem.  Ind.  6,  272  (1908). 

Zeteck,  Ebtctrvchem.  7,  290  (1900).        -  Zcitsch.  Angew,  Chem.  17,  985  (1904). 


156    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY 


powdered  magnesia,   gypsum,   etc.,   etc.     Some  we  shall  meet  witHl 
again. 

Testing  of  Diaphragms.—  Chemical  questions  apart,  we  are  con-1 
cerned  with  the  efficiency  of  the  diaphragm  in  preventing  the  mixing 
of  the  liquids  on  its  two  sides,  and  with  the  extra  resistance  introduced 
into  the  electrolyte  because  of  its  use.  These  properties  should  be! 
investigated  for  every  diaphragm,  and  the 
way  of  doing  so  has  been  clearly  pointed  out 
by  Guye  and  Tardy.1  Besides  the  dimensions 
of  a  diaphragm,  something  should  be  known 
of  the  volume  of  its  pores,  of  its  action  in] 
preventing  the  passage  of  a  liquid  through  it, 
of  its  action  in  preventing  diffusion  of  a  dis- 
solved substance  from  one  side  to  the  other, 
and  finally  of  its  electrical  resistance.  The 
volume  of  the  pores  is  readily  determined  by 
weighing  the  diaphragm  dry,  and,  after 
soaking  it  in  water,  reweighing.  The  frac- 
tion of  the  total  volume  of  the  diaphragm 
due  to  the  pores  may  vary  widely  —  between 
20  per  cent,  and  70  per  cent. 

The  permeability  of  the  diaphragm, 
measuring  the  ease  with  which  a  liquid  passes 
through  it,  can  be  determined  by  the  apparatus 
in  Fig.  41.  The  diaphragm  A  is  clamped  in  a 
horizontal  position  between  the  two  vessels 
BB.  The  upper  one  of  these  is  filled  with 
a  liquid  (water  is  suitable),  and  the  liquid 
head  above  the  surface  of  the  diaphragm 
measured  on  the  scale  C.  Water  percolates 
through,  and  is  collected  in  the  vessel  D. 
The  quantity  v  which  has  passed  will  vary 

directly  as  the  area  of  the  diaphragm  a,  as  its  permeability  K,  as  the 
head  of  liquid  h,  and  as  the  density  of  the  same  S.  It  will  vary  in- 
versely as  the  viscosity  of  the  liquid  77,  and  we  get 

h.  B  .  a 


Fio.  41. — Measurement 

of  Permeability  of 

Diaphragm. 


The  measurement  of  the  diffusion  coefficient  is  carried  out  with  a 
similar  apparatus,  in  which  the  diaphragm  is  now  placed  vertically,  and 
t  hr  head  of  the  electrolyte  is  the  same  on  both  sides.  The  concentra- 
tions of  solute  on  the  two  sides  of  the  diaphragm  are  different,  and  the 
quantity  which  passes  through  in  a  certain  time  is  measured.  It  is 
best  to  stir  the  liquid  on  both  sides  of  the  diaphragm,  to  keep  the  more 

1  Jour.  Chim.  Phys. 


XL]  THE  ELECTROLYSIS  BATH  157 

concentrated  solution  saturated,  and  to  run  pure  water  continually 
through  the  other  side.  The  concentration  difference  between  the  two 
sides  is  thus  kept  constant.  If  now  the  same  salt  be  always  used  (to 
avoid  differences  of  diffusion  coefficients),  and  if  the  experiment  be 
continued  always  for  the  same  length  of  time,  the  quantity  which 
passes  through  will  be 

m-K'.?, 

a 

where  a  is  the  area  and  d  the  thickness  of  the  diaphragm,  whilst  _, 

IV. 

gives  the  relative  resistance  to  diffusion  of  the  diaphragm  material. 

The  measurement  of  electrical  resistance  is  simply  carried  out  by 
determining  the  resistance  between  two  electrodes  kept  at  a  fixed 
distance  apart  in  an  electrolyte  when  both  stand  on  the  same  side  of 
the  diaphragm,  and  then  when  they  are  on  opposite  sides.  Subtraction 
gives  us  the  apparent  resistance  of  the  diaphragm,  which  varies  very 
considerably  in  different  cases.  The  acid-resisting  diaphragms  of  Le 
Blanc— 5  mm.  in  thickness— absorbed  Q'15-0'2  volt  with  a  current 
density  of  O02  amp.  /cm.2,  which  is  a  somewhat  low  figure. 


CHAPTER  XII 

MOLTEN   ELECTROLYTES 

THE  electrolysis  of  fused  salts  is  of  considerable  importance  technically. 
In  this  chapter  we  shall  deal  with  their  general  behaviour  as  electro- 
lytes, noting  particularly  how  it  differs  from  that  of  aqueous  solutions. 
The  chief  worker  in  this  field  during  the  last  fifteen  years  has  been 
Lorenz,  and  most  of  the  results  quoted  below  are  the  work  of  him  and 
his  pupils. 

Conductivity.—  Molten  salts  are  good  conductors  of  electricity,  and 
their  conductivity  is  electrolytic.  At  anode  and  cathode  are  liberated 
the  products  we  should  expect  to  result.  Thus  PbCl2  gives  lead  and 
chlorine,  NaN03  gives  sodium  together  with  oxygen  and  nitrous  gases. 
Table  XXV  contains  the  specific  conductivities  of  a  number  of  fused 
salts  a  few  degrees  above  their  melting-points. 

TABLE  XXV 

K  (m-iprocal  ohms  per 

Salt  9  centimetre  cube) 

NaM>  318°  1-022 

KXO,  :u:i  0-646 

CaCl,  (in.  p.  774°)  800°  1-90 


XnCl,  (nearly   pure)  :HMi  O'OOlSli 

XnCl.  (impure)  258°  (i-JlM 

•~>t*  1  •:>!><> 

NaCl  (m.p.  800°)  850°  :{  •;,<> 

If  \ve  e.  .injure  the  table1  of  conductivities  of  aqueous  solutions  at  18° 
we  see  tli;it  tin-  liiMiic.  for  molten  salts  are  usually  considerably  ureater. 
though  of  the  same  order.  As  with  aqueous  solutions,  t  lie  conductivity 
increases  considerably  with  rise  of  temperature,  indeed  for  moderate 
temperatures  almost  linearly.  At  higher  temperatures  the  increase  of 
conductivity  falls  behind  the  rise  of  temperature. 

1  P.  r,}. 
158 


MOLTEN  ELECTROLYTES  159 

Tables  XX VI  and  XXVII  give  results  obtained  by  Lorenz  and  Kal- 
inus l  and  by  Arndt  and  Gessler.2 

TABLE  XXVI 
A'-Ci207  (L^and  K.)  XaX03  (L.  and  K.) 

e                          K                                 0  K 

397°                         0-1959                                   308°  0-965 

407°                         0-2198                                   318°  1-022 

417°                         0-2381                                   328°  1-065 

4.57                           0-274.-,                                   338°  1-108 

I  .-,7                           0-3109                                   348°  1-151 

477°                         0-3473                                   358°  1-195 

497°                         0-3837                                   368°  1-239 

507°                         0-4019                                   378°  1-283 

TABLE  XXVII. 

AgBr  (A.  and  G.)  C«C72  (.4.  and  G.) 

6  * 


450°  2-93 

500°  3-02 

550°  3-10 

600°  3-18 

7006  3-34 

800°  3-50 

900°  3-68 


800°  1-90 

850°  2-12 

900°  '2-3:1 

950°  2-50 

1000°  2-66 

1050°  2-76 


The  measurement  of  the  conductivity  of  fused  salts  usually  offers 
no  particular  difficulties.  The  arrangement  used  is  essentially  that 
described  on  p.  60.  Unpkttinised  platinum  electrodes  must  be 
employed,  as  platinum-black  at  high  temperatures  quickly  changes  its 
structure.  With  particularly  good  conducting  salts,  the  electrodes  must 
be  separated  by  a  capillary  tube,  in  order  to  increase  the  resistance  to 
a  convenient  amount. 

Current  Efficiency.— The  next  question  is— Is  Faraday's  Law 
valid  for  the  electrolysis  of  molten  salts  ?  That  it  is  has  been  rigorously 
proved  by  Lorenz  and  Helfenstein8  and  by  T.  W.  Richards  and  Stull.* 
But  it  happens  that  the  causes  which  lower  current  efficiencies  at 
room  temperature  are  far  more  active  at  high  temperatures.  Velocity 
of  chemical  reaction  and  velocity  of  diffusion  are  both  much  greater. 
Hence,  unless  the  anodic  and  cathodic  products  are  carefully  kept 
separated  from  one  another  and  from  the  action  of  the  electrolyte  and 
the  air,  the  yields  obtained  will  be  less  than  those  calculated  from 
Faraday's  Law.  Thus  Lorenz  and  Helfenstein,  in  the  electrolysis  of 

1  Zdttch.  Phys.  Chan.  59,  17  (1907). 

2  Zeitsch.  Elektrochem.  14,  662  (1908). 

3  Zeitsch.  Anorg.  Chem.  23,  255  (1900). 

4  Zeitsch.  Phys.  Chem.  42,  621  (1903). 


160    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

lead  chloride,  where  the  disturbing  effects  are  comparatively  small, 
obtained  the  following  figures  for  the  cathodic  current  efficiency 
at  520°  :— 

(a)  anode  protected  from  electrolyte,  97*95  per  cent.  ; 

(6)  cathode  protected  from  electrolyte,  99 '46  per  cent. ; 

(c)  both  electrodes  protected  from  electrolyte,  99 '98  per  cent. 

The  chief  sources  of  loss  of  cathodic  metal  in  the  electrolysis  of 
molten  salts  are  volatility,  diffusion  of  anode  products  to  cathode, 
formation  of  metal  *  fog/ *  and  action  of  atmospheric  oxygen.  With 


100 


10 


\\ 


\ 


\  \ 


\ 


600°          600°  700°          800°  900°          1000' 

Temperafazre,  degrees  C. 

FIG.  42. 


increase  in  temperature,  the  yield  must  therefore  fall  rapidly.  This  is 
well  shown  in  Fig.  42,  where  the  different  curves  hold  for  different  current 
densities.  The  effect  of  increased  current  density  on  current  efficiency 
simply  depends  on  the  fact  that,  whilst  the  absolute  losses  of  cathodic 
product  do  increase  to  a  certain  degree  owing  to  increased  diffusion 
from  the  anode,  yet  the  quantity  produced  in  unit  time  increases  still 
more  quickly,  and  the  current  efficiency  must  rise.2  At  very  low  current 
densities,  the  quantity  of  product  formed  in  a  given  time  may  not 
exceed  the  amount  absorbed  by  the  different  sources  of  loss,  and  the 
current  efficiency  may  consequently  fall  to  zero.  The  increase  of 
•  urront  density  is  only  limited  by  considerations  of  voltage  and  develop- 
ment of  Joule  heat  near  the  electrodes,  and  by  the  entrance  of  an 
'  anode  effect/  *  resulting  in  a  great  rise  in  voltage.  Occasionally  losses 
may  occur  owing  to  the  formation  of  a  subsalt  between  the  precipitated 
metal  and  the  melt.  This  can  happen  under  certain  conditions  in  the 
electrolysis  of  CaCl, :  red  crystals  of  CaCl  are  produced.  Or,  again,  the 

1  See  below. 

2  Of.  the  effect  of  current  concentration  (p.  30). 

3  See  below. 


xii.]  MOLTEN  ELECTROLYTES  161 

melt  may  actually  dissolve  the  metal,  forming  a  true  solution.  This 
occurs  during  the  electrolysis  of  molten  NaOH.  Such  cases  are, 
however,  rare. 

Metal  Fog.— But  there  is  onecathodic  source  of  loss  quite  peculiar 
to  molten  salts— the  '  metal  fog '  first  noticed  by  Lorenz.  If  a  metal 
such  as  zinc  or  lead  be  melted  under  one  of  its  own  fused  salts 
(e.g.  ZnCl2  or  PbCl2),  the  molten  salt  will  remain  unaffected  if  the 
temperature  be  kept  low.  But  if  it  is  raised,  dark  clouds  rise  up  from 
the  metal  and  apparently  dissolve  in  the  melt,  and  this  continues 
until  a  state  of  equilibrium  is  reached.  With  lead  in  PbCl2,  a  yellow 
melt  results,  and  at  higher  temperatures  a  brown  one.  Zinc  gives  a 
bluish-coloured  fog,  silver  a  black  one.  When  the  temperature 
falls,  the  clouds  settle  down  slowly,  and  finally  re-enter  the  metal. 
If  the  excess  of  massive  metal  be  removed  and  air  kept  excluded, 
then  the  coloured  melt  appears  stable.  But  if  oxygen  be  admitted, 
or  a  trace  of  an  oxidising  agent  added,  the  colour  disappears. 

On  the  other  hand,  it  can  be  produced  in  the  absence  of  the  massive 
metal  by  the  addition  of  a  small  quantity  of  a  reducing  agent.  Essen- 
tially the  same  phenomena  must  occur  during  the  electrolysis  of  molten 
salts,  and  will  adversely  affect  the  yield,  as  the  metal  present  in  that 
form  is  far  more  readily  attacked  chemically  than  the  massive 
metal.  It  is  true  that  only  a  small  quantity  of  metal  «  0*1  per  cent.) 
is  present  in  such  melts,  even  in  those  which  have  a  very  dark 
appearance.1 

The  exact  nature  of  these  metal  fogs  is  a  matter  of  conjecture.  It  is 
most  probable  that  they  are  analogous  to  colloidal  solutions — are  in 
fact  colloidal  suspensions  of  very  finely  divided  drops  of  fused  metal. 
In  this  connection  it  is  interesting  to  notice  the  effect  on  the  metal  fog 
formation  of  additions  of  certain  neutral  salts  to  the  original  melt. 
It  largely  prevents  fog  formation,  just  as  the  addition  of  electrolytes 
to  an  aqueous  colloidal  solution  may  precipitate  the  colloid.  The  mode 
of  action  in  the  two  cases  can  hardly  be  the  same,  however.  Lorenz  sup- 
poses 2  that  when  fused  metal  and  fused  salt  come  into  contact,  a  certain 
amount  of  complex  cation  is  produced,  thus  perhaps  Pb"  -j-  Pb  *~~^  Pb2" 
(like  the  Hg2"  ion).  The  metal  fog  would  then  represent  metal 
dissociated  off  from  this  complex  and  in  equilibrium  with  it.  The 
effect  of  the  added  salts  he  explains  by  saying  that  they  remove  the 
cation  of  the  first  salt  from  the  melt  as  complex  anion3— thus  perhaps 

Pb"  +  2C1'  +  KC1 — >  K*  +  PbCV. 

The  concentration  of  complex  cation,  and  hence  the  metal  fog,  must 
therefore  become  less. 

1  Lorenz,  v.  Hevesy,  and  Wolff,  Zeitsch.  Phys.  Chem.   76,  732  (1911). 
-  Loc.  cit.  3  See  p.  57. 


PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 


The     effect     of     this     addition     of 
be    studied    by    measuring    its    effect 


1211    10    987654     3210 

MoUcnLes.FbCljg 
01234     5678     9    10   11  12 


FIG.  43. 

Per  cent.  FeCl3  added 
0-0 
0-005 
0-03 
0-1 
0-4 
0-8 
1-6 
3-2 


PbCk  at  600° 


neutral  electrolytes  can 
on  the  cathodic  current 
efficiency  during  electro- 
lysis. As  the  fog  forma- 
tion is  one  of  the  chief 
sources  of  loss  in  the 
preparation  of  metal,  the 
two  effects  will  run  more 
or  less  parallel.  The  in- 
fluence of  the  addition 
of  different  salts  on  the 
course  of  electrolysis  of 
molten  PbCl2  is  shown  in 
Fig.  43.1  In  other  cases, 
where  the  electrolysis 
losses  under  normal  con- 
ditions are  greater,  the 
effects  produced  are  even 
more  marked.  But  not 
every  addition  to  the 
electrolyte  increases  the 
current  efficiency.  Thus, 
if  FeCl3  be  added  to 
PbCl2;  the  yield  diminishes 
considerably,  even  if 
the  addition  be  very 
small. 

Appelberg  obtained  the 
following  figures 2 : 

Current  efficiency 
96-3 
95-6 
87-6 
76-8 
70-5 
51-7 
22-3 
19-8 


The  effect  here  can  doubtless  be  explained  by  the  FeCl3  being 
reduced  at  the  cathode,  either  electrochemically  or  by  the  action  of 
the  metal  fog.  The  FeCl2  is  reoxidised  at  the  anode,  and  the  losses 
thus  become  continuous.  Traces  of  iron  entering  a  fused  melt  from 

1  Lorenz,  Zeihch.  Elektrochem.  18,  582  (1907). 
7  Zeittch.  Anorg.  Chem.  36,  'M  (I'.m::). 


XIL]  MOLTEN  ELECTROLYTES  163 

the  material  of  the  containing  vessel  may  thus  exert  considerable 
influence  on  the  electrolysis. 

Voltage  Relations.— When  discussing  the  E.M.F.'s  of  cells  with 
aqueous  electrolytes,  we  saw  that  the  values  depended  on  the  concen- 
trations of  the  solutions.  With  pure  fused  salts,  questions  of  con- 
centration do  not  enter.  The  reversible  E.M.F.  of  a  primary  cell 
depends  simply  on  the  nature  of  electrodes  and  electrolyte  and  on  the 
temperature.  So  does  the  corresponding  decomposition  voltage.  And 
these  are  of  course  equal.  For  example,  the  decomposition  voltage 
of  fused  PbCl2  at  570°  is  1'25  volts,  whilst  the  E.M.F.  of  the  cell 
Pb  PbCL  C12  at  the  same  temperature  is  T24  volts.  But  with 

fused 

mixtures  of  salts  it  can  be  shown  that  the  E.M.F.'s  depend  on 
the  concentration  of  the  salt  corresponding  to  the  metal  used  as 
electrode.  Thus  Gordon1  measured  cells  made  up  as  follows  : 


Ag 


AgN03  in  a 

mixture    of 

KN03+NaNO; 


AgN03  in  a 

mixture    of 

KN03+NaN03 


Ag, 


the  AgN03  concentration  in  the  melts  surrounding  the  two  electrodes 
being  different.  He  found  that  the  E.M.F.'s  of  such  cells  could  be 
expressed  by  the  well-known  formula  for  concentration  cells,2 


where  n  is  of  course  one  in  this  case  and  [Cj]  and  [C2]  are  the  molecular 
concentrations  of  the  AgN03  (in  aqueous  cells  [CJ  and  [C2]  represent 
the  gram-ionic  concentrations). 

We  know  that  the  reversible  E.M.F.  of  a  primary  cell  is  a  measure  of 
the  useful  work  which  the  corresponding  chemical  reaction  can  furnish. 
This  varies  with  temperature,  and  as  a  large  range  of  working  tempera- 
tures is  possible  with  fused  salts,  the  corresponding  E.M.F.'s  or  decom- 
position voltages  can  show  considerable  variation.  With  cells  of  the 
type  metal  salt  halogen,  or  with  electrolytic  processes  corresponding 
to  the  reaction  salt  --  >  metal  +  halogen  or  non-metal  (the  kind  of 
process  with  which  we  shall  principally  have  to  deal),  the  E.M.F.  or 
decomposition  voltage  diminishes  as  the  temperature  rises,  corresponding 
to  the  tendency  of  the  compound  to  dissociate  more  and  more  with  rise 
of  temperature,  the  free  energy  liberated  during  the  combination  of  its 
constituents  consequently  becoming  less.  As  examples  we  may  take 
the  cells  Ag  AgCl  C12  and  Pb  j  PbBr2  Br2.3 

1  ZeitscJi.  Phys.  Chem.  28,  302  (1899). 

2  See  p.   104. 

3  Czepinski,  Zeitsch.  Anorg.  Chem.  19,  208  (1899). 

M  2 


164    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 


TABLE  XXVIII 


Ag  |  AgCl  \  CL 


6  E.M.F. 

480° :  0-903  volt 

520°  0-891 

560°  0-882 

600°  0-871 

620°  0-865 

660°  0-854 


Pb\PbBr,\Br. 


E.M.F. 


310°  1-132  volt 

411°  1-095 

502°  1-022 

580°  0-960 

680°  0-873 

740°  0-808 


From  the  point  of  view  of  voltage  alone,  it  would  be  advantageous 
to  use  high  temperatures  in  technical  fused-salts  work,  but  this  gain 
would  be  more  than  neutralised  by  the  increased  wear  and  tear  of  the 
apparatus  and  by  the  heat  expenditure  necessary  to  compensate  for 
radiation  losses.  Decreased  current  efficiencies  would  also  result. 

Anode  Effect. — Irreversible  voltage  effects,  etc.,  are  far  less  important 
with  molten  salts  than  with  aqueous  solutions.  Such  effects  ultimately 
depend  on  the  low  velocity  of  some  stage  of  the  electrode  process,  and, 
in  view  of  the  known  effect  of  temperature  on  reaction  velocity,  it  is 
natural  that  at  high  temperatures  they  should  be  in  general  very  slight. 
But  nevertheless,  when  the  current  density  becomes  excessive,  they 
may  make  their  appearance  ;  and  particularly  what  is  known  as  the 
anode  effect  is  observed  at  carbon  anodes  in  fused  metallic  halides. 
The  electrode  becomes  covered  with  a  film  of  gas — C12,  F2,  Br2  or  I2 — 
through  which  the  current  can  only  pass  as  an  arc  discharge.  The 
voltage  rises  very  considerably  and  the  anode  appears  to  glow,  as  a 
consequence  of  the  number  of  tiny  arcs  which  are  passing.  The 
anodic  current  densities  above  which  this  happens  will  vary  with 
the  nature  of  the  electrolysis  and  the  temperature  ;  they  average  about 
4-5  amp.  /cm.2  with  hard  carbon  and  7-8  amp.  /cm.2  with  graphite. 

The  phenomenon  has  been  observed  with  chlorides  of  Pb,  Cd,  Ag,  Ca, 
Mg,  Al ;  with  bromides  of  Ag  and  Pb  ;  with  PbI2 ;  and  with  Na3AlF6 
(cryolite)  and  AlFg.1  It  is  most  pronounced  with  fluorides,  least  so 
with  iodides.  It  is  sometimes  accompanied  by  loss  in  weight  of  the 
anodes,  owing  to  formation  of  halides  of  carbon,  the  more  the  higher  the 
temperature  and  the  heavier  the  current  density.  The  cause  of  this 
behaviour  is  that  the  gas  is  liberated  at  the  electrode  more  quickly 
than  it  can  stream  away,  and  that  the  current  can  then  only  pass  as 
an  arc  discharge  between  electrolyte  and  anode.  It  should  be  men- 
tioned that  by  stirring,  or  by  raising  the  anode  from  the  melt  for  a 
moment,  or  by  reversing  the  current— in  other  words,  by  removing 

1  See,  amongst  other  references,  Hulin,  Zeitsch.  Angew.  Chem.  11,  159  (1898); 
Lorenz  and  Czepinski,  Zeitsch.  Anorg.  Chem.  19.  246  (189ft)  ;  Muthmann,  Hofer 
an. I  Weiss,  Lieb.  Ann.  820,  237  (1901)  ;  Wohler,  Zeitsch.  Elektrochem.  11,  612 
(1905);  Arndt  and  Willner,  Ber.  40,  3,025  (J907) ;  A.  Oettel,  Dissertation  (Dresden, 
1908);  Kailan,  Zeitsch.  Anorg.  Chem.  68,  141  (1910);  particularly  Frary  and 
Badger,  Trans.  Amer.  Electrochem.  Soc.  16, 


XII.] 


MOLTEN  ELECTROLYTES 


165 


the  gas  layer— the  effect  can  be  made  to  disappear.  Kailan  found  that 
an  increase  of  temperature  at  constant  current  density  acted  similarly. 

The  only  other  appreciable  irreversible  effect  is  the  concentration 
polarisation  which  occurs  at  high  current  densities.  Apart  from  any 
formation  of  gas  layer  on  the  electrode,  the  voltage  of  a  working  cell 
increases  with  current  density,  just  as  with  aqueous  electrolytes.  In 
the  latter  case  the  cause  is  exhaustion  of  ions  near  the  electrode,  and 
if  we  assume  the  presence  of  ions  in  molten  salts,  we  can  suppose  the 
same  cause  to  be  active  there. 

The  reversible  E.M.F.  of  a  cell  can  be  determined  by  either  decom- 
position voltage  or  polarisation  discharge  method.  In  the  latter  case, 
the  curve  obtained  falls  far  more  rapidly  than  a  similar  curve  got 
with  an  aqueous  electrolyte,  owing  to  the  high  temperature  and  the 
rapid  rate  of  diffusion  of  products  away  from  the  electrodes. 

Owing  to  the  differences  in  decomposition  potential  shown  by 
different  fused  salts,  a  separation  of  the  several  metallic  constituents 
from  a  mixture  is  possible  in  the  molten  state,  just  as  in  aqueous 
solution.  The  relations  of  course  may  be  rendered  rather  complex  by 
the  varying  tendencies  of  the  metals  to  alloy  amongst  themselves,  and 
thus  to  depolarise  one  another's  discharge.  With  a  mixture  of  the 
chlorides  of  silver,  lead,  and  zinc,  Lorenz  x  obtained  results  shown  in 
Fig.  44.  The  abscissse  represent  the  quantity  of  electricity  passed  since 
the  commencement  of  electrolysis,  and  the  ordinates  the  composition 


A 


80         120        160        200        240        280 
Ampere  -mtnat&s. 
FIG.  44*. 


320 


360       400 


of  the  cathode  product  taken  from  the  cell  at  the  corresponding  in- 
tervals of  time.  The  silver  is  preferentially  deposited,  then  the  lead, 
finally  the  zinc. 

Constitution  of  Fused  Electrolytes.— An   interesting   point  which 
we  cannot  discuss  here  is  how  far  the  theory  of  electrolytic  ionic 

1  ZeiUcTi.  Anorg.  Chem.  10,  78  (1895). 


166      PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY 

dissociation  is  applicable  to  fused  electrolytes.  We  can  only 
say  that  all  the  evidence  renders  it  probable  that  ions  as  we 
know  them  in  aqueous  solution  are  here  also.  But  as  to  what 
<-xt<>nt  that  is  so,  and  as  to  what  laws  govern  their  equilibrium 
with  the  rest  of  the  melt,  nothing  definite  can  be  stated.  It  is  certain 
that  the  molecular  condition  of  a  fused  salt  is  very  complex.  Thus, 
fused  lead  chloride,  besides  simple  molecules  PbCL  and  simple  ions 
Pb"  and  Cl',  contains  without  doubt  complex  neutral  molecules  (PbCl2)n, 
and  also  very  probably  complex  anions,  such  as  PbCl3'  or  Pb2Cl5'. 
This  last  statement  is  rendered  likely  by  the  results  of  migration 
experiments  of  Lorenz  and  Ruckstuhl x  on  mixtures  of  PbCl2  and  KC1. 
n,,h  was  found  to  be  negative  when  excess  of  KC1  was  present,  and  this 
indicates  the  presence  of  lead  as  anion.2 

Apparatus. — As  far  as  possible  a  fused-salt  bath  is  worked  so  that 
its  temperature  is  kept  up  by  the  Joule  heat  supplied  by  the  current. 
No  external  heating  is  used,  and  the  walls  become  lined  with  a  solidified 
layer  of  the  electrolyte.  The  longer  life  of  the  plant  thus  secured  com- 
pensates for  the  use  of  the  more  expensive  electric  heating.  The  walls 
of  the  vessel  are  made  of  iron,  or  of  some  suitable  refractory,  depending 
on  the  nature  of  the  electrolyte.  The  cathode  employed  often  consists 
of  the  precipitated  metal  itself.  In  other  cases  (e.g.  with  MgCl2  or 
NaOH)  iron  can  be  used,  whilst  graphite  is  suitable  in  halide  melts  if 
the  current  density  be  not  too  high,  in  which  case  it  disintegrates.3 
As  anode  material,  when  a  halogen  is  being  evolved,  graphite  is  by  far 
the  best.  At  high  current  densities,  however,  such  as  are  responsible 
for  the  anode  effect,  it  can  combine  with  the  halogen  and  rapidly  lose 
weight.  With  NaOH,  iron  or  nickel  can  be  used,  and,  in  the  manu- 
facture of  aluminium,  where  oxygen  is  evolved  from  a  cryolite-alumina 
bath  at  950°,  carbon  anodes  which  burn  away  continuously  are 
employed. 


Literature 
Lorenz.    Die  Elektrolyse  geschmohener  Sake.    Vols.  II  and  III. 

>*ch.  Anorg.  Ghent.  52,  41  (7507). 
8  See  p.  162  ;  also  p.  57.. 

K< -1111111  TIT,  Trans.  Amer.  Electrochcm .  Xor.  9,  117  (1906). 


CHAPTEK  XIII 

GENERAL  PRINCIPLES  OF   ELECTROTHERMICS 
1.  Electric  Heating 

So  far  we  have  dealt  only  with  electrolytic  processes.  But  an 
exceedingly  important  branch  of  applied  electrochemistry  is  that  of 
electrothermics,  the  chemical  effect  resulting  from  a  heat  effect,  itself 
produced  by  electrical  means.  On  p.  4  we  briefly  noticed  some 
of  the  advantages  of  electric  heating,  and  must  now  consider  the  matter 
in  more  detail. 

We  can  distinguish  three  types  of  electric  heating.     In  the  first, 


I       I       I       I        I 


II       I 


%  I  '  I  '  I 


FIG.  45. — Resistance  Furnace. 

the  current  passes  through  some  resistant  material,  and  the  heat 
thus  produced  is  imparted  to  the  reacting  substances  (Fig.  45).  These 
may  surround  the  resistor  where  the  heat  is  produced,  or  may  actually 
themselves  entirely  or  in  part  form  the  resistor.  Examples  are  the 
carborundum  and  cyanamide  furnaces.  Secondly  (Fig.  46),  the  heat 
may  be  produced  by  means  of  a  low  voltage  arc  discharge,1  in  which 
case  it  is  transmitted  to  the  reacting  substances  by  radiation.  The 
Stassano  steel  and  the  de  Laval  zinc  furnaces  work  on  this  principle. 
In  many  technical  furnaces,  including  some  of  the  most  important, 
the  arc  is  the  chief  heating  agent,  but  a  certain  amount  of  resistance 
heating  also  occurs.  The  ferro-silicon  furnace  is  an  example.  Thirdly, 
we  have  induction  heating,  only  possible  with  alternating  currents. 
Here  the  furnace  consists  essentially  of  a  step-down  transformer, 

1  Strictly  a  special  case  of  resistance  heating. 
167 


FIG.  46. — Arc  Furnace. 


168    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

with  a  short-circuited  secondary  in  which  enormous  currents,  and  con- 
sequently powerful  heating  effects,  are  produced.  The  well-known 
Kjellin  steel  furnace  is  an  example.  As  induction  heating  differs  con- 
siderably from  other  forms  of  electrical 
heating,  we  will  defer  its  consideration  to 
Chap.  XXIV,  dealing  with  the  electro- 
metallurgy of  iron  and  steel. 

Let  us  now  compare  electrical  and  fuel 
heating,  assuming  at  first  that  the  fuel  or 
products  of  combustion  come  into  direct 
contact  with  the  charge. 

Temperature  Range.— The  first  point 
is  the  available  range  of  temperature, 
and  there  electrical  heating  has  a  great  advantage.  The  temperature 
attainable  by  the  combustion  of  a  fuel  depends  essentially  on  its  heat 
of  combustion  and  on  the  specific  heats  of  the  products.  Consider 
the  combustion  of  pure  carbon  by  means  of  the  theoretical  quantity 
of  oxygen  to  C02.  When  12  kilos  of  carbon  are  burnt,  44  kilos 
of  C02  result,  and  the  heat  liberated,  if  both  starting  materials 
and  final  product  are  at  room  temperature,  is  97,200  Cals.  If  all  this 
heat  were  imparted  to  the  C02,  we  should  have,  putting  the  mean 
molecular  specific  heat  of  the  latter  between  room  temperature  and 
0°  C.  at  constant  pressure  as  7'5  +  0'0037  0, 

97,200  =  (7-5  +  0-0037  0}  0, 

whence  0  =  4210°. 

4200°  C.  is  thus  the  highest  temperature  attainable  by  the  combus- 
tion of  carbon,  assuming  it  to  be  complete,  to  consume  the  theoretical 
minimum  quantity  of  oxygen,  and  the  heat  produced  to  be  imparted 
solely  to  the  C02  resulting. 

But  in  practice  these  conditions  are  never  fulfilled.  Firstly,  the 
combustion  is  not  complete.  C02  dissociates  as  the  temperature  is 
raised  into  CO  and  oxygen,  and  appreciably  above  2000°  C.  Thus  at 
2000°  and  atmospheric  pressure  6  per  cent,  is  dissociated,  at  2800°, 
16  per  cent.  Consequently,  if  carbon  on  burning  actually  gave  pro- 
ducts at  such  high  temperatures,  the  heat  liberated  would  not  be  that 
furnished  by  its  complete  combustion  to  C02,  but  rather  that  produced 
by  its  combustion  partly  to  C02  and  partly  to  CO.  Further,  the 
heat  actually  liberated  is  not  entirely  imparted  to  the  gaseous  products, 
but  in  part  is  lost  by  conduction  through  the  furnace  hearth  and  walls. 
Lastly,  and  of  most  importance,  the  fuel  is  not  burnt  by  oxygen,  but 
by  air,  and  apart  from  any  excess  of  air  which  may  be  used,  the  heat 
generated  heats  up  four  volumes  of  nitrogen  for  every  one  volume  of 
COj  formed,  the  maximum  possible  temperature  being  thus  corre- 
spondingly depressed.  -  Under  these  circumstances  it  is  not  surprising 


xiii.]  ELECTROTHERMICS  169 

that  the  maximum  temperature  attainable  with  ordinary  fuels  is  about 
1800°  C. 

In  electric  heating,  the  quantity  of  heat  produced  per  second  in  a 

T2T5 

given  system  is  equal  to  --—  Cals.,  where  I  is  the  current  flowing  in 
4:*j.y 

amperes,  and  R  the  resistance  in  ohms  of  the  system.  The  temperature 
is  determined  by  the  heat  capacity  of  the  system  and  by  the  conduction 
and  radiation  losses,  etc.  etc.  If  the  latter  could  be  entirely  avoided, 
it  should  be  possible  to  increase  the  temperature  indefinitely  by  merely 
increasing  the  current  passing. 

But  in  practice  a  limit  is  set  by  these  losses,  and  particularly  by  the 
volatility  of  the  resistor  or  of  the  electrodes  between  which  the  arc  is 
passing.  At  this  boiling  or  volatilisation  point,  as  the  case  may  be, 
the  heat  produced  will  be  largely  absorbed  as  latent  heat  of  vaporisation 
or  volatilisation  respectively,  and  the  temperature  will  rise  no  further. 
This  limit  is  generally  far  higher  than  is  the  case  with  fuels.  With  an  arc 
between  carbon  electrodes,  for  example,  it  is  about  3500°  C.  Hence, 
when  operations  are  to  be  carried  out  demanding  exceedingly  high 
temperatures,  electrical  heating  can  be  employed  where  fuel  heating 
would  be  useless.  Thus  it  becomes  possible  to  treat  highly  refractory 
ores  and  to  carry  out  endothermic  reactions  (which  take  place  best  at 
high  temperatures1),  to  eliminate  rapidly  impurities  which  are  only 
removed  slowly  at  lower  temperatures,  as  in  steel  refining,  and  further, 
to  procure  good  castings  of  very  difficultly  fusible  metals  and  alloys. 

Then  there  are  other  important  advantages.  By  a  simple  regulation 
of  the  power  consumed  in  a  furnace,  effected  by  altering  the  applied 
voltage  or  the  position  of  the  electrodes,  the  temperature  can  be  varied 
within  wide  limits  and  also  kept  very  constant.  Further,  the  heat 
generated  is  localised  and  produced  exactly  where  wanted.  In  all 
electric  furnaces  the  reacting  substances  and  the  source  of  heat  are 
very  close  together  :  often  the  charge  actually  envelops  the  region  where 
the  heat  is  produced.  It  is  obvious  that  the  utilisation  of  the  heat 
must  be  far  more  perfect  than  with  fuel,  in  which  case  the  heat  is 
developed  in  a  more  diffused  manner,  and  where  there  are  consider- 
able losses  owing  to  the  hot  waste  gases. 

The  fact  that  in  many  forms  of  furnace  the  heat  is  generated  in  the 
centre  of  the  charge  has  another  important  advantage,  in  that  the  walls 
of  the  furnace  are  thereby  protected  from  its  direct  action,and  expenses 
of  repairs  are  consequently  lessened.  It  is  a  mistake  to  suppose,  as  is 
often  done,  that  electric  furnaces  require  linings  of  most  exceptional 
refractory  properties.  If  we  except  carbon  itself,  no  refractory  could 
stand  the  direct  action  of  the  electric  arc,  and  with  this  therefore 
excluded,  it  becomes  possible  to  use  the  ordinary  technical  materials. 

1  P.  23. 


170    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

Finally,  the  furnace  product  is  free  from  those  impurities  which  are 
necessarily  present  when  fuel  is  mixed  with  the  charge  or  when  fuel 
gases  have  played  on  its  surface. 

Several  of  the  above  statements  also  hold  good  for  a  comparison 

'•n  fuel  heating  with  the  heat  transmitted  through  the  walls  of  a 
it-tort  or  muffle,  and  electrical  heating.  In  the  latter  case  the  utilisation 
of  heat  is  far  better,  and  the  retort  or  muffle  is  not  exposed  to  the 
action  of  hot  flue  gases. 

Comparative  Cost.—  Electrical  heating  has  only  one  essential  dis- 
advantage as  compared  with  fuel  heating,  but  that  is  often  insuperable. 
It  is  generally  considerably  more  expensive.  For  our  present  purpose, 

ill  define,  with  J.  W.  Richards,  the  efficiency  of  a  furnace  as  the 
ratio  of  the  heat  usefully  employed  in  heating  or  causing  chemical  or 
physical  changes  to  the  total  heat  used.  The  efficiency  of  electric 
furnaces  varies  considerably  according  to  the  process  in  question  and 
the  design  and  size  of  the  furnace.  Generally  speaking,  the  larger  the 
latter  is,  the  smaller  are  the  radiation  losses  in  comparison  to  its  load, 
and  the  higher  the  efficiency.  Indeed,  results,  which  on  a  small  scale 
appear  to  be  hopeless,  may  prove  to  be  quite  satisfactory  with  a  large 
commercial  unit.  One  can  perhaps  set  the  average  efficiency  (so  defined) 
of  a  well-designed  technical  furnace  at  about  70  per  cent.  It  depends 

,  <>„  //„•  tt  n.furature,  being  greater  the  lower  this  is.  Thus  Snyder  x 
describes  a  35  K.W.  electric  resistance  furnace  used  for  the  distillation 
of  turpentine  from  wood,  in  which  the  maximum  temperature  is  190° 
and  the  efficiency  about  96  per  cent.  The  efficiency  of  a  fuel-heated 
furnace  shows  similar  but  greater  variations.  If  fuel  and  charge  are 
mixed,  the  efficiency  will  be  comparatively  good,  though  lower  than 
that  of  an  electric  furnace.  Let  us  assume  50  per  cent.  But  if  a  high 
temperature  is  needed,  involving  the  use  of  a  powerful  blast,  then  the 
efficiency  falls  off.  Whilst  if  the  question  is  one  of  external  heating 
of  muffles  or  crucibles,  the  utilisation  of  heat  is  small,  and  at  high 

ratures—  1500°  C.  and  over—  very  small.  It  may  easily  fall 
to  5  per  cent,  or  less.  For  purposes  of  comparison  with  electrical 

K,  this  rapid  decrease  of  the  efficiency  of  fuel  heating   with  rise 

M})'-]  at  ure  must  be  strongly  emphasised. 
Now  the  heat  equivalent  of  a  H.P.  year  is 


of  which  we  assume  70  per  cent.,  or  3,930,000  Cals.,  are  utilised  by  the 
•  •Icct  He  furnace.  As  a  mean  value  for  the  calorific  power  (units  of  heat 
generate,!  |,v  combustion  of  one  unit  mass)  of  solid  fuel  we  can  take 
7,800  Cals.  per  kilo,  or  7,800,000  Cals.  per  ton.  The  fraction  of  this  total 

<.  Amer.  Eleclrochem.  Soc.  13,  371  (I'.KiH)  ;   19,  191  (/.'///). 


xiii.]  ELECTROTHERMICS  171 

usefully  utilised  varies  between  3  per  cent,  and  50  per  cent.  One 
ton  of  fuel  therefore  furnishes  230,000-3,900,000  Gals,  for  the  carrying 
out  of  the  process.  Hence,  other  conditions  apart,  with  some  processes 
it  will  be  more  economical  to  employ  electrical  heating  only  when  a  H.P. 
year  can  be  got  for  less  than  the  price  of  a  ton  of  fuel ;  whilst  in  other 
cases,  electrical  heating  is  more  economical  when  a  H.P.  year  costs  less 
than  13-14  tons  of  fuel.  Obviously  the  first  condition  corresponds  to 
a  country  where  fuel  is  dear  and  power  cheap  ;  whilst  for  the  second 
extreme  case  mentioned,  it  should  be  advantageous  to  use  electric 
heating  even  where  fuel  is  very  cheap.  In  many  processes  it  would 
probably  pay  to  carry  out  the  first  part  of  the  heating,  during  which 
the  temperature  is  low  and  the  thermal  efficiency  high,  by  means  of 
fuel  firing  ;  and  then  employ  the  more  expensive  electric  heating  for 
the  final  high  temperature  stages  when  the  efficiency  of  fuel  heating  is 
correspondingly  low. 

Conclusions. — We  thus  arrive  at  the  following  conclusions  : 
(a)  electrical  heating  is  essential  for  very  high  temperature  work  ; 
(6)  it  is  often  more  economical  for  temperatures  which,  though  high, 
can  nevertheless  be  attained  by  fuel  firing  ; 

(c)  it  is  to  be  recommended  if  any  specially  pure  product  is  needed  ; 

(d)  for  most  purposes  where  temperatures  lower  than  1400°  C.  are 
required,  its  use  is  only  economical  in  certain  exceptional  regions  where 
power  is  cheap  and  fuel  expensive.1 


2.  General  Principles  of  Electric  Furnace  Design 

Electric  furnaces  may  work  continuously  or  discontinuously.  Arc 
heating  can  be  used  in  both  cases,  but  resistance  heating  by  itself  is 
practically  only  applied  to  discontinuous  processes.  To  enter  at  all 
precisely  into  electric  furnace  design  would  be  impossible  ;  here  we 
can  only  mention  a  few  sufficiently  obvious  general  principles.  Thus 
an  electric  furnace  should  be  as  simple  in  construction  as  possible. 
This,  of  course,  is  an  excellent  rule  for  the  design  of  all  large-scale 
chemical  plant,  but  particularly  so  for  apparatus  exposed  to  high 
temperatures,  or  through  which  an  electric  current  passes.  Every 
extra  complication  means  more  care  in  handling,  and  increases  the 
possibility  of  a  breakdown.  An  electric  furnace  should  be  so  con- 
structed that  all  parts  of  it,  particularly  those  likely  to  need  frequent 
repair,  are  readily  accessible.  To  minimise  these  necessary  repairs, 
the  arc  (assuming  an  arc  furnace)  must  not  be  allowed  to  play  directly 
on  any  part  of  the  hearth,  walls,  or  roof.  If  possible  it  should  be 

1  One  or  other  of  the  distinctive  characteristics  of  electric  heating  may  lead  to 
its  introduction  in  certain  cases  ;  for  example,  in  the  turpentine  furnace  men- 
tioned above,  the  ease  with  which  a  constant  and  regular  low  temperature  can  be 
produced.  A  higher  temperature  Avould  destroy  the  turpentine. 


172    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

entirely  surrounded  by  the  charge.  But  the  charge  should  never  be 
in  contact  with  the  electrodes  at  the  point  where  the  latter  pass  through 
the  furnace  lining.  If  this  occurs,  it  is  difficult  with  high  current 
densities  to  avoid  fusing  the  refractory  and  destroying  the  furnace 
wall.  The  electrodes  themselves  must  be  protected  against  overheating 
and  oxidation  by  water- jacketing  or  other  means. 
To  reduce  radiation  losses,  the  ratio 

outside  exposed  surface  of  furnace 
volume  of  furnace 

must  be  small,  and  the  walls,  or  some  thickness  of  them,  made  of  a 
good  heat  insulator.  Further,  the  lining  should  have  a  high  electrical 
resistance  at  its  working  temperature,  as  the  heat  produced  by  any 
current  passing  along  the  lining  between  two  electrodes  (often  a  con- 
siderable amount)  is  mostly  lost.  To  secure  a  proper  utilisation  of  the 
heat  supplied  the  charge  must  be  sufficiently  great,  and  suitably 
placed  with  respect  to  that  part  of  the  furnace  where  the  heat  is  being 
generated.  A  paper  by  Collins l  on  the  design  of  intermittent  resistance 
furnaces  deals  particularly  with  these  last  points.  Finally,  for  con- 
tinuously working  furnaces,  the  charging  and  tapping  arrangements 
must  be  convenient,  and  if  valuable  gaseous  products  are  evolved,  as 
in  the  pig-iron,  ferro-silicon,  and  carbide  furnaces,  they  should  be 
collected  and  utilised.  It  is  only  recently  that  steps  have  been  taken 
to  recover  these  gases.8 

We  have  stated  that  when  possible  the  charge  itself  or  else  the 
cooled  product  should  form  the  effective  furnace  lining,  in  which  case 
the  ordinary  materials — firebrick,  silica,  calcined  magnesite  or  dolomite, 
etc. — can  be  used  for  the  furnace  walls.  But  this  is  not  always  prac- 
ticable. In  ferro-silicon  furnaces  the  hearth  is  filled  with  the  corrosive 
liquid  product ;  in  steel  furnaces  walls  and  bed  undergo  the  action  of 
liquid  slag  and  steel,  and  the  roof  is  exposed  to  slag  vapours  and  more 
or  less  to  direct  radiation  from  the  arc  (not  of  course  in  induction 
furnaces).  In  such  cases  it  may  be  necessary  to  use  particularly 
resistive  linings,  according  to  the  temperature  and  the  chemical  pro- 
perties of  the  substances  present.  But  not  always  so.  A  dolomite 
hearth  and  a  silica  or  magnesite  roof  are  found  to  serve  excellently  in 
most  steel  furnaces.  Of  more  resistive  refractories,  several  are  them- 
selves products  of  the  electric  furnace.  Alundum,3  carborundum,4  and 
siloxicon  5  are  examples.  This  last  substance,  though  oxidised  at  1500°, 
is  stable  up  to  3000°  in  absence  of  air,  and  is  not  attacked  by  molten 
metals  as  carborundum  is.  Another  good  refractory  is  crystallised 
and  ground  magnesia,  made  by  fusion  in  the  electric  furnace.  Finally, 

1  Trans.  Amer.  Electrochem.  Soc.  0,  31  (1!KM>). 

2  Trans.  Farad.  Soc.  5,  2.14  ( /.w). 

3  P.  495.  <  P.  488.  I'.  492. 


xm.]  ELECTROTHERMICS  173 

for  the  highest  furnace  temperatures  attainable,  carbon  is  the  only 
possible  material.  Air  must  naturally  be  excluded.  Carbon  is  also 
the  only  possible  lining  for  the  hearth  of  a  ferro-silicon  furnace.  Every- 
thing else  is  attacked  and  dissolved. 

Whilst  the  lining  of  a  furnace  must  be  sufficiently  refractive,  the 
materials  of  which  the  walls  are  constructed  should  be  poor  conductors 
of  heat  and  electricity.  It  does  not  always  happen  that  these  pro- 
perties are  found  combined  in  the  same  substance  with  adequate 
refractoriness.  Thus  carborundum  and  carbon  conduct  heat  and 
electricity  well.  The  best  practice  is  to  make  the  refractory  lining 
of  as  small  a  thickness  as  is  practicable,  and  to  back  it  with  bricks  which 
are  primarily  poor  conductors  of  heat  and  electricity.  It  has  indeed 
been  found  practically  that  the  thickness  of  refractory  necessary  to 
prevent  melting  of  the  more  fusible  low-conductivity  bricks  behind 
it  is  usually  exceedingly  small.  It  is  generally  possible  to  apply  it  in 
the  form  of  a  wash,  thus  minimising  both  the  current  losses  between 
electrodes  embedded  in  the  walls,  and  also  its  effect  on  the  heat  lost 
by  conduction  outwards  through  the  walls. 

The  quantity  of  heat  passing  in  unit  time  through  a  plate  of  a 
given  substance  is  expressed  by  the  formula  — 


where  a  is  the  area  of  the  plate,  I  its  thickness,  6  the  difference  in  tem- 
perature between  the  two  sides,  and  k  the  specific  conductivity  of  the 
substance.  This  formula  cannot  be  directly  applied  to  technical 
furnaces,  because  a  varies  between  the  limits  of  the  inside  and  outside 
surfaces  of  the  furnace,  and  to  use  the  mean  of  these  values  by  no  means 
always  gives  a  good  approximation.  Hering  x  has  worked  out,  for  the 
more  simple  shapes  of  furnace,  simple  and  accurate  expressions  by 

which  -  in  the  above  equation  can  be  replaced.     His  paper  brings 

out  the  extreme  importance  for  effective  and  economical  insulation  of 
(1)  making  the  load  per  single  unit  as  heavy  as  possible,  when  the 
relative  loss  is  reduced,  and  (2)  for  a  furnace  of  given  load,  of  making 
its  inside  area  as  small  as  possible. 

Our  only  accurate  knowledge  of  the  heat  conductivities  of  furnace 
materials  at  high  temperatures  we  owe  to  Wologdine,2  who  carried  out 
measurements  on  bricks  of  many  materials  up  to  temperatures  of 
1,000°,  and  discovered  several  interesting  relations  between  tempera- 
ture of  firing,  conductivity,  permeability  to  gases,  etc.  Some  of  his 
results  are  given  in  the  following  Table  XXIX.  The  conductivities 

1  Trans.  Amer.  Electrochem.  Soc.  14,  215  (1908). 
-  Electrochem.  2nd.  7,  383  (1909). 


174    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 


calories  X  c.m. 

are  expressed  in  -         ,  — 2      10  p 

second  X  c.m.*  X  1    O. 

range  0°  to  1000°. 

TABLE   XXIX 
Material  Conductivity 


Graphite  brick 
Carborundum  brick 
Magnesia  brick 
Chromite  brick 
Firebrick 
Gas  retort  brick 
Bauxite  brick 
Silica  brick 
Kieselguhr  brick 


0-025 

0-0231 

0-0071 

0-0057 

0-0042 

0-0038 

0-0033 

0-0020 

0-0018 


fo]. 


Relative 
Conductivity 

100 
92-4 
28-4 
22-8 
16-7 
15-2 
13-2 

7-8 

7-1 


As  far  as  the  size  of  furnaces  is  concerned  we  appear  to  be  in  a 
period  of  rapid  transition.  Taussig  l  points  out  that  the  practical  limit 
with  uncovered  arc  furnaces  is  reached  at  about  3000  K.W.  per  hearth, 

above  this  the  difficulties  of 
charging  and  working  and  the 
fume  nuisance  becoming  too 
great.  But  with  closed  fur- 
naces, such  as  those  of  Hel- 
fenstein2  (Fig.  47),  great 
developments  may  be  antici- 
pated. Of  considerable  ad- 
vantage will  be  the  possibility 
of  collecting  and  utilising 
the  furnace  gases,  often  of 
very  high  calorific  power. 
These  remarks  apply  parti- 
cularly to  carbide  furnaces. 
But  the  new  Heroult  steel 
furnaces  of  the  United  States 
Steel  Corporation  represent 
another  big  advance,  and  we 
may  also  expect  interesting  results  during  the  development  of  the 
electrothermal  pig-iron  industry,  still  in  its  infancy. 

It  is  noticeable  that  all  the  furnaces  concerned  in  these  developments 
are.  of  the  arc  type—  far  more  suitable  than  those  of  the  resistance 
type  for  large  commercial  units  by  reason  of  their  ease  of  regulation, 
the  flexible  way  in  which  load  and  rate  of  production  can  be  varied, 
and  the  possibility  of  continuous  working. 


Fio.  47. — Closed  He  If  enste in  Furnace. 


1  Trans.  Farad.  Nor.  5,  2.71 

-  See  also  Zeitsck.  Elektrochem.  17,  642  (1911). 


xiii.]  ELECTROTHERMICS  175 

3.  Electrical  Aspects  of  Electric  Furnace  Design 

The  Electrodes.— Of  electrode  materials  we  need  consider  only  two 
—carbon  and  graphite.  We  have  seen1  that  for  electrolysis,  graphite 
has  very  considerable  advantages  over  carbon,  owing  to  its  greater 
chemical  resistivity.  But  in  furnace  processes  the  case  is  somewhat 
altered.  At  the  high  temperatures  employed,  electrodes  of  either 
material  usually  disintegrate  or  oxidise  comparatively  quickly,  and 
their  cost  is  often  very  important,  amounting  to  even  20  per  cent,  or 
more  of  the  total  charges.  Under  these  circumstances,  though  the 
rate  of  oxidation  of  graphite  is  undoubtedly  less  than  that  of  carbon, 
and  its  electrical  conductivity  greater,  its  high  price  generally  renders 
its  use  inadmissible,  and  carbon  electrodes  are  employed.  The  manu- 
facture of  large  carbon  electrodes  for  furnace  work  has  been  described 
by  Louis.8  The  raw  material — anthracite,  retort  carbon,  coal  tar — 
must  be  as  free  as  possible  from  ash,  including  sulphur  and  phosphorus, 
in  order  to  avoid  subsequent  contamination  of  the  furnace  product. 
It  is  first  distilled  in  closed  vessels,  and  powdered  to  a  grain  not 
exceeding  O'5-l  mm.  Then  10-15  per  cent,  of  finely  divided  dry 
pitch  is  added,  and  the  whole  thoroughly  incorporated  at  70°-90°. 
After  compressing  into  moulds,  the  mixture  is  very  carefully  heated 
to  a  temperature  not  exceeding  1200°  for  8-10  hours.  A  very  slow 
cooling  succeeds,  without  which  the  electrode  is  fragile,  and  liable  to 
break. 

The  maximum  permissible  current  density  is  put  as  3-4  amps./ cm.2, 
but  in  practice  this  is  often  exceeded,  values  of  10  amps./ cm.2  being 
not  uncommon.  Of  course  under  these  circumstances  the  electrode 
becomes  exceedingly  hot,  and  oxidises  rapidly  if  air  be  present.  The 
permissible  value  really  depends  somewhat  on  the  cross-section  of  the 
electrode,  being  greater  the  smaller  the  latter  is.  The  reason  is  that 
the  smaller  electrodes  can  be  made  of  a  far  more  regular  structure  and 
freer  from  flaws.  It  is  indeed  often  advantageous  to  use  a  bundle  of 
electrodes  of  small  cross-section  instead  of  one  large  electrode  for 
carrying  heavy  currents.  With  graphite  electrodes,  a  current  density 
of  20  amps./ cm.2  can  be  easily  maintained. 

Heat  Losses  in  Electrodes.— But  electrodes,  besides  their  function 
of  conducting  the  current  into  the  furnace,  also  act  in  another  way. 
A  certain  amount  of  heat  is  produced  in  them  by  the  passage  of  the 
current,  and  some  of  this  will  be  conducted  outside  the  furnace.  More- 
over, an  electrode,  being  a  good  conductor  of  electricity,  is  also  a  good 
conductor  of  heat.  One  end  is  at  the  temperature  of  the  interior  of 
the  furnace,  the  other  end  is  usually  cooled  by  water.  So  the  electrode 
will  also  conduct  heat  from  the  reaction  zone  of  the  furnace  to  outside,  and 
thus  in  two  ways  will  cause  heat  losses.  The  sum  of  these  two  losses 
i  P.  152.  2  Jour.  Four  Elect.  19,  247  (1910). 


176    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

is  known  as  the  electrode  loss,  and,  when  all  the  electrodes  in  a  furnace 
are  reckoned,  may  assume  considerable  dimensions.   In  a  small  futnace, 
indeed,  it  may  form  a  large  percentage  of  the  total  energy  consumption. 
This  subject  has  been  much  discussed  in  America  during  the  last 
few  years 1  and  the  chief  conclusions  arrived  at  are  stated  below.     The 
minimising  of  the  electrode  losses  depends  essentially  on  a  correct 
proportioning  of  the  electrodes.     This  is  plain  when  we  see  that  an 
increase  in  length  of  an  electrode  increases  the  electrical  losses,  but 
decreases  the  amount  of  heat  conducted  out  from  the  interior  of  the 
furnace,  whilst  an  increase  in  its  cross-section  has  the  opposite  effect. 
Moreover,  as  a  result  of  their  different  electrical  and  heat  conductivities, 
the  correct  proportions  of  an  electrode  for  one  material  will  not  be  the 
correct  proportions  for  another  material.     And  for  the  same  electrode 
material,  the  proportions  will  depend  on  the  difference  of  temperature 
between  the  two  ends  of  the  electrode,  and  on  the  current.     Suppose 
the  heat  produced  in  an  electrode  by  the  current  to  be  PR.    If  we 
assume  that  the  resistance  of  the  electrode  is  independent  of  tempera- 
ture, that  the  heat  is  thus  uniformly  produced  along  its  length,  and 
that  none  leaves  the  electrode  along  its  length  but  only  at  the  ends  (both 
-    obviously  very  approximate  assumptions),  then  this  heat  will  tend  to 
flow  out  equally  at  both  ends.    Imagine  also  that  an  amount  of  furnace 
heat,  Q,  enters  the  hot  end  of  the  electrode.     Then  the  net  amount 
of  heat  entering  the  hot  end  of  the  electrode  is  Q  —  JPR,  whilst  the 
total  amount  of  heat  leaving  the  cold  end — i.e.  the  electrode  loss — is 
Q  -f-  JPR.     Assuming   the   furnace   temperature   to   be    fixed,    the 
minimum  electrode  loss  will  coincide  with  no  interchange  of  heat 
between  furnace  and  the  hot  end  of  the  electrode.     The  condition  is 
therefore  that  Q  =  JPR,  giving  an  electrode  loss  of  PR.      This  con- 
dition is  fulfilled  when  the  temperatures  of  the  furnace  and  the  hot  end 
of  the  electrode  are  identical.     If  not  so,  the  electrode  will  either 
convey  heat  out  of  the  interior  of  the  furnace  or  else  will  pour  it  in, 
the  loss  at  the  cold  end  being  in  either  case  correspondingly  greater. 

On  the  above  basis  Hering  has  deduced  certain  simple  formula?, 
from  which  the  best  proportions  for  electrodes  in  any  given  case  can 
be  readily  calculated.  He  makes  one  or  two  assumptions — either  the 
conductivities  entering  the  calculation  are  independent  of  temperature, 
or  they  can  be  represented  by  certain  mean  values,  in  which  case  the 
electrode  temperature  gradient  is  regarded  as  uniform.  He  has  sue-  , 
ceeded  in  showing  that  these  arbitrary  and  obviously  only  very  approxi- 
mately true  assumptions  bring  no  great  errors  into  the  final  results. 
Suppose  the  total  electrode  loss  per  second  be  X,  the  heat  flowing 

1  See  many  papers  by  Hering  and  by  Hansen  in  Metall.  Chem.  Engin. 
(Ekctrochem.  Ind.)  and  Trans.  Amer.  Electrochem.  Soc.  1909-1911.  Particularly 
EUctrochem.  Ind.  7,  442  (1909);  Metall.  Chem.  Engin.  8,  276,  471  (1910) ; 
Trans.  Amer.  Electroctem.  Soc.  17,  171  (Win). 


xm.]  ELECTROTHERMICS  177 

into  the  electrode  Q,  the  heat  produced  in  the  electrode  by  the  current 
W,  all  values  expressed  in  joules,  which  allow  of  simple  treatment. 
Then 


And  Q  =  k  .  6  .  -,  where  a  and  I  are  the  section  and  length  J  of  the  elec- 

trode, either  in  inches  or  cm.,  k  the  thermal  conductivity  in  joules 
and  the  chosen  unit  of  length,  and  0  the  temperature  difference  between 
the  two  ends.  Also 

W  _  P          l_9 

2"  =~  2"  '     "a 

where  r  is  the  specific  resistance  in  ohms  per  unit  cube.    We  get  then 


I        2         a 

This  is  a  minimum  when  the  two  right-hand  terms  are  equal,  and  we 
have 


^minimum  = 

The  minimum  loss  depends  on  the  product  of  the  heat  conductivity 
and  electrical  resistance  of  the  material,  on  the  current,  and  on  the 
temperature  difference  between  the  ends  of  the  electrodes.  Also 
on  the  correct  proportioning  of  the  electrodes.  For  the  most  favourable 
proportions  we  have  the  equation 


T~ 

It  thus  appears  that  as  far  as  the  losses  are  concerned  the  absolute 
dimensions  of  the  electrode  are  of  no  account—  only  th^cta'o  of  cross- 
section  to  length,  determined  in  any  given  case  by  tJ^Rurrent,  the 
temperature  difference  between  the  ends  of  the  electrode,  and  the 
ratio  of  electrical  resistivity  and  thermal  conductivity  of  the  material. 
In  designing  electrodes  one  therefore  proceeds  as  follows  :  From 
imber  of  kilowatts  the  furnace  is  to  consume  the  voltage  and 
tions  are  calculated,  the  voltage  being  made  as  large:and 
small  as  other  conditions  render  practicable.  In  that 
way  the  heat  nerated  in  the  electrode  is  minimised.  Then,  knowing 
the  furnace  temperature  and  the  values  of  r  and  k,  the  most  favourable 

ratio  of  -  is  calculated  by  the  last  formula.    Finally  I  is  made  as  short 
as  is  practicable. 

!  The  length  is  taken  as  that  part  of  the  electrode  below  the  water-  jacket. 

ti 


178    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 
The  application  of  these  formulae  is  illustrated  by  the  following  two 
simple  examples.    The  values  of    rk   and  •=  are  taken  from  one  of 
Bering's  papers,  and  refer  to  inch  units. 

(1)  A  300  K.W.  furnace  takes  60  volts  and  5000  amps.     Current  is  led  both  in 
and  out  of  the  furnace  by  two  paralleled  carbon  electrodes.     What  are  the  best 
proportions  for  these,  and  what  are  then  the  electrode  losses  of  the  furnace  ?    The 
temperature  difference  between  the  ends  of  the  electrodes  is  2000°. 
For  carbon,  rk  =  (18500  +  1'900)10-7 

£  =  0-00080  above  1100° 

For  6  =  2000°,  rk  becomes  22'3  x  10~4. 

As  2500  amps,  are  carried  by  each  electrode  we  have 

'o-ooos 

A  normal  current  density  in  a  carbon  electrode  is  25  amps.  /inch2.     The  cross- 
section  of  each  electrode  must  accordingly  be  100  sq.  in.     Whence  we  get  that  the 

length  is  yqo  =  90  inches.     This  would  probably  be  inconveniently  long.      It 

might,  however,  be  possible  to  double  the  current  density,  in  which  case  the  length 
of  the  electrode  would  be  reduced  to  45  inches.  The  power  loss  in  each  electrode 
is 

2500v/22-3  x  10"4  x  4000  watts 

=  7-6  K.W.  per  electrode 

=  30  K.W.  for  the  furnace. 

As  the  load  of  the  latter  is  300  K.W.  the  electrode  losses  amount  to  1.0  per 
cent. 

(2)  A  small  40  K.W.  furnace  works  with  graphite  electrodes  at  40  volts  and 
1000  amps.,  6  being  1500°.     According  to  Hering,  for  graphite 

rk=  (12800  -3-560)10-7 
j[  =  (5500  +  5-86(9)  10-8  above  800° 

For  6  *  1500°  rk  =  74'6  x  10~5 

•jj  =  142-9  x  10-* 

Whence 


Working  at  100  ,         ,  the  cross-section  of  the  electrode  is  10  sq.  inches,  and 

10 

ita  length  0>22  =  45  inches. 

The  loss  per  electrode  becomes 


21500  .  74-6  .  1(       watts 
=  1-5  K.W., 

the  total  loss  being  3  K.W.     As  the  furnace  load  is  40  K.W.,  the  electrode  losses 
amount  to  7*6  per  cent. 

Of  course  it  may  easily  happen  that  the  best  conditions  with  respect 


XIIL] 


ELECTROTHERMICS 


179 


to  electrode  losses  do  not  coincide  with  the  other  conditions  for  the 
highest  efficiency  or  convenience  of  working.  For  example,  a  correctly 
designed  carbon  electrode  always  tends  to  be  short  and  stumpy  with 
furnaces  of  small  capacity.  And  what  is  necessary  is  a  maximum 
efficiency  for  the  furnace  as  a  whole,  not  for  any  particular  part.  For 
further  details  on  this  subject,  including  experimental  determinations 
and  many  numerical  f  electrode  constants/  the  reader  is  referred  to 
the  original  papers. 

The  specifically  electrical  or  mechanical  features  of  the  electric 
furnace  do  not  here  demand  a  full  treatment.  Provided  there  is  no 
chance  of  a  disturbing  electrolysis,  alternating  or  direct  current  can 
be  indifferently  used.  In  calcium  carbide  manufacture,  for  example, 
the  output  per  unit  of  energy  is  the  same  in  the  two  cases.  Whether 
then  alternating  or  direct  *  current  is  adopted,  and,  if  alternating  current, 
whether  single-  or  poly-phase,  depends  on  such  factors  as  the  size  of  the 
furnace  in  question,  the  relative  costs  of  generators,  transformers, 


(a) 


cables,  electrodes,  etc.     It  would  be  impossible  to  discuss  these  matters 
adequately  here. 

Power  Factor.— One  important  point,  however,  arises  when  an  alter- 
nating current  is  used — that  of  the  power  factor  of  the  furnace;  and  as 

1  The  use  of  direct  current  in  practice  is  exceedingly  rare. 


180    PRINCIPLES  OP  APPLIED  ELECTROCHEMISTRY    [CHAP. 

this  question  frequently  appears  to  present  difficulty  to  chemists  we 
will  briefly  discuss  it.  The  power  in  a  given  circuit  at  any  moment  is 
the  product  of  the  instantaneous  current  and  the  instantaneous  voltage. 
Using  an  alternating  current,  at  one  instant  the  current  flows  in  one 
direction,  during  the  next  instant  in  the  opposite  direction,  whilst  the 
voltage  applied  to  the  circuit  simultaneously  changes  in  sign  and 


A    A    A 


WWW 


FIG.  49. 

magnitude.  If  the  circuit  is  a  pure  resistance  circuit,  containing  no 
capacity  or  inductance,  then  current  and  voltage  remain  in  phase— i.e. 
their  maximum  positive  values  are  reached  at  the  same  instant,  as  also 
the  zero  values  and  the  maximum  negative  values,  etc.  See  Fig.  48,  in 
which  time  is  plotted  along  the  abscissa?,  current,  voltage,  and  power 
along  the  ordinates.  The  power  in  the  circuit,  the  product  of  voltage 
and  current,  is  always  positive  or  zero,  never  negative.  But  if  the 
circuit  contains  a  capacity l  or  an  inductance  such  as  is  produced  by  the 
presence  of  other  conductors  in  its  neighbourhood,  then  a  phase  differ- 
ence arises  between  voltage  and  current.  Their  maxima  and  minima  no 
longer  correspond  (Fig.  49a  and  6),  and  consequently  the  product,  at 

1  The  presence  of  capacities  is  important  in  ozonisers  (Chap.  XXVIII),  but 
plays  no  appreciable  role  in  electrothermal  furnaces.  (See,  however,  footnote  on 
p.  457.)  It  may  be  mentioned  here  that  capacities  and  inductances,  though  both 
separately  lowering  the  power  factor  of  a  circuit,  mutually  destroy  one  another's 
effect  if  in  the  same  circuit.  Cf.  pp.  511,  526. 


XIIL]  ELECTROTHERMICS  181 

any  instant,  of  current  and  voltage  is  no  longer  always  positive  or  zero 
(Fig.  49c).  There  is  a  surging  of  power.  Energy  at  one  instant  passes 
from  the  generator  into  the  circuit,  at  the  next  instant  in  the  opposite 
direction  ;  and  only  the  difference  between  these  two  quantities  of 
energy  is  consumed  in  the  circuit — i.e.  the  sum  of  areas  A  minus  sum  of 
areas  B  (Fig.  49c).  This  does  not  mean  that  power  is  lost  (it  passes 
back  into  the  generator)  ;  it  only  means  that  the  power  consumption  is 
not  equal  to  the  product  of  the  voltage  and  amperage  of  the  circuit, 
but  is  less  than  this,  and  that  a  larger  generator  must  be  used, 
and  cables  and  leads  must  be  provided  which  will  carry  larger 
currents  than  would  be  necessary  if  the  power  factor,1  denned  as 

watts  in  circuit 

:—  — -. were    unity.      A   low   power 

volts  in  circuit  X  amperes  in    circuit 

factor  means  therefore  higher  costs  for  electrical  appliances.  Suppose 
we  wish  a  circuit  to  consume  630  K.W.  and  its  power  factor  is  0'7.  The 

value  of  the  product  volts  X  amps,  must  be  —  X  630000  =  900000. 

Suppose  120  volts  to  be  available,  the  necessary  current  will  be  7500 
amps.,  whereas  with  a  power  factor  1*0,  only  5250  amps,  would  have 
been  needed. 

We  cannot  here  fully  discuss  the  phenomena  of  inductance,3  and 
must  simply  quote  some  of  the  more  important  formulae  and  expres- 
sions. Instead  of  having,  as  in  a  pure  resistance  circuit,  the  relation 
E  =  IR,  we  have 


E  =  I  yR2  +  47rVIA 

Here  n  is  the  number  of  complete  cycles  or  periods  the  alternating 
current  makes  per  second,  the  periodicity  (2?rn  is  termed  the  fre- 
quency) ;  L  is  the  inductance  or  coefficient  of  self-induction  of  the 
circuit,  a  measure  of  the  ease  with  which  an  opposing  E.M.F.  is  produced 
when  the  external  alternating  voltage  is  applied,  and  is  essentially  a 
magnetic  property.  If  L  is  zero,  then  we  have  again  E  =  IR.  At 
constant  voltage,  I  becomes  small  if  L  is  very  great.  Its  effect  is 

1  Usually  written  cos  6. 

-  As  the  voltage  and  amperage  of  an  alternating  current  circuit  are  varying 
from  instant  to  instant,  it  is  obvious  that  the  figures  used  in  this  expression  must 
be  average  values,  which  give  the  same  mean  result  over  a  stretch  of  time  as  would 
the  instantaneous  figures  if  suitably  summed.  They  are  here  in  fact  the  r.m.s. 
(root  mean  square)  values,  got  by  taking  the  square  root  of  the  average  value  for 
the  square  of  the  alternating  current  or  voltage.  This  particular  mean  value  is 
taken  because  the  useful  effect  produced  by  an  alternating  current  at  any  moment 
is  usually  measured  by  the  square  of  the  current  or  voltage  (e.g.  in  electrical  heating 

I2R,  or      )•     And  these  r.m.s.  values  are  those  read  off  on  alternating  current  (e.g. 

R 

hot  wire)  measuring  instruments. 
3  See,  however,  also  p.  451. 


182       PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY 

practically  equal  to  the  introduction  of  a  new  resistance  into  the  circuit, 
a  resistance  however  which  does  not  absorb  power.     The  expressions 


2?rnL  and  xR*  +  4:7r2n2L2  are   termed  the  reactance  and  impedance 
of  the  circuit  respectively.     Further  we  have 

watts 


cos 


e  = 


volts  X  amps. 

PR 
EX  I 
IR 

E 

IR 


R 

=  X/R2   +  47T2tt2L2' 

We  see  that  the  power  factor  is  greater  the  higher  the  resistance  of  the 
circuit,  the  lower  the  self-inductance,  and  the  smaller  the  frequency  of  the 
alternating  flux. 

In  designing  and  installing  technical  furnaces,  inductance  effects 
must  naturally  be  avoided.  There  should  be  no  large  masses  of  metal 
in  the  neighbourhood,  as  these  cause  actual  power  losses,  not  merely 
a  lowering  of  cos  0 ;  and  the  nature  of  the  leads  and  the  way  they  are 
connected  to  the  furnace  should  be  carefully  thought  out.1  The 
power  factors  of  technical  furnaces  vary  between  G'5-0'95. 


Literature. 

Stansfield.     The  Electric  Furnace. 

Rodenhauser  and    Schoenawa.     EleTdrische    Ofen    in  der   Eisen- 
industrie. 

Borchers.    Electric  Furnaces. 

1  For  further  on  this  point  and  on  other  details  of  furnace  construction,  see 
Conrad,  Electrochem.  Ind.  6,  397  (1908). 


CHAPTER  XIV 

ELECTRICAL  DISCHARGES  IN  GASES 
1.  Gas  Ions 

BY  suitably  varying  the  conditions  under  which  electricity  discharges 
through  air,  either  ozone  or  nitric  oxide  can  result.  Both  reactions  are 
important  technically,  and  in  this  chapter  we  shall  discuss  generally 
the  different  types  of  electrical  discharge  in  air,  their  production,  and 
associated  phenomena. 

The  conduction  of  electricity  through  gases  is  now  universally 
regarded  as  due  to  the  presence  of  charged  gaseous  ions,  a  theory  pro- 
pounded by  Sir  J.  J.  Thomson.  Even  in  the  normal  state  every  gas 
contains  a  very  small  number  of  these  ions,  which  may  be  either  posi- 
tively or  negatively  charged.  A  positive  gas  ion  is  a  positively  charged 
atom,  usually  associated  with  one  or  more  neutral  gas  molecules.  Nega- 
tive ions  can  similarly  consist  of  negatively  charged  atoms  associated 
with  neutral  molecules. 

But  they  can  also,  on  the  other  hand,  be  subatomic  in  nature, 
and  identical  with  those  discrete  particles  of  negative  electricity  out 
of  which  we  presume  atoms  of  '  matter  '  to  be  constituted.  These  sub- 
atomic negative  particles  are  called  electrons.  Owing  then  to  the 
presence  of  these  ions,  a  gas  has  a  very  slight  electrical  conductivity 
even  in  the  normal  condition,  and  if  a  potential  difference  be  maintained 
between  two  insulated  metal  plates  set  up  in  air,  a  small  current  will 
traverse  the  intervening  air  space.  Suppose  now  that  the  voltage 
difference  between  these  plates  be  raised.  The  greater  it  becomes,  the 
steeper  the  potential  gradient  along  the  path  which  the  ions  take,  and 
consequently  the  more  rapid  the  rate  at  which  they  are  urged  along. 
Collisions  between  ions  and  neutral  molecules  are  constantly  occurring, 
and  with  a  sufficiently  high  potential  gradient,  the  velocity  of  the 
ions,  and  hence  their  kinetic  energy,  may  increase  to  such  an  extent 
that,  by  their  collision  with  the  neutral  particles,  the  latter  are  split 
up  themselves  into  two  or  more  ions,  which  at  once  begin  to  conduct. 
When  this  happens  the  current  will  rise  rapidly,  the  temperature  also. 

183 


Low  tension, 
arc 


184    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

This  process  of  ionisation  by  impact  is  accompanied  by  luminescence 
of  the  gas. 

As  the  voltage  difference  between  the  two  electrodes  is  steadily 
raised,  the  potential  gradient  necessary  for  ionisation  by  collision  is  not 
reached  simultaneously  in  all  parts  of  the  path.  Consequently,  ionisa- 
tion and  luminescence 
do  not  commence  in  all 
parts  of  the  gap  between 
the  electrodes  at  once, 
but  perhaps  at  first  in 
one  region  only,  with  a 
correspondingly  smaller 
increase  in  current.  This 
first  change  will  be 
followed  by  subsequent 
ones.  The  current  volt- 
age relations  in  a  gaseous 
FIG.  5k  electrical  discharge  are 

in   fact   very  complex, 

as  the  following  curves  show.  These  characteristic  curves  are  obtained 
by  plotting  voltage  against  a  steadily  increasing  current.  The 
simplest  cases  are  those  where  direct  current  is  used  and  the  influence 
of  one  electrode  eliminated.  This  can  be  effected  by  allowing  the 
discharge  to  pass  from  a  small  pointed  electrode  to  a  large  earthed 
plate.1  The  phenomena  observed  are  essentially  due  to  the  charged 
point,  and  vary  somewhat 
with  its  sign.  If  it  be 
positively  charged,  we 
obtain  a  discontinuous 2 
curve  as  in  Fig.  50 ;  if 
negatively  charged,  as  in 
Fig.  51. 

Suppose  the  current 
passing  through  air  from 
a  positively  charged  point 
to  earth  be  gradually  in- 
creased. The  voltage  will 
first  rise  very  rapidly  to 
A.  At  this  point  there 
will  be  a  slight  discontinuity,  the  voltage  will  rise  rather  less  rapidly, 
and  the  discharge  will  become  luminous.  At  B  the  voltage  gradient 
is  great  enough  to  cause  considerable  impact  ionisation.  The  con- 
ductivity very  rapidly  rises,  the  voltage  falls,  and  a  new  type  of 

1  Toepler,  Drud.  Ann.  7,  477  (/.'**.')• 

2  Shaded  parts  denote  discontinuity  of  discharge. 


Current. 
FIG.  51. 


XIV.] 


ELECTRICAL  DISCHARGES  IN  GASES 


185 


discharge,  the  brush  discharge,  sets  in,  the  current  being  greater 
and  the  gas  more  luminous  than  in  the  previous  glow  discharge. 
This  transition  is  accompanied  by  sparking.  At  C  there  is 
more  discontinuity,  and  the  character  of  the  discharge  alters  radi- 
cally. It  has  now  become  a  high-tension  arc,  the  whole  of  the 

current  path  is  strongly  luminous,  the  ratio  is  much  higher 

voltage 

than  before,  and  the  characteristic  has  become  negative,  the  voltage 
decreasing  whilst  the  current  increases.  This  means  that  owing  to 
very  powerful  impact  ionisation,  caused  by  the  large  number  of  charged 
particles  present,  the  resistance  of  the  gap  decreases  more  quickly  than 
the  current  increases,  and  hence  an  increase  of  current  is  accompanied 
by  a  decrease  of  voltage.  As  the  current  becomes  greater  and  the  tem- 
perature rises,  even  this  form  of  discharge  becomes  unstable.  Charged 
particles  and  vapours  are  given  out  by  the  hot  electrodes,  there  is 
another  discontinuity,  and  the  final  form  of  discharge,  the  low-tension 
arc,  is  reached.  The  characteristic  is  here  again  negative  at  first. 

The  results  obtained  with  a  negatively  charged  point  are  similar. 
High  voltages  are  involved.  The  points  A  and  B  correspond  to  thou- 
sands or  tens  of  thousands  of 
volts  for  a  discharge  distance 
measured  in  centimetres ; 
whilst  the  currents  are  small, 
only  amounting  to  a  few 
amperes  even  in  the  low- 
tension  arcs.  If  now,  instead 
of  one  point  and  one  plate 
electrode,  we  consider  two 
similar  electrodes— e.g.  rods — 
we  get  a  curve *  such  as  in 


Law  tension 
cere 


Current. 
FIG.  52. 

Fig.     52.       This     is    merely 

drawn  diagrammatically,  as  the  influence  of  the  various  experi- 
mental conditions— nature  and  size  of  electrodes,  nature  of  gas, 
length  of  discharge,  etc.— is  enormous.  An  alternating  discharge 
gives  a  similar  characteristic.2 


2.  Different  Types  of  Discharge 

We  will  now  consider  the  different  types  of  discharge  in  more  detail. 
The  non-luminous  discharge  need  not  detain  us.  Very  small  currents 
pass  and  it  produces  no  chemical  effects.  At  a  certain  voltage,  de- 
pending on  the  pressure,  electrode  distance,  etc.,  there  is  discontinuity, 
and  the  discharge  becomes  luminous. 

1  Brion,  Zeitsch.  Elektrochem.  14,  245  (1908). 
^  2  Cramp  and  Hoyle,  Electrochem.  2nd.  7,  74  (1909). 


186    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

Silent  Discharge. — The  appearance  and  exact  nature  of  this 
luminous  discharge  varies  considerably.  It  depends  on  whether  the 
discharge  is  a  direct-current  point  discharge  (and,  if  so,  on  its  sign)  or 
whether  it  is  an  alternating  current  discharge.  It  depends  on  the 
current  and  on  the  shape  of  the  electrodes.  For  our  purpose  we  can 
group  together  all  these  different  kinds  of  discharge  (glow,  brush,  strip) 
by  the  facts  that  their  characteristic  is  almost  always  positive,  the  voltage 

increasing  with  increasing  current,  and  that  the  ratio  -       —  is  always 

current 

high.  When  passing  through  air  they  generate  ozone,  and  the 
commonly  used  generic  term,  '  silent  discharge/  is  often  applied  to  them 
(frequently  a  misnomer).  Electrically  considered  they  are  stable.1 
The  truth  of  this  statement  is  readily  seen.  As  an  increase  of  current 
necessitates  increased  voltage,  such  a  change  cannot  commence  spon- 
taneously if  the  voltage  is  kept  constant. 

When  luminous  discharge  begins  from  a  positively  charged  point, 
there  appears  first  a  thin  reddish  glowing  layer  on  the  electrode.  With 
increasing  current,  small  '  twigs  '  of  light  appear,  and  finally  a  strongly 
luminous  brush  discharge.  If  continuously  used,  the  point  gradually 
loses  its  power  of  giving  this  brush  discharge,  and  sparking  occurs 
instead.  If  the  discharge  is  intermittent  (thus,  if  there  is  a  tiny  spark- 
gap  in  series)  the  point  does  not  tire.  With  a  negatively  charged  point 
there  appears  first  a  luminous  star  round  the  electrode,  and  as  current 
and  voltage  rise,  a  reddish  conical  brush  (the  positive  column),  separated 
from  the  light  already  present  by  a  dark  space,  starts  out  towards  the 
anode.  We  shall  see2  that  the  chemical  action  of  the  discharge  is 
closely  connected  with  these  phenomena.  The  appearance  with  an 
alternating  current  is  naturally  more  complex,  and  depends  on  the 
frequency  as  well  as  on  other  factors.  In  all  cases  the  voltage  necessary 
at  ordinary  gas  pressures  for  any  given  effect  increases  with  the  diameter 
of  the  electrodes,  their  distance  apart,  and  the  pressure. 

High-tension  Arc. — The  next  form  of  discharge  is  the  high-tension 
arc.  This  is  the  name  it  now  generally  receives,  and  refers  more 
particularly  to  heavy  current  discharges.  Produced  on  a  small  scale, 
in  a' gas  at  low  pressure,  it  is  the  well-known  glow  current  (with  its 
Faraday  dark  space,  positive  column,  etc.).  It  is  this  discharge  which 
is  the  most  active  in  producing  nitric  oxide  when  burning  in  air,  and 
is  essentially  the  one  used  in  the  various  successful  technical  processes. 
We  have  seen  that  it  results  in  a  steady  increase  of  the  current  in  an 
ordinary  brush  discharge.  At  a  certain  point,  depending  on  conditions, 
the  brush  discharge  breaks  down,  sparking  occurs,  and  the  high-tension 
arc  begins  to  pass.  The  first  noticeable  point  about  this  arc  is  its 

'Except  at  their  extreme  upper  limits,   where   both  voltage   and   current 
(comparatively)  'are  high. 
I'.  523. 


xiv.]  ELECTRICAL  DISCHARGES  IN  GASES  187 

negative  characteristic  ;  that  is,  as  the  current  allowed  through 
increases,  the  voltage  falls.  The  reason  is  that  the  increased  current 
means  an  increased  temperature  and  a  greater  velocity  of  the  particles 
in  the  arc.  Further,  the  hot  electrodes  (with  direct  current  the  cathodes) 
begin  to  emit  electrons  in  large  numbers.  The  rate  of  production  of  ions 
by  impact  consequently  increases,  and  hence  the  resistance  falls.  It, 
moreover,  falls  more  quickly  than  the  current  increases,  and  therefore 
the  product  IR  =  E,  where  R  is  the  apparent  resistance,  decreases 
as  I  increases. 

Suppose  now  the  arc  to  be  fed  by  a  source  of  constant  voltage. 
If  any  small  increase  of  current  occurs  the  arc  voltage  drops,  and,  the 
external  voltage  remaining  constant,  a  still  larger  current  passes. 
The  tendency  is  therefore  for  the  current  flowing  to  increase  enor- 
mously ;  that  is,  the  discharge  becomes  unstable,  sparking  takes  place, 
and  a  final  form  of  discharge,  the  low-tension  or  lighting  arc,  begins  to 
pass.  If,  on  the  contrary,  a  slight  decrease  in  current  occurs,  the 
temperature  will  fall  and  the  resistance  rise,  and,  as  there  is  no  reserve 
of  voltage  on  which  to  draw,  the  current  will  decrease  indefinitely, 
and  the  arc  will  go  out.  To  render  the  arc  stable,  means  must  there- 
fore be  adopted  to  lower  the  voltage  across  it  simultaneously  with  any 
increase  in  current,  and  vice  versa.  This  is  effected  by  placing  in  series 
a  resistance  (with  direct  current)  or  an  inductance  (with  alternating 
current)  of  high  value.  An  increased  current  then  automatically 
increases  the  voltage  drop  across  the  resistance  or  inductance,  and 
therefore  decreases  the  same  across  the  arc.  A  decreased  current  in- 
creases the  voltage  available  for  the  arc.  A  slight  alteration  in  current 
produces  therefore  a  corresponding  alteration  in  voltage. 

The  greater  the  resistance  or  impedance,  the  more  powerfully  will 
any  change  of  current  be  checked,  and  the  more  stable  the  arc  will  be. 
The  series  resistance  used  with  direct  current  means  of  course  con- 
siderable consumption  of  electrical  energy  outside  the  arc.  With  an 
alternating  current  an  impedance  of  very  low  resistance  may  be  used, 
and  the  losses  thus  minimised.  Using  such  an  impedance  it  is  occa- 
sionally possible  with  high-tension  alternating  current  arcs  to  increase 
the  current  so  much  that  the  characteristic  changes  sign  and  becomes 
positive.  An  increase  in  current  is  then  accompanied  by  an  increase  in 
voltage. 

Another  difference  between  direct  and  alternating  current  arcs  is 
the  fact  that,  although  a  very  high  initial  voltage  is  necessary  to  start 
a  direct-current  arc  at  atmospheric  pressure,  when  once  it  has  passed, 
assuming  it  stable,  a  much  lower  voltage  suffices  to  keep  it  discharging. 
The  current  in  an  alternating  arc,  on  the  contrary,  sinks  to  zero  every 
half-period,  the  temperature  falls,  the  ionisation  decreases,  the  resistance 
rises,  and  consequently  a  higher  voltage  is  needed  to  restart  the  arc 
as  the  current  again  increases.  An  alternating-current  arc  requires 


188    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

therefore  a  higher  external  voltage  than  does  a  direct-current  arc,  and 
this  must  be  controlled  by  powerful  choking  coils.1  If  the  electrodes 
do  not  cool  overmuch  at  the  end  of  each  half-period,  this  excess  voltage 
may  be  largely  avoided.  A  third  point  to  be  reckoned  with  in  alter- 
nating-current arcs  is  the  power  factor.  In  most  technical  arc  circuits 
this  is  low,  O'G-0'8,  chiefly  because  of  the  big  series  inductances  used. 

In  appearance  the  high-tension  arc,  if  burning  quietly,  consists 
of  a  glowing  column  of  gas  —  the  positive  column  —  practically  joining 
the  two  electrodes.  In  technical  arcs,  which  are  subjected  to  various 
deforming  influences,  the  appearance  presented  is  sometimes  very 
different.  The  effect  of  pressure  changes  is  similar  to  that  in  the 
case  of  the  brush  discharge.  For  pressures  greater  than  a  small  fraction 
of  an  atmosphere,  the  voltage  necessary  for  the  arc  to  strike  and  play 
increases  with  the  pressure.  An  increase  in  the  length  of  the  arc  causes 
both  an  increase  in  the  voltage  necessary  for  a  constant  current,  and 
also  in  the  current  furnished  by  a  given  voltage.  The  following  approxi- 
mate formula,  which  takes  no  account  of  the  nature  of  the  electrodes, 
holds2: 


where  a  and  b  are  constants,  and  for  air  at  atmospheric  pressure  are 
about  350  and  100-150  respectively.  Thus,  if  the  arc  is  200  cm. 
long,  and  the  current  100  amps., 


=  2850  volts. 

It  will  be  noticed  that  a  is  the  minimum  voltage  necessary  for  the 
striking  of  a  high-tension  arc.  The  voltage  drop  across  such  a  direct- 
current  arc,  consisting  of  a  uniform  column  of  glowing  gas,  is  com- 
posed of  three  parts  —  the  cathode  and  anode  falls,  and  the  voltage 
drop  along  the  positive  column.  The  structure  of  this  column  being 
uniform,  so  is  also  the  potential  gradient  along  it.  In  the  example 

given  it  is  12'5  —  • 
cm. 

Fig.  53  represents  diagrammatically  for  the  same  case  the  dis- 
tribution of  voltage  along  the  length  of  the  arc.  The  minimum 
value  necessary  for  the  arc  to  strike  is  given  by  the  sum  of  the 
anode  and  cathode  falls.  Here,  where  two  metal  electrodes  are 
assumed,  the  cathode  is  responsible  for  about  300,  and  the  anode  for 
50,  of  the  350  volts.  By  using  a  hot  cathode,  covered  with  some 

1  A  choking  coil  is  an  impedance  of  high  inductance  and  low  resistance. 
-'  Urion,  Ziitsch.  Elektrochem.  14,  245 


XIV.] 


ELECTRICAL  DISCHARGES  IN  GASES 


189 


metallic  oxide  (e.g.  lime),  the  cathode  fall  can  be  very  considerably 
reduced,  owing  to  the  large  number  of  electrons  such  a  cathode 
emits.  The  temperature  of  the  high-tension  arc  varies  in  different 
parts,  and  also  depends  on  the  current  and  voltage  relations.  Using 


Ca&wcie 


-Anode 


FIG.  53. 


alternating  current,  it  varies  also  with  the  time.  It  will  be  higher 
at  the  electrodes,  owing  to  the  bigger  potential  gradients.  In  the 
positive  column  it  may  perhaps  be  taken  as  2200°-2500°  C. 

Low-tension  Arc.— The  low-tension  arc  discharge,  the  kind  used  in 
steel  and  carbide  furnaces,  etc.,  needs  brief  treatment  only.  It  results 
from  the  breakdown  of  the  high-tension  arc  when  the  latter  is  driven 
with  too  great  a  current.  Its  characteristic  is  first  negative,  then, 
with  increasing  current,  positive.  The  equation  connecting  current, 
voltage,  and  length  of  arc  differs  slightly  in  form  from  that  given  for 
high-tension  arcs,  a  is  here  only  about  30  volts,  and  is  chiefly  due  to 
the  anode  fall;  6  is  also  much  smaller  than  with  high-tension  arcs. 

The  reasons  why  these  low-tension  arcs  are  not  used  for  technical 
nitric  oxide  syntheses  are  that  the  potential  gradient  along  the  positive 
column  is  much  lower  than  in  high-tension  arcs l  and  that  the  tempera- 
ture is  too  high.2  When  metals  and  many  metallic  oxides  are  made 
incandescent,  they  shoot  out  large  quantities  of  electrons.  Further, 
certain  metals  at  lower  temperatures  can  give  off  positively  charged 
particles,  of  sizes  comparable  to  that  of  an  ordinary  chemical  atom. 
The  breakdown  of  the  high-tension  to  the  low-tension  arc  is  really 
caused  by  the  temperature  of  the  electrode  rising  high  enough  for  these 
processes  to  set  in  strongly.  Electrons  are  shot  off  from  the  cathode, 
and  positively  charged  ions  from  the  anode.  The  impact  of  these 
particles  on  the  gas  molecules  increases  the  ionisation  enormously. 
The  conductivity  rapidly  rises,  the  character  of  the  discharge  changes, 
and  the  temperature  increases  very  considerably.  In  fact,  a  low- 
tension  arc  passes  through  an  atmosphere  very  largely  consisting  of  the 
vapours  of  the  electrode  material.  It  is  chiefly  the  electrons  dis- 
charged from  the  cathode  which  bring  about  the  rapidly  increasing 


See  pp.  191,  509. 


See  pp.  192,  510. 


190    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

ionisation  in  the  gas.  If  the  cathode  has  no  spot  sufficiently  hot  to 
emit  these  electrons  in  quantity,  the  low-voltage  arc  will  not  pass. 
The  temperature  of  this  arc  varies  essentially  with  the  conditions. 
With  carbon  electrodes  it  is  about  3500°  C. 


3.  Chemical  Effects 

We  must  now  consider  the  chemical  effects  produced  by  electrical 
discharges  through  gases,  discussing  more  particularly  the  mechanism 
of  the  reactions.  In  the  silent  discharge  C02  is  partly  decomposed  to 
CO  and  02,  ammonia  to  nitrogen  and  hydrogen,  and  HC1  to  H2  and  C12. 
Oxygen  is  ozonised,  and  traces1  of  nitrogen  oxides  are  formed  in  a 
nitrogen -oxygen  mixture.  In  a  high-tension  arc  the  same  mixture 
gives  larger  quantities  of  oxides,  whilst  in  a  low-voltage  arc  the  pro- 
portion of  nitrogen  oxides  again  decreases.  All  these  reactions,  and 
others,  result  from  two  superimposed  effects,  varying  greatly  in 
different  cases  according  to  the  conditions — the  electrical  and  the 
thermal  effects  of  the  discharge. 

Electrical  Effect.— Consider  first  the  electrical  effect.  We  have 
seen  that  the  power  of  gases  to  conduct  electricity  depends  on  the 
presence  of  free  ions  or  charged  particles,  which  traverse  the  path  of 
the  discharge  between  the  electrodes.  Only  when  a  discharge  passes 
are  these  ions  numerous.  If  the  current  ceases  they  quickly  disappear, 
oppositely  charged  ions  neutralising  one  another,  reforming  neutral  gas 
molecules.  We  must  assume  that  this  recombination  of  oppositely 
charged  particles  is  continually  proceeding  whilst  the  current  is  passing. 
In  an  electrical  discharge,  indeed,  a  dynamic  ionic  equilibrium  exists. 
In  a  given  time  interval,  the  sum  of  the  ions  formed  by  impact  and 
those  coming  from  the  electrodes  must  be  equal  to  those  lost  by  recom- 
bination plus  those  giving  up  their  charges  to  the  electrodes.  Now,  in 
a  gas  containing  several  different  kinds  of  gas  ions,  the  products  formed 
during  the  neutralisation  may  differ  considerably  from  the  original 
neutral  molecules.  Suppose  an  oxygen  molecule  ionised  by  impact  into 
a  negative  electron  and  a  positively  charged  oxygen  molecule,  and 
another  oxygen  molecule  resolved  into  two  oppositely  charged  oxygen 
atoms.  Then,  if  positively  charged  oxygen  molecule  and  negatively 
charged  oxygen  atom  come  into  contact,  combination  may  occur, 
resulting  in  a  neutral  ozone  molecule.  Similarly,  if  oppositely  charged 
oxygen  and  nitrogen  atoms  come  together,  neutralisation  may  take 
place,  giving  a  neutral  molecule  of  nitric  oxide. 

In  this  way  we  can  account  for  those  chemical  effects  noticed  in 
electrical  discharges  in  gases  which  are  specifically  due  to  electrical 
action.  Such  chemical  action  will  only  occur  to  any  extent  when 

Under  suitable  condition!,  far  larger  quantities  of  nitrogen  oxides  are  formed. 


xiv.]  ELECTRICAL  DISCHARGES  IN  GASES  191 

there  is  a  lively  impact  ionisation.  For  this  not  only  the  presence  of 
ions  is  necessary,  but  also  ions  moving  with  a  sufficient  velocity. 
And  this  is  only  attainable  when  using  a  discharge  of  steep  potential 
gradient. 

Electrical  Equilibria.— The  new  products  thus  formed  by  electrical 
discharge  are  subject  to  ionisation  and  decomposition  by  ionic  impact 
just  like  the  original  molecules.  At  first  more  molecules  will  be  formed 
in  a  given  time  than  will  be  decomposed,  but  finally  these  quantities 
will  become  equal,  and  the  composition  of  the  gas  will  assume  a  definite 
value,  corresponding  to  a  definite  electrical  equilibrium.  By  this  term 
is  denoted  a  state  stable  only  during  the  passage  of  an  electrical  dis- 
charge, and  depending  on  the  nature  of  this  discharge.  When  the 
discharge  is  interrupted,  the  concentrations  of  the  gaseous  components 
correspond  to  a  state  unstable  with  respect  to  the  state  defined  by  the 
ordinary  thermal  equilibrium  constant.  The  composition  of  the  gas 
will  hence  tend  to  change.  Whether  it  can  do  so  with  appreciable 
velocity  depends  on  another  factor  which  we  shall  presently  discuss. 

The  general  qualitative  relation  existing  between  electrical  and 
thermal  equilibria  is  simple.  WTien  an  electrical  discharge  passes 
through  a  gas  which  is  in  thermal  equilibrium,  the  new  electrical  equi- 
librium resulting  corresponds  to  a  thermal  equilibrium  at  a  higher 
temperature — i.e.  to  a  shift  from  the  thermal  equilibrium  in  the  endo- 
thermic  sense.  Thus  the  amount  of  ozone  in  thermal  equilibrium  with 
oxygen  at  room  temperature  is  infinitesimal.  At  1300°  C.  it  is  Ol  per 
cent.,  at  2000°  1  per  cent.,  and  at  4500°  perhaps  10  per  cent.  But 
by  means  of  a  silent  discharge,  oxygen  can  be  10  per  cent,  ozonised  at 
room  temperature.  Similarly  the  quantities  of  CO  and  02  in  thermal 
equilibrium  with  C02  at  room  temperature  are  very  small,  as  are  also 
the  quantities  of  nitrogen  and  hydrogen  in  equilibrium  with  ammonia. 
But  when  a  silent  discharge  passes  through  the  respective  gases,  decom- 
position products  in  electrical  equilibrium  are  formed  to  an  extent 
corresponding  to  considerably  higher  temperatures. 

Finally,  we  may  take  the  equilibrium  between  N2,  02,  and  NO.  The 
reaction  N2  +  02  — >  2NO  is  endothermic.  NO  exists  in  considerable 
quantities  in  thermal  equilibrium  with  N2  and  02  only  at  very  high 
temperatures.  At  room  temperature  the  equilibrium  concentration  of 
NO  is  exceedingly  low.  If  a  silent  discharge  be  passed  through  air, 
some  NO  results,  which,  though  small  in  amount,  corresponds  to  a 
thermal  equilibrium  of  a  higher  temperature.  Complications  are 
present,  due  to  the  ozone  simultaneously  produced  forming  higher  oxides 
of  nitrogen.  But  at  2000°  C.,  in  a  high-tension  arc,  where  such  com- 
plications are  impossible,  NO  can  be  produced  in  amounts  which  would 
not  be  in  thermal  equilibrium  below  4500°-5000°  C. 

Thermal  Effects.— But,  besides  the  electrical  effect  of  the  discharge, 
we  must  consider  its  thermal  effect.  The  electrical  equilibrium  only 


192      PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY 

persists  whilst  the  discharge  is  actually  passing.  When  it  ceases  the 
system  tends  to  lapse  into  thermal  equilibrium,  and  reaches  this  state 
the  more  quickly  it  can  react—  i.e.  the  higher  the  temperature.  Thus, 
highly  ozonised  air  or  oxygen  is  apparently  stable  at  room  temperatures 
simply  because  the  rate  of  decomposition  of  ozone  under  such  conditions 
is  very  slow.  But  no  ozone  results  from  a  high-tension  arc  in  oxygen 
or  air  because  it  decomposes  rapidly  already  at  moderate  temperatures. 
On  the  other  hand,  the  rate  of  decomposition  of  NO  (experimentally 
determined)  is  much  lower  than  that  of  ozone.  Hence  NO  can  be 
produced  in  concentrations  corresponding  to  electrical  equilibria  by 
using  forms  of  electric  discharge  of  comparatively  high  temperature, 
followed  by  sufficiently  rapid  cooling  to  freeze  the  equilibrium  when 
the  gases  have  been  removed  from  the  influence  of  the  electric  discharge. 
A  gas  is  thus  obtained  containing  several  per  cents,  of  NO  or  its  further 
reaction  products  (N02,  N204,  etc.),  a  quantity  far  in  excess  of  that 
corresponding  to  the  thermal  equilibrium  either  at  room  temperature 
or  at  the  temperature  of  the  arc.  The  higher  the  temperature  of  the 
discharge,  the  more  rapidly  the  thermal  equilibrium  will  be  reached  in 
the  arc  itself,  and  the  nearer  will  electrical  and  thermal  equilibria 
approach  one  another.  The  greater,  too,  will  be  the  change  in  con- 
centration whilst  cooling. 

It  is  obvious  that  the  gaseous  product  obtained  by  means  of  an 
electrical  discharge  is  determined  by  several  complicated  factors.  In 
the  particular  case  of  nitric  oxide,  the  important  role  of  electrical 
influences  has  only  been  recently  recognised.  It  had  previously  been 
thought  that  thermal  effects  only  came  into  play.  The  contrary  view 
was  first  suggested  by  Warburg,1  and  shown  experimentally  to  be 
correct  by  Haber  and  Koenig.1 

One  last  point  should  be  mentioned.  Can  the  mass-action  law 
be  applied  to  electrical  as  to  thermal  equilibria  ?  The  answer  is  un- 
certain, but  is  very  probably  no.  In  cases  where  the  difference  between 
electrical  and  thermal  equilibrium  is  not  great,  and  where  the  tempera- 
ture is  high,  the  law  apparently  holds.  But  in  other  cases  that  is  not 
so.  Nor  can  we  expect  it  otherwise  when  we  consider  the  complexity 
of  the  phenomena  involved.  The  conditions  in  a  high-tension  arc  are 
favourable,  and  Le  Blanc  and  Niiranen  and  Grau  and  F.  Russ  have 
accordingly  found  the  mass-action  law  to  be  applicable  to  the  nitric 
oxide  synthesis  under  most  conditions.  But  in  the  silent  discharge, 
the  electrical,  and  hence  other,  conditions  are  obviously  complex 
and  varied,  and  Le  Blanc  and  Da  vies  consequently  found  that  the 
NHa  —  Na  —  Hj  equilibrium  was  not  governed  by  the  mass-action 
equation. 

1  Zeitsch.  Elektrochem.  12,  540  (!'.>< »;}.  2  gee  pp.  507-510. 


PART   II 

SPECIAL  AND  TECHNICAL 


CHAPTER  XV 

PRIMARY   CELLS 

1.  General  Considerations 

APPLIED  electrochemistry  is  chiefly  concerned  with  the  production  of 
substances  by  means  of  the  electric  current,  and  the  inverse  operation, 
the  conversion  of  chemical  into  electrical  energy,  is  much  less  im- 
portant. If  it  were  possible  to  liberate  the  chemical  energy  obtainable 
from  the  combustion  of  fuels  as  electrical  energy1  this  statement 
would  be  incorrect.  But,  as  matters  stand,  the  use  of  primary  cells 8  is 
practically  restricted  to  the  intermittent  production  of  small  amounts  of 
electrical  energy,  under  conditions  in  which  the  installation  of  a  generator 
or  the  taking  of  energy  from  a  distributing  system  would  be  impossible 
or  disproportionately  expensive.  Thus  primary  cells  are  largely  used 
for  electrotherapeutic  purposes,  electric  bells,  small  private  telephone 
systems,  motor  ignition,  etc.  The  more  efficient  forms  can  be  employed 
for  small  lighting  installations  in  remote  buildings,  and  for  driving 
small  motors  and  lathes.  But  the  high  cost  renders  the  production  of 
electricity  in  bulk  from  primary  cells  impracticable. 

For  an  element  of  high  E.M.F.  we  must  combine  two  electrode 
systems,  one  with  a  high  positive  or  oxidising  potential,  the  other  with  a 
strongly  negative  or  reducing  potential.  The  E.M.F.  of  the  resulting 
primary  cell  is  then  potential  positive-electrode  minus  potential  negative- 
electrode.  From  the  table  on  p.  96  we  see  that  if  it  were  possible,  for 
example,  to  combine  a  sodium  electrode  in  a  n.  Na*  solution  with  a 
gold  electrode  in  a  normal  Au*  solution,  an  E.M.F.  of  4'2  volts  would 
result ;  while  a  cell  consisting  of  copper  in  a  normal  Cu"  solution 
and  a  chlorine  electrode  in  a  n.  Cl'  solution  would  have  an  E.M.F.  of 
1-023  volts  (in  both  cases  neglecting  the  liquid  potential  difference). 
Further,  by  decreasing  the  ionic  concentration  in  the  electrolyte  the 
potential  becomes  more  positive  if  the  ion  concerned  is  negative  and 
more  negative  if  the  ion  is  a  positive  one.  This  furnishes  a  means  of 

1  P.  209.  2  p.  3. 

195  o  2 


196    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

still  further  increasing  the  E.M.F.  of  a  cell.     Thus  zinc  in  —  K2ZnCy4 

solution,  in  which  the  Zn"  concentration  is  very  low,  has  a  potential 
of  —  1-033  volts,  and,  combined  with  copper  in  n.  CuS04  solution,  will 
give  an  E.M.F.  of  1'35  volts,  0*25  volt  higher  than  that  of  the  ordinary 
Daniell  cell. 

Equally  important  for  primary  cells  are  the  potentials  of  oxidation - 
reduction  electrodes,1  some  of  which  are  here  collected. 

TABLE  XXX 

Electrode  potential  against 
Combination.  .    ,.„  L 

indifferent  electrode. 

0*2  n.  potassium  stannite  —  0*58  volt. 

CuO  in  n.  NaOH  -  0-1 

MnO2  in  0-1  n.  KOH  +  0-42 

Ni2O:,  in  2-8  n.  KOH  +  0-48 

6  per  cent.  HNO.»  +  0-92 

n.  KC10  +  0-94 

n.  KBrO  +  1-08 

35  per  cent.  HNO;»  +  1-09 

66  per  cent.  HNO;,  .  +     -12 

n.  CrO;,  in  H2S04  +    -20 

95  per  cent.  HNO;<  +    -22 


n.  HC10;,  + 

Mn02  in  0'5  n.  HoS04  +  0-05  n.  MnS04  + 

n.  KMn04  + 

n.  Ce(S04)2  in  H«S04  + 


PbOc  in  n.  HoS04  +  1-595 

All  but  two  of  these  combinations  give  positive  potentials,  and  the 
majority  are  known  chemically  as  oxidising  agents.  The  fact  is  that 
the  reducing  potentials  given  even  by  the  most  powerful  reducing  agents 
seldom  approach  the  negative  potential  values  for  such  metals  as  zinc, 
and,  apart  from  the  questions  of  relative  cheapness,  convenience  of 
use,  or  atmospheric  oxidation,  they  are  consequently  never  used  as  the 
anode  system  in  primary  cells.  With  oxidising  agents  it  is  different, 
as  we  shall  see.  At  present  we  may  notice  that  the  combination 
Zn  |  n.  Zn"  solution  |  n.  H2S04  |  Pb02  would  give  an  E.M.F.  of  2-36 
volts ;  the  combination  Fe  |  n.  Fe"  solution  J  n.  KMn04  Pt  1-98 
volts,  etc.,  etc. 

Irreversible  Effects.— All  these  values  assume  the  given  combina- 
tions to  work  reversibly.  This  is  very  desirable,  for  otherwise  the  full 
amount  of  the  available  free  energy  of  the  chemical  reaction  which 
takes  place  in  the  cell  is  not  obtained  as  electrical  energy,  but  some  is 
turned  into  heat,  and  thus  lost.  We  already  know  that  a  cell  or 
an  electrode  never  behaves  absolutely  reversibly,  and  that  often  the 
degree  of  irreversibility  is  considerable.  We  have  further  (Chap.  X) 

1  P.  100. 


xv.]  PRIMARY  CELLS  197 

discussed  in  detail  in  what  ways  irreversibility  can  enter  into  anodic 
and  cathodic  processes.  In  primary  cells  we  must  consider  both  these 
possibilities,  and  also  concentration  changes  in  the  electrolyte.  As 
anode,  a  soluble  metal  is  invariably  used  in  practice.  Oxygen  evolu- 
tion never  occurs,  and  the  only  irreversible  effect  to  fear  is  passivity. 
It  is  therefore  important  that  metal  and  electrolyte  be  so  chosen  that, 
under  working  conditions,  the  former  does  not  appreciably  become 
passive. 

Cathodic  Depolarisers.— At  the  cathode  the  matter  is  more  complex. 
In  some  cases  the  cathodic  process  is  the  discharge  of  a  metallion  to 
metal.  We  know  that  such  changes  usually  take  place  very  nearly 
reversibly,  though  there  are  exceptions.1  But  more  often  the  cathode 
system  is  an  oxidation-reduction  electrode,  consisting  of  an  oxidising 
agent  in  contact  with  an  indifferent  electrode,  and  serving  to  depolarise 
the  discharge  of  H'  ions.2  Suppose  for  the  moment  the  primary  cell 
to  be  Zn  |  n.  Zn"  solution  |  n.  H'  solution  |  indifferent  metal.  Then 
zinc  would  tend  to  dissolve  and  H'  ions  to  be  discharged,  and  the  rever- 
sible E.M.F.  would  be  0*77  volt  (neglecting  liquid  potential  difference). 
But  in  general,  though  the  zinc  enters  solution  almost  reversibly,  a 
considerable  overvoltage3  is  needed  for  H'  discharge,  and  this  figure 
must  be  subtracted  from  O77  to  get  the  working  voltage  of  the  cell. 
Thus,  if  a  copper  cathode  be  used,  at  which  the  current  density  is 
O01  amp./ cm.2,  then  the  overvoltage  will  amount  to  about  0'35 
volt,  and  the  E.M.F.  of  the  resulting  cell  is  only  CK2  volt. 

But  the  result  is  different  with  a  depolariser.  It  increases  the  E.M.F. 
of  the  cell  above  the  value  corresponding  to  reversible  cathodic  H* 
discharge,  and  it  effects  this  by  reacting  so  quickly  with  the  discharged 
hydrogen  that  the  electrode  never  becomes  saturated  with  the  gas. 
The  latter  does  not  therefore  exert  its  normal  electrolytic  solution 
pressure,  and  H*  discharge  takes  place  at  a  correspondingly  less  negative 
cathode  potential.  It  is  essential  for  the  efficiency  of  a  depolariser 
that  its  interaction  with  hydrogen  should  be  rapid.4  The  more  rapid 
it  is,  the  lower  is  the  hydrogen  concentration  in  the  electrode,  the 
more  positive  becomes  the  cathode  potential  and  the  more  closely  it 
approaches  the  equilibrium  value  (such  as  those  given  on  p.  196). 

As  the  table  indicates,  both  solid  and  liquid  depolarisers  come  into 
question.  Generally  speaking,  liquids  depolarise  more  quickly  than 
solids,  and  hence  give  working  potential  values  more  nearly  approaching 
the  reversible  figures.  On  the  other  hand,  their  power  of  diffusion 
towards  the  anode,  which  in  most  cases  they  strongly  attack,  usually 
renders  it  necessary  to  employ  some  kind  of  diaphragm  between  catho- 
lyte  and  anolyte.  This  increases  the  resistance  of  the  cell,  and  com- 
plicates its  construction.  A  satisfactory  solid  depolariser,  besides 

1  P.  120.  -  P.  128.  3  P.  118.  4  P.  128. 


198    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

reacting  rapidly  with  the  discharged  hydrogen,  should  have  a  high  elec- 
trical conductivity,  and  be  in  good  contact  with  the  metallic  part  of 
the  electrode.  Where  the  conductivity  is  too  low,  it  may  be  necessary 
to  mix  with  it  finely  divided  graphite  or  some  indifferent  metal.  Whilst 
having  such  a  mechanical  structure  that  it  does  not  crumble  or  flake 
away,  it  should  nevertheless  be  so  porous  that  the  under  layers  can  take 
part  in  the  depolarisation  without  difficulty,  and  the  reaction  with 
hydrogen  should  not  produce  crusts  or  in  any  way  tend  to  choke  up 
these  pores. 

The  polarisation  resulting  from  concentration  changes  is  less  im- 
portant. Round  the  dissolving  electrode  there  will  be  an  increased 
concentration  of  metallion,  and  at  the  cathode,  assuming  a  liquid 
depolariser,  there  may  be  complicated  concentration  changes.  With  a 
solid  depolariser,  the  only  change  usually  noted  is  that  the  electrolyte 
becomes  less  acid  or  more  alkaline,  the  magnitude  of  this  effect  depending 
on  the  porosity  of  the  solid.  But,  except  at  high  current  densities,  the 
effect  of  these  changes  on  the  E.M.F.  is  generally  small,  and  they  are 
largely  neutralised  by  diffusion.  In  cells  of  the  Daniell  type,  with 
two  nearly  reversible  electrodes,  concentration  changes  are,  however, 
responsible  for  the  greater  part  of  the  small  polarisation.  One  further 
cause  lowers  the  working  voltage  of  a  cell,  viz.  the  internal  resistance. 
This  must  be  as  low  as  possible.  Diaphragms  should  be  avoided  (a 
consideration  in  favour  of  solid  depolarisers),  the  electrodes  should  be 
as  close  together  as  possible,  and  the  electrolyte  of  high  conductivity. 

Deterioration. — But  galvanic  cells,  besides  behaving  satisfactorily 
when  working,  must  do  so  when  standing  idle.  They  must  not  dete- 
riorate owing  to  chemical  changes  in  electrolyte  or  at  the  electrodes. 
Such  changes  can  be  of  several  kinds.  The  soluble  anode  may  itself 
tend  to  dissolve  spontaneously  in  the  electrolyte,  not  doing  so  because 
of  the  high  overvoltage  needed  for  H2  discharge  at  its  surface.  If  now 
the  electrolyte  contains  a  dissolved  trace  of  a  more  electronegative  or 
noble  metal,  this  metal  will  be  chemically  replaced  from  solution  and 
deposited  on  the  electrode.  If  the  hydrogen  overvoltage  is  sufficiently 
less  at  this  second  metal  than  it  is  at  the  electrode  metal,  the  latter  may 
commence  to  dissolve  owing  to  the  formation  of  a  small  local  element — 
anode  metal  |  electrolyte  |  electronegative  metal.  The  anode  metal 
dissolves,  and  hydrogen  is  evolved  at  the  electronegative  metal.1  To 
avoid  this,  the  electrolyte  must  therefore  be  free  from  traces  of  such 
salts.  In  the  particular  case  of  zinc  anodes,  any  such  action  is  further 
avoided  by  amalgamating  the  electrode  with  mercury,  which  tends  to 
dissolve  any  trace  of  foreign  metal  deposited.  At  the  smooth  mercury 
or  amalgam  surface  hydrogen  overvoltage  is  very  high. 

We  have  already  noticed  the  disadvantage  of  a  liquid  cathodic 

1  Pp.  201,  227. 


xv.]  PRIMARY  CELLS  199 

depolariser  owing  to  its  tendency  to  attack  the  anode  chemically  when 
the  cell  is  standing.  This  is  sometimes  avoided  by  lifting  the  anode  out 
of  the  electrolyte  during  idle  periods.  Atmospheric  influences  can  also 
cause  deterioration.  The  oxygen  may  attack  the  soluble  anode  if  the 
electrolyte  is  suitable  ;  and  with  alkaline  solutions  the  cell  should 
be  protected  against  the  action  of  C02. 

Then  comes  the  all-important  question  of  price  of  materials.  Many 
excellent  depolarisers  cannot  be  used  on  account  of  their  high  cost. 
Such  depolarisers  as  can  be  regenerated  after  cathodic  reduction  by 
means  of  atmospheric  oxygen  are  naturally  very  advantageous.  CuO, 
which  is  much  used,  is  thus  regenerated.  Ceric,  titanic,  and  vanadic 
sulphates,  which  however  must  be  used  in  the  liquid  form,  would  be 
excellent  from  this  point  of  view.  Other  points  to  be  weighed  are 
simplicity  and  durability  of  construction  and  ease  of  working.  Dia- 
phragms should  be  as  far  as  possible  avoided,  and  refilling  and 
recharging  rendered  easy. 

Owing  to  one  or  more  of  the  above  considerations,  a  number  of 
electrode  systems,  which,  judging  from  their  reversible  potentials, 
appear  very  suitable  for  building  up  primary  cells,  are  never  used  for 
that  purpose.  The  alkali  metals  cannot,  of  course,  be  used  as  soluble 
anodes  because  of  their  chemical  reactivity.  Magnesium  is  also  very 
reactive  under  certain  conditions  and  is  expensive  ;  aluminium  is 
sometimes  strongly  reactive  chemically  and  sometimes  electrochemi- 
cally  passive,  and  the  same  is  true  of  iron  ;  cadmium  and  chromium  are 
too  expensive,  and  the  latter  becomes  easily  passive.  Of  depolarising 
systems,  Pb02  in  H2S04  is  too  expensive,  as  it  is  not  readily  regenerated 
chemically  ;  NaCIO,  NaBrO  and  KMn04  solutions  do  not  depolarise 
quickly  ;  and  solutions  of  HBr03  and  HC103  suffer  from  both  disadvan- 
tages. In  practice  zinc  is  invariably  the  soluble  anode.  The  anolyte 
can  consist  of  a  solution  of  ZnS04,  MgS04,  ZnCl2,  NH4C1,  H2S04,  Gr03 
in  H.^SO^  or  NaOH.  As  cathode  systems  we  have  copper  in  CuS04 
(Ag  in  AgN03  would  be  expensive  and  inconvenient  for  other  reasons) 
and  carbon,  whilst  chromic  and  nitric  acids  are  used  as  liquid,  and 
Mn02  and  CuO  as  solid  depolarisers. 


2.  Primary  Cells  in  General  Use 

We  will  now  consider  the  different  primary  cells  still  in  general  use  ; 
they  are  : — 

(a)  Daniell  cells. 

(6)  Grove-Bunsen  nitric  acid  cells. 

(c)  Poggendorff  bichromate  cells. 

(d)  Lalande  cells.     (Cupron  cell  :    Neotherm  or  Wedekind  cell.) 

(e)  Leclanche  cells. 
(/)  Dry  ceUs. 


200    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

Daniell  Cell.—  We  have  several  times  already  met  with  this  well- 
known  cell.  We  have  seen  that  it  consists  of  a  zinc  rod  in  zinc  sulphate 
coupled  with  a  copper  rod  in  copper  sulphate.  The  chemical  reaction 
which  takes  place  when  it  gives  current  is 

Zn  +  CuS04  -  >  Cu  +  ZnS04. 

The  salts  must  be  regarded  as  reacting  in  the  dissolved  state  if  the 
cell  electrolytes  are  unsaturated,  in  the  solid  state  when  using  saturated 
solutions.  We  have  further  seen  x  that  the  voltage  is  given  approxi- 
mately by  the  formula 

E  =  1-1  +  0-029  log  J5?J  volts, 
[Zn  J 

where    (  -  J  is   the   ratio   of  the  molar    Cu"   concentration   in   the 

[Zn"J 

catholyte  to  the  molar  Zn"  concentration  in  the  anolyte.  If  the 
concentrations  of  zinc  and  copper  sulphates  do  not  widely  differ,  the 

ratio  ^      J  can  be  replaced  by  ~      J^,    as  the  degrees  of  dissocia- 
[Zn  ' 


tion  of  the  two  salts  are  very  similar  under  similar  conditions. 
We  know  also  2  that  the  electrical  energy  liberated  by  the  cell  when 
working  is  almost  equal  to  the  change  in  total  energy  which  takes 
place,  and  that  consequently  the  E.M.F.  is  nearly  independent  of 
temperature.  The  actual  sign  of  its  temperature  coefficient  (like  the 
E.M.F.  itself)  depends  on  the  concentrations  of  the  electrolytes,  becoming 

more  positive  with  increase  of  the  ratio  p=-J« 

[Zn  J 

In  its  original  form  the  Daniell  cell  consists  of  an  outer  glass  vessel 
in  which  a  porous  pot  stands.  The  outer  vessel  contains  the  copper 
sulphate  solution,  which  is  usually  kept  saturated  (the  E.M.F.  is 
thereby  kept  up)  ;  the  inner  porous  pot  contains  the  ZnS04  solution. 
A  cylindrical  rod  serves  as  zinc  electrode,  and  a  cylindrically  bent  piece 
of  copper  foil  surrounding  the  porous  pot  as  copper  electrode,  though 
thin  sheet  lead  on  which  a  layer  of  copper  has  been  deposited  is  also 
used.  If  the  porous  pot  be  properly  chosen  the  resistance  of  the 
cell  will  be  low.  The  E.M.F.  is  also  very  constant,  showing  only  a 
slight  decrease  owing  to  the  accumulation  of  zinc  ions.  But  in  most 
forms  the  maximum  current  is  very  small,  not  exceeding  0'2  ampere. 
The  great  drawback  of  the  Daniell  cell,  however,  is  the  severe  chemical 
corrosion  of  the  zinc.  The  diffusion  of  CuS04  through  the  wall  of  the 
porous  cell  cannot  be  avoided  except  during  continuous  working  (when 
the  cationic  migration  away  from  the  anode  chamber  overcomes  this 

|P.  99. 

P.  82.      For  a  more  detailed  r  •nnsideration  of  the  thermodynamics  of  this  cell 
•ee  Cohen.  Chattaway  and  Tnml.n.r-k.  Z>  if  <>•/,.  Phys.  Chem.  80,  706  (/.W7). 


xv.]  PRIMARY  CELLS  201 

tendency).  This  means  chemical  replacement  of  a  certain  amount 
of  zinc  and  destruction  of  far  larger  quantities  owing  to  local  action 
started  by  the  deposited  copper.  Hydrogen  overvoltage  at  this 
metal  is  much  less  than  at  zinc,  and  dissolved  oxygen  also  plays  a  part 
in  the  corrosion.  According  to  Haber,1  even  under  favourable  conditions 
only  30  per  cent,  of  the  zinc  is  electrochemically  utilised. 

Various  improved  forms  of  the  cell  have  been  suggested  and  widely 
used  in  America  and  Germany.  They  are  particularly  designed  to  give 
a  higher  E.M.F.,  larger  currents  and  capacities,2  and  lower  resistances. 
To  avoid  using  porous  pots,  the  difference  in  densities  between  the 
solutions  employed  has  been  utilised.  Saturated  copper  sulphate  is 
covered  with  a  lighter  solution  of  MgS04,  in  which  the  zinc  electrode 
dips.  Even  after  the  cell  has  been  working  for  some  time  the  Zn" 

concentration  is  low,  and  the  ratio  -= — i  and  consequently  the  E.M.F., 

[Zn"J 

high.  In  the  Meidinger  element,  for  example,  the  voltage  at  the  start 
is  1*18  volts,  though  rapidly  falling  to  1*12-1 '13  volts.  Its  resistance 
is,  however,  not  low.  Other  designs  provide  a  large  electrode  surface, 
thus  allowing  larger  currents  to  be  taken  off  without  undue  polarisation. 
But  in  no  case  has  it  proved  possible  to  exclude  local  action,  and  this 
is  quite  sufficient  to  prevent  a  wider  use  of  the  cell. 

Grove-Bunsen  Cell.— This  is  also  a  two-fluid  cell.  The  anode 
consists  of  amalgamated  zinc,  usually  in  8  per  cent.  H2S04.  As  cathode 
depolariser,  nitric  acid  is  used,  usually  66  per  cent.  In  the  first  form 
of  the  cell,  due  to  Grove,  the  cathode  consisted  of  platinum.  Bunsen 
replaced  this  by  the  cheap  retort  carbon,  which  alteration  hardly 
affects  the  E.M.F.  In  most  Bunsen  cells,  the  carbon  electrode  (a 
rectangular  plate)  and  the  HN03  are  contained  in  a  rectangular-shaped 
porous  pot,  which  stands  in  an  outer  larger  rectangular  glass  jar  con- 
taining the  H2S04.  The  zinc  is  used  in  the  form  of  a  single  sheet,  bent 
parallel  to  the  sides  of  the  porous  pot,  and  underneath  it. 

The  anode  process  in  the  Bunsen  cell  consists  in  zinc  ions  entering 
the  solution.  The  cathodic  process  depends  essentially  on  the  concen- 
tration of  the  nitric  acid.  The  matter  has  not  been  fully  investigated, 
but  we  know  that,  with  the  strongest  acid,  N02  first  results,  and  as  the 
acid  becomes  weaker  NO  appears.  With  still  more  dilute  acid,  N20  is 
formed,  then  nitrogen,  and  finally  ammonia.  Pure  aqueous  HN03, 
whether  strong  or  dilute,  is  not  a  reversible  depolariser.  Its  rate  of 
reaction  with  the  discharged  hydrogen  is  very  slow,  and  such  an  electrode 
becomes  rapidly  polarised.  A  number  of  substances,  however,  catalyse 
this  depolarisation,  and  such  substances  are  usually  added  to  the 

1  Grundriss  der  techniscJien  Elektrochemie,  p.  156  (1898). 

-  The  capacity  of  a  cell  under  given  conditions  is  the  quantity  of  electricity 
it  can  furnish,  usually  expressed  in  ampere-hours. 


202    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

cathodic  nitric  acid  of  the  Bunsen  cell.  HC1,  H,S04,  H2Cr207  all  act 
thus.  Most  important  is  nitrous  acid,  itself  formed  in  the  first  stages 
of  reduction.  It  increases  enormously  the  rate  of  oxidation  of  the 
hydrogen,  and  thus  keeps  the  E.M.F.  near  to  the  equilibrium  value.1 
It  is  particularly  effective  if  the  acid  has  a  concentration  of  <  30  per 
cent.,  as  polarisation  is  then  very  easy.  As  a  matter  of  fact,  these 
different  catalysts  render  the  depolarisation  in  the  Bunsen  cell  very 
good,  and  large  currents  can  be  taken  from  it  without  any  serious  drop 
in  voltage. 

The  E.M.F.  is  about  T96  volts  if  fuming  HN03  be  used,  but  with 
45  per  cent,  to  65  per  cent,  acid  it  is  1-8-1-9  volts.  This  figure  is 
composed  of  the  difference  of  the  anode  potential  difference,  about  —  0*8 
volt,  and  the  cathode  potential  difference,  +  1*0  to  1*1  volts.  The 
conductivities  of  the  electrolytes  are  high,  and  the  resistance  conse- 
quently low— O'l  to  0'2  ohm.  An  objection  to  the  cell  is  the  evolution 
of  nitrous  fumes  when  working. 

Poggendorff  Bichromate  Cell. — This  cell  resembles  the  above  in 
using  a  liquid  depolariser,  but  differs  from  it  in  containing  only  one 
liquid  and  in  dispensing  with  a  diaphragm.  The  electrolyte  is  a  solution 
containing  H2S04  and  either  potassium  or  sodium  bichromate.  It  of 
course  evolves  no  fumes.  The  total  reaction  at  the  cathode  can  be 
expressed  thus— 

Crt07"  +  14H*  — >  2Cr'"  +  7H20  +  6  ©, 
and  that  in  the  whole  cell  by 

3Zn  +  Cr20/  +  HH'  — >  2Cr'"  +  3Zn"  +  7H20. 

The  electrolyte  very  slowly  attacks  the  zinc  electrode  chemically, 
and,  in  order  completely  to  avoid  local  action  during  long  idle  periods, 
the  latter  is  usually  arranged  so  that  it  can  be  lifted  out  of  the  solution 
or  immersed  in  it  at  will.  The  use  of  Na2Cr207  in  the  electrolyte  is 
preferable  to  that  of  the  potassium  salt,  owing  to  its  greater  solubility. 
This  means  a  higher  capacity,  and  a  lesser  tendency  to  depolarisation. 
If  potassium  bichromate  be  used,  the  solution  contains  8-10  parts 
of  this  salt  and  10-18  parts  H2S04  per  100  parts  water  ;  and  with 
sodium  bichromate,  17-20  parts  of  salt  and  20-24  parts  of  H2S04  per 
100  parts  water.  The  resistance  of  the  cell  is  about  0'3  ohm— low, 
though  higher  than  that  of  the  Bunsen  cell.  The  cathodes  are  of 
carbon. 

We  have  seen  *  that  the  potential  of  a  chromic  acid  solution  in  H2S04 
is  higher  than  the  potential  of  66  per  cent.  HN03.  It  follows  that  the 
E.M.F.  of  the  Poggendorff  cell  should  exceed  that  of  the  Bunsen  cell, 
which  is  the  case.  It  is  usually  2*0  volts.  On  the  other  hand,  it 
polarises  more  easily.  This  is  partly  due  to  sluggishness  in  oxidising 

1  Ihle,  Zeitoch.  Elektrochem.  2,  174  (1895).  "  Table,  p.  196. 


xv.]  PRIMAKY  CELLS  203 

the  discharged  hydrogen,  but  also  to  concentration  polarisation.  The 
Cr207"  ions  around  the  cathodes  and  in  their  pores  become  exhausted 
and  are  replaced  by  Or'"  ions.1  On  standing,  these  concentration 
changes  are  neutralised  by  diffusion  and  the  cell  recovers.  The  polar- 
isation due  to  slow  action  between  the  chromic  acid  and  hydrogen  can 
be  largely  removed  by  adding  some  soluble  chloride,  which  acts  as  a 
catalyst.  It  unfortunately  also  increases  the  rate  of  chemical  solution 
of  the  zinc  by  the  electrolyte.  Owing  to  the  high  voltage,  and  its  com- 
paratively ready  polarisability,  the  bichromate  cell  is  particularly 
suitable  for  furnishing  large  currents  for  short  periods,  between  which 
it  is  allowed  to  recover. 

Lalande  Cells. —  These  cells  also  have  a  soluble  zinc  anode.  But, 
unlike  those  already  considered,  the  electrolyte  is  alkaline,  not  acid, 
and  the  cathodic  depolariser  a  solid,  viz.  CuO.  The  system  is 

Copper  |  Oxides  of  copper  .  Alkali  |  Zinc. 

First  invented  by  Lalande,  they  were  worked  on  by  Edison  and  others, 
and  successful  forms  developed  by  Messrs.  Umbreit  and  Matthes  of 
Leipzig  (Cupron  element)  and  by  Wedekind  (in  England  known  as 
the  Neotherm  cell).  These  latest  types  are  undoubtedly  the  best  and 
most  efficient  primary  cells  we  possess. 

In  construction  the  Cupron  element  consists  of  a  rectangular  glass 
trough,  with  an  ebonite  lid  through  which  pass  the  nickel  terminals 
of  the  plates  inside.  These  number  three,  and  hang  parallel  to  one 
another.  The  two  outer  are  of  amalgamated  zinc,  the  inner  is  the 
porous  copper  oxide  plate.  This  is  constructed  by  rilling  a  flat  cage 
of  copper  gauze  with  cupric  hydroxide,  compressing  strongly,  and 
subsequently  baking.  15  per  cent,  to  18  per  cent.  NaOH  solution  is  used. 
The  Cupron  cells  have  an  E.M.F.  (open  circuit)  of  about  1*0-1 'I  volts. 
This  very  rapidly  falls  when  current  is  furnished  to  about  0*9  volt, 
and  then  slowly  decreases  to  0'75  volt  during  the  discharge.  At  this 
point  it  commences  to  again  rapidly  fall  and  the  cell  polarises.  As  long, 
however,  as  the  active  material  is  still  not  all  reduced,  very  considerable 
currents  can  be  given  without  causing  the  working  voltage  to  appre- 
ciably fall.  The  depolariser  acts  quickly,  and  the  internal  resistance 
is  very  low — 0*03  to  0*05  ohm.  When  run  down,  fresh  zinc  plates  and 
NaOH  are  put  in,  and  the  cupric  oxide  plate  regenerated  by  heating 
it  to  150°— in  an  oven  for  example. 

The  Neotherm  element  consists  of  a  rectangular  iron  containing 
vessel,  closed  with  an  indiarubber  gland  and  an  iron  lid.  Through 
the  lid  pass  ebonite  bushes,  insulating  the  terminals.  There  is  one 
zinc  electrode,  a  vertical  plate  suspended  in  the  middle  of  the  cell. 

1  In  the  Bunsen  cell,  although  the  HN03  is  consumed,  there  is  no  accumulation 
of  other  products,  as  the  gases  formed  stream  away. 


204    PKINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

The  copper  oxide  depolariser  is  caused  by  a  special  process  to  adhere 
to  the  cell  wall,  and  this  is  directly  connected  with  the  positive  terminal. 
Like  the  Cupron  cell,  the  E.M.F.  of  the  Neotherm  cell  is  l'0-l'l  volts, 
and  its  discharge  voltage  0'90-0-75,  though  it  is  customary  to  take  it 
down  to  0'5-0*6  volt  when  working.  Below  that  limit  it  drops  very 
rapidly.  The  internal  resistance  is  even  smaller  than  that  of  the  Cupron 
cell.  The  copper  oxide  is  regenerated  by  heat,  the  whole  cell  body 
being  warmed  up. 

In  spite  of  its  low  voltage,  the  Lalande  cell,  now  that  difficulties 
inherent  in  the  CuO  plate  have  been  overcome,  has  won  considerable 
favour,  and  is  undoubtedly  to  be  preferred  to  any  other  type  of  primary 
cell.  It  is  compact  in  structure,  and  has  a  high  capacity  per  unit- 
weight.  (A  Neotherm  cell  weighing  12  Ibs.  will  give  150  ampere-hours 
if  discharged  at  1  amp.)  The  zinc  consumption  is  practically  theore- 
tical during  discharge,  and  local  action  whilst  standing  is  negligible, 
as  the  solubilities  of  CuO  and  Cu20  (particularly  the  latter)  in  the 
electrolyte  are  very  small.  The  alkali  is  well  protected  against  atmo- 
spheric C02.  The  positive  plate  depolarises  rapidly,  and  admits  of 
large  currents,  whilst  the  regeneration  is  quick  and  convenient.  There 
are  no  diaphragms  or  similar  complications,  and  the  resistance  is  very 
low. 

In  the  Lalande  element  the  zinc  dissolves  as  a  complex  anioii, 
probably  according  to  the  equation 

Zn  +  30H'  — >  Zn02H'  +  H20  +  20. 

Owing  to  the  exceedingly  low  Zn"  concentration  in  equilibrium  with 
this  complex,  the  anode  potential  will  have  a  high  negative  value. 
Measured  against  a  normal  calomel  electrode,  Lorenz *  found  a  voltage 
difference  of  T55  volts,  which  would  correspond  to  a  single  potential 
of  about  —  1'3  volts.  We  know  that  such  an  electrode  is  0-4  volt  more 
negative  than  a  hydrogen  electrode  in  the  same  solution,  and  about 
-  1'24  volts  is  a  more  probable  average  figure  for  this  single  potential 
(assuming  Zn  |  n.  Zn"  =  —  0*77  volt). 

The  change  of  the  copper  oxide  plate  potential  during  working 
lias  been  studied  by  Johnson,2  and  his  results  are  expressed  in  Fig.  54. 
The  initial  potential  is  1'4  volts  positive  to  hydrogen  in  the  same 
solution  (this  is  probably  too  high),  but  very  rapidly  falls  to  0'6  volt. 
At  this  value  it  keeps  constant  for  some  time,  then  falls  quickly  to 
0-4  volt,  and  remains  nearly  constant  during  the  greater  part  of  the 
discharge.  Towards  the  end  there  is  another  rapid  drop  in  potential, 
and  the  point  of  reversible  hydrogen  evolution  is  passed.  But  no 

Zctoch.  EUktrochem.  4,  308  f 

•  Tran*.  Amrr.  Klrrlrnrhnn.  Nor.  1,  187  (W<K).      The  'charge'  curve  refers  to 
the  attempted  use  of  the  copper  oxide  j>lai<  in  MOODcUzy  oeHs,     S«  p.  221. 


XV.] 


PRIMAKY  CELLS 


205 


hydrogen  is  evolved  until  a  potential  corresponding  to  an  overvoltage 
of  about  0'3  volt l  is  reached.  During  the  first  horizontal  stage  of  the 
discharge,  CuO  is  reduced  to  Cu20.  Before  all  the  CuO  has  disappeared, 
however,  polarisation  has  lowered  the  potential  to  the  point  at  which 
Cu20  is  reduced  to  copper.  Along  the  main  horizontal  part  of  the 
curve  this  is  the  chief  process,  though  the  further  reduction  of  CuO  to 
Cu20  occurs  simultaneously.2  When  the  Cu20  in  the  plate  approaches 


Volts. 


C0          40          80         120        160        200        240        280        320       360 
Time  in  Hours. 

FIG.  54. — Discharge  Curve  of  Lalande  Cell  Positive^Electrode. 

exhaustion,  the  potential  rapidly  falls  to  the  value  necessary  for 
hydrogen  evolution.  During  this  stage  Johnson  noticed  that  the  small 
quantities  of  Cu20  left  suffered  further  reduction  to  green  copper 
quadrantoxide  —  Cu40.  The  potentials  of  the  two  stages  of  the 
discharge  (0'6  and  O'l  volt  respectively,  referred  to  a  hydrogen  electrode 
in  the  same  solution)  lie  somewhat  below  the  equilibrium  values.  These 
are3  0'75— O66  volt  for  the  CuO— Cu20  mixture,  depending  on  the 
method  of  preparation  of  the  CuO,  and  O47  volt  for  the  Cu20— Cu 
electrode.  The  discrepancies — up  to  0'15  volt — are  due  to  concen- 
tration polarisation,  as  the  amounts  of  the  oxides  dissolved  in  the 
electrolyte  at  any  moment  are  exceedingly  small. 

Johnson  found  for  the  system  Cu  [  CuO  KOH  Pt  |  H2  the  E.M.F. 
0-73  volt.  This  value  is  probably  too  high,  owing  to  the  presence  of 
dissolved  oxygen.  The  most  probable  figure  is  -f-  0'68  volt  referred 
to  hydrogen  in  the  same  solution.  If  we  subtract  from  this  —  0*41 
volt,  the  potential  of  the  zinc  anode,  we  get  1'09  volt  as  the  E.M.F.  of 
the  cell,  which  is  the  value  actually  obtained.  Its  working  voltage 

1  Ci.  values  for  copper  on  pp.  118-119. 

-  The  Lalande  cell  cannot  be  strictly  regarded  as  a  form  of  the  Daniell  cell, 

as  is  sometimes  done.  The  process  Cu  * >  Cu  does  not  take  place,  but  instead 

we  have  the  two  successive  reactions  Cu" >  Cu*  and  Cu* >  Cu. 

3  Allmand,  Trans.  Chem.  Soc.  95,  2151  (1909) ;  97,  603  (1910). 


206    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

during  the  first  stages  is  somewhat  lower,  and  during  the  chief  part 
of  the  discharge  is  made  up  of  the  difference  of  the  potentials  of  the 
CuO  plate  +  0*4  volt  and  the  zinc  plate  —0*4  volt,  i.e.  0'8  volt.  Lorenz  x 
has  shown  that  the  zinc  electrode  behaves  nearly  reversibly  except  at 
very  high  current  densities.  Finally  the  initial  high  voltage  stages  of 
the  discharge  are  due  to  depolarisation  by  air  dissolved  in  the  elec- 
trolyte and  adsorbed  on  the  positive  plate.  Its  presence  keeps  the 
Cu'  concentration  low  and  the  potential  high.  The  highest  possible 
initial  voltage  thus  obtainable  would  correspond  to  the  air  potential 
(-}-  1*22  volts  with  respect  to  hydrogen  in  the  same  solution)  and 
would  be  about  1'6  volts.  In  practice  figures  above  T2  volts  are  never 
obtained. 

Leclanche'  Cell.—  Like  the  Lalande  element,  the  Leclanche  cell2 
uses  a  zinc  anode  and  a  solid  cathodic  depolariser,  but  the  efficiencies 
and  capabilities  of  the  two  cells  are  very  different.  The  electrolyte 
in  the  Leclanche  is  a  strong  NH4C1  solution  (perhaps  20  per  cent.). 
Various  hygroscopic  substances,  such  as  glycerine,  zinc  chloride  or 
calcium  chloride,  may  also  be  added,  thus  lessening  the  tendency  of  the 
cell  to  lose  water  on  standing.  The  depolariser  is  Mn02,  which  becomes 
reduced  to  manganese  sesquioxide,  OH'  ions  being  liberated.  We  can 
suppose  the  cathodic  processes  to  be  successively 

Mn02  +  2H20  —  »  Mn""  +  40H' 

Mn""  —  *  Mn"*  -f  © 
Mn-  +  30H'  —  >  JMn203  +  fH20. 

The  total  result  is 


Mn02  +  JH20  —  >  JMnA  +  OH'  +  ©. 

The  exact  form  in  which  the  Mn203  is  precipitated  is  not  known.  This 
depolariser  has  several  disadvantages.  It  does  not  react  as  rapidly  as 
the  CuO  plate  of  the  Lalande  cell.  One  or  other  of  the  above  reactions 
takes  place  comparatively  slowly,  and,  unless  the  current  density 
be  kept  low,  the  polarisation  increases  to  such  an  extent  that  the 
potential  necessary  for  H*  discharge  is  reached.  The  cathode  potential  3 
also  becomes  more  negative  owing  to  the  formation  of  OH7  ions.  This 
formation  occurs  in  all  the  primary  cells  we  have  considered  with  the 
exception  of  the  Daniell  cell.  In  elements  with  acid  depolarisers  it  is 
neutralised,  and  in  the  Lalande  cell  its  effect  is  small  in  comparison  with 

1  Loc.  cit. 

»  Friedrich,  EUktrochem.  Zeitsch.  16,  219,  252,  287  (1W9-10). 

3  In  the  present  case 

g  =  (E.P,  +  0.058  .o 


=  constant  —  0'058  log  [OH'] 
See  p.  102. 


xv.]  PRIMARY  CELLS  207 

the  OH'  ions  already  present.  But  in  the  Leclanche  element,  with  a 
neutral  electrolyte,  the  effect  on  the  cathode  potential  can  be  marked. 
On  standing,  the  greater  part  of  the  alkali  of  course  diffuses  away. 

Finally  the  specific  resistance  of  the  Mn02  is  high.  Not  only  does 
this  mean  an  increased  cell  resistance,  but  it  renders  it  difficult  to 
make  the  mass  so  porous  that  the  full  capacity  of  the  depolariser  can 
be  utilised.  It  is  always  necessary  to  mix  some  highly-conducting 
powdered  graphite  or  carbon  with  the  Mn02  on  this  account. 

At  the  anode  zinc  dissolves,  forming  ZnCl2.  The  OH7  ions  produced 
at  the  cathode  give  undissociated  NH4OH  with  the  NH4*  ions,  which 
in  its  turn  furnishes  ammonia  gas.  Some  of  this  escapes  into  the  atmo- 
sphere. But  the  greater  part  remains  dissolved  and  sets  itself  into 
equilibrium  with  the  ZnCl2  as  follows  : 

ZnCl2  +  2NH3  ±^  Zn(NH3)2Cl2. 

This  last  salt,  the  chloride  of  a  complex  zinc-ammonium  cation,  is 
sparingly  soluble  and  gradually  separates  out.  The  equation  expressing 
the  main  reaction  in  a  Leclanche  cell  is  therefore  Zn  -f-  2NH4C1  -f-  2 
Mn02  — >  Mn203  -f  Zn(NH3)2Cl2  -f  H20.  Whether  water  actually 
separates  or  not  depends  on  the  form  in  which  the  Mn203  is  precipitated. 
Besides  Zn(NH3)2Cl2,  basic  insoluble  zinc  ammonium  salts  can  be 
formed.  Another  effect  of  the  cathodic  production  of  OH"  ions  is  to 
precipitate  zinc  hydroxide  where  their  concentration  is  greatest,  i.e. 
in  the  porous  Mn02  mass.  This  precipitation  naturally  still  further 
raises  th^  resistance  of  the  depolariser.  The  conductivity  of  the 
cathodic  mass  of  a-n  exhausted  element  is  exceedingly  low,  due  to  this 
cause  and  to  the  production  of  the  manganese  sesquioxide. 

There  is  one  further  complication.  In  a  working  Leclanche  cell  it 
is  usually  observed  that  the  upper  end  of  the  zinc  electrode  is  far  more 
attacked  than  the  lower  end.  The  cause  is  as  follows.  The  ZnCl2  solu- 
tion formed  anodically  is  heavier  than  the  rest  of  the  electrolyte,  and 
falls  to  the  bottom  of  the  vessel.  We  have  then  a  zinc  rod,  its  two  ends 
dipping  into  solutions  of  Zn012  of  different  strengths,  in  other  words  a 
concentration  cell.1  This  cell  will  furnish  a  current  that  will  tend  to 
neutralise  the  concentration  difference.  Zinc  is  deposited  at  the 
bottom  end  of  the  rod  and  dissolves  at  the  upper  end,  whilst  positive 
electricity  flows  along  the  rod  from  bottom  to  top.  The  result  is  the 
phenomenon  observed. 

The  actual  construction  of  the  Leclanche  cell  shows  many  variations, 
designed  to  increase  capacity  and  lessen  resistance.  A  common  type 
is  a  glass  vessel  containing  the  NKtCl  solution.  The  zinc  anode  is  an 
amalgamated  rod,  whilst  the  cathode  is  a  carbon  rod  surrounded  by  a 
cylindrical  mass  of  the  depolariser.  This  consists  of  powdered  pyrolusite 
intimately  mixed  with  excess  of  graphite  or  carbon.  Various  additions 

1  P.  103. 


208    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

are  sometimes  made,  such  as  MgO,  CaO,  Fe203,  but  their  function 
is  doubtful.  The  mass  is  held  together  by  a  suitable  cement.  In  other 
types  the  depolarising  mixture  is  packed  round  the  carbon  into  a 
porous  pot. 

The  voltage  of  the  Leclanche  on  open  circuit  is  T4-1-65,  but,  owing 
to  polarisation,  drops  to  ri-1'2  volts  when  furnishing  even  small 
currents  (0'1-0'2  amperes  for  an  ordinary  cell).  The  value  1-4  volts 
corresponds  to  the  Mn02-Mn203  electrode ;  the  higher  figure,  1-65 
volts,  is  due  to  depolarisation  by  air  dissolved  and  adsorbed  in  the 
active  mass,  and  is  only  given  for  a  very  short  time  by  fresh  cells.  The 
air  potential  is  about  +  O81  volt,  and  the  potential  of  the  zinc  elec- 
trode at  the  prevailing  low  Zn"  concentration  can  be  taken  as  —  0*84 
volt.  The  current  furnished  depends  on  the  active  area  of  the  depo- 
lariser.  Generally  (to  avoid  hydrogen  evolution)  it  should  not  exceed 
0*1  amps,  /d.m.2,  i.e.  O'l— 0*2  amps,  for  an  average  cell.  The  resistance 
varies  enormously  with  the  construction  (0*05-10  ohms),  but  averages 
0*4-2  ohms.  Despite  its  many  disadvantages,  the  Leclanche  is 
widely  used  for  purposes  requiring  small  intermittent  currents  (bells, 
telephones,  etc.).  The  reasons  are  that  it  is  readily  set  up,  requires 
but  little  attention,  and  is  cheap.  For  larger  continuous  currents  it  is 
useless. 

Dry  Cells.— These  cells1  are  Leclanche  elements  in  which  the 
electrolyte  is  contained  in  some  porous  material  with  which  the  cell  is 
filled.  Such  cells  can  be  placed  in  any  position  without  losing  liquid, 
often  a  great  convenience.  Millions  are  made  yearly,  all  of  small 
capacity,  and  used  for  small  handlamps,  telephones,  door  bells  and 
motor  ignition.  The  outer  containing  vessel  can  be  of  zinc  and  form 
the  anode,  or  of  impregnated  cardboard,  enamelled  iron,  celluloid,  etc. 
The  zinc  is  best  amalgamated,  taking  the  form  of  a  rod  or  a  cylindrical 
piece  of  foil.  The  carbon  rod  (cathode)  is  surrounded  with  the  depo- 
lariser,  a  mixture  of  graphite  or  carbon  powder  and  Mn02,  the  latter  as 
pure  as  possible.  The  electrodes  are  placed  closely  together. 

As  porous  material  are  used  paper-pulp,  sawdust,  cotton-wool, 
cocoanut  charcoal,  clay,  infusorial  earth,  etc.  Gypsum,  magnesia,  and 
ZnO  have  the  disadvantage  of  setting  into  a  solid  mass  under  the 
action  of  the  electrolyte  and  increasing  the  cell  resistance.  The  electro- 
lyte contains  usually  NH4C1  (25  per  cent.)  and  ZnCl2.  The  latter 
decreases  the  local  action  at  the  zinc  electrode  and  also  the  polarisa- 
tion. As  in  the  Leclanche,  hygroscopic  substances  such  as  CaCl2  and 
glycerine  are  also  added,  it  being  essential  that  the  cell  does  not 
become  too  dry. 

The  cells  being  intended  for  intermittent  use,  and  having  to  stand 
idle  for  long  periods,  great  care  must  be  taken  in  their  construction 

»  Trans.  Amer.  Electrochem.  Soc.  16,  97  (1M9) ;  17,  341  (1910). 


xv.j  PRIMARY  CELLS  209 

to  avoid  defects  that  may  lead  to  local  action,1  and  the  materials  used 
must  be  very  pure.  A  trace  of  copper  is  fatal.  The  cells  are  usually 
closed  up  with  hard  pitch,  an  opening  being  left  for  the  escape  of  the 
ammonia  when  working. 

On  open  circuit  the  voltage  of  a  dry  cell  is  equal  to  that  of  a  Le- 
clanche,  perhaps  1*6  volts.  But  if  discharged  at  an  average  rate  only 
about  one  volt  is  obtained.  The  capacity  depends  largely  on  the  rate  of 
discharge.  A  normal  cell  under  normal  conditions  will  furnish  perhaps 
30  ampere-hours  before  its  voltage  falls  to  0*5  volt.  The  resistance 
increases  continually  during  working,  the  evaporation  of  water  and 
the  deposition  of  basic  zinc  compounds  being  responsible. 

3.  Fuel  Cells 2 

The  Problem.  —  When  mechanical  energy  (or  electrical  energy)  is 
produced  from  fuel  by  means  of  boiler  and  steam  engine  (and  dynamo), 
only  some  10  per  cent,  to  15  per  cent,  of  the  total  amount  of  energy 
liberated  by  the  complete  combustion  of  the  fuel  is  obtained,3  and  with  a 
gas  engine  some  25  per  cent.  One  of  the  most  important  technical 
problems  is  a  better  utilisation  of  this  available  chemical  energy  of 
oxidation  of  fuel,  and  many  attempts  have  been  made  to  convert  it 
directly  into  electrical  energy  without  the  intermediate  production  of 
heat,  i.e.  to  bring  about  the  combination  of  the  fuel  and  atmospheric 
oxygen  electrochemically.  If  it  were  possible  to  do  this,  and  reversibly, 
not  merely  a  small  fraction  of  the  free  energy  of  combustion  of  the 
fuel  would  become  available  as  mechanical  energy,  but  the  whole  of  it, 
and  this  naturally  would  mean  an  industrial  revolution  only  comparable 
with  the  first  introduction  of  steam  engines.  Besides  the  enormous 
drop  in  the  cost  of  power,  resulting  in  a  great  extension  of  its  use  and  in 
the  unquestioned  supremacy  of  coal  as  against  water-power  (this  fact 
would  be  of  particular  economic  importance  to  industrial  electro- 
chemistry), a  successful  fuel  cell  would  mean  a  great  mitigation  of  the 
smoke  nuisance  and  vast  changes  in  the  engineering  and  chemical 
industries.  Obviously  all  schemes  so  far  proposed  have  been  fruitless, 
but  in  view  of  the  extreme  importance  of  the  subject  we  must  consider 
it  at  some  length  here. 

The  chief  constituent  of  our  fuels  is  carbon.  If  burnt  to  C02  at 
room  temperature,  one  gram-atom  (12  grams)  liberates  97,650  cals* 
This  is  U,4  the  decrease  of  total  energy.  A,  the  decrease  of  free  energy, 
the  measure  of  the  maximum  amount  of  useful  work  obtainable  from 

1  In  some  types  the  water  necessary  is  only  added  immediately  before  use. 
Such  cells  can  be  stored  indefinitely  till  required  without  fear  of  deterioration. 

-  Ostwald,  Elektrotech.  Zeitsch.  15,  329  (1894).  Haber,  Grundriss  der  Tech- 
nischen  Elektrochemie,  p.  178  (1898).  Zeitsch.  Elektrochem.  11,  264  (1905). 

3  P.  10.  <  P.  78. 


210    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

the  combustion,  is  rather  less,  96,635  cals.  Carbon  is  tetravalent,  and 
its  electrochemical  combustion  would  necessitate  the  passage  of  four 
faradays  per  gram-atom.  If  E  is  the  voltage  of  a  C  —  02  cell  at  room 
temperature,  we  have  then 

E  X  96540  X  4  =  4-19  X  96635 
E  =  1-05  volts. 

If  air  be  used  instead  of  oxygen,  the  E.M.F.  will  be  about  O'Ol  volt 
lower,  i.e.  1'04  volt,  and  the  reversible  combustion  of  one  kilo,  of  carbon 
in  this  way  will  furnish  about  8,940  ampere-hours  at  this  pressure. 
The  coulombs  are  high  and  the  voltage  low. 

The  Difficulties.— To  effect  the  electrochemical  combination  of 
carbon  and  oxygen,  the  obvious  procedure  is  to  construct  a  cell  with  a 
carbon  anode,  an  oxygen  cathode,  and  a  suitable  electrolyte.  Oxygen 
or  oxygen-containing  substances  and  carbon  must  of  course  be  kept 
out  of  contact  with  one  another.  The  earlier  cells  (Becquerel ;  Jabloch- 
koff)  erred  in  this  way,  other  considerations  apart.  But  we  at  once 
encounter  two  main  difficulties.  It  is  essential  that  the  carbon  should 
ionise  (not  necessarily  as  carbon  ions,  for  ions  containing  carbon  and 
some  constituent  of  the  electrolyte  would  suffice),  otherwise  the  electro- 
chemical solution  of  the  carbon  anode  cannot  take  place.  Now,  carbon 
stands  in  the  middle  of  the  first  horizontal  series  of  the  Periodic  System, 
and  one  characteristic  of  these  horizontal  series  is  that  the  end  members 
show  very  marked  electrochemical  properties,  and  have  high  electrolytic 
solution  pressures,  whilst  this  tendency  steadily  diminishes  towards  the 
middle  of  the  series.  Carbon  is  almost  electrically  neutral,  and  has 
resisted  all  attempts  to  make  it  ionise,  whatever  the  electrolyte. 

The  second  difficulty  is  that  the  forms  of  carbon  which  constitute  our 
fuels *  are  by  no  means  pure.  The  presence  of  inorganic  ash  constituents 
apart,  the  carbon  itself  does  not  really  occur  as  elementary  carbon, 
but  rather  as  highly  complex  substances,  containing  also  hydrogen, 
oxygen,  and  nitrogen.  Considering  that  carbon  itself  does  not  ionise, 
it  is  difficult  to  imagine  what  ions  these  carbon-rich  complexes  could 
give.  The  anodic  impurities  would  also  mean  a  foul  electrolyte,  and 
we  have  already  seen  how  essentially  important  it  is  for  an  electrolytic 
process  that  this  fouling  be  minimised. 

The  recognition  of  these  facts  has  led  later  experimenters  to  employ 
a  different  principle.  This  consists  in  first  allowing  the  carbon  to 
react  chemically  with  other  substances,  forming  an  electromotively 
active  product,  and  using  this  product  in  the  primary  cell.  The  final 
result  is  that  the  fuel  is  burnt  to  C02  (also  water,  etc.).  For  example, 
one  could  imagine  the  carbon  to  be  used  to  produce  zinc  from  ZnO, 
giving  at  the  same  time  C02,  and  the  zinc  subsequently  electrochemically 
oxidised  to  ZnO  (assuming  these  reactions  to  be  possible).  Or  Fe208 

1  These,  and  not  electrode  carbons,  would  form  the  active  anodes  in  a  fuel  cell ! 


xv.]  PRIMARY  CELLS  211 

could  be  reduced  by  carbon  to  iron  and  afterwards  electrochemically 
regenerated  from  the  iron  and  air.     The  equations  would  be 

(a)  2ZnO  +  C      —  >  C02  +  2Zn. 

2Zn  +  02   -—  >2ZnO. 

(b)  2Fe203+3C  --  >4Fe+3C02 

4Fe+302  -  ^2Fe203. 

The  above  examples,  if  feasible,  would  of  course  furnish  a  large 
quantity  of  electrical  energy  during  the  second  stage  of  the  reaction, 
but  the  first  stage  of  the  cycle  is  strongly  endothermic,  absorbing  heat 
and  requiring  a  high  temperature,  and  the  wastage  of  fuel  involved 
would  render  the  processes  impracticable.  To  be  successful,  the  first 
(chemical)  stage  of  the  cycle  must  not  take  place  with  any  great  absorp- 
tion of  heat.  If,  on  the  contrary,  it  take  place  with  evolution  of  heat, 
the  smaller  this  evolution,  other  things  being  equal,  the  better  ;  for 
then  a  greater  fraction  of  the  original  heat  of  combustion  of  the  carbon 
will  be  available  for  the  production  of  electrical  energy  in  the  second 
part  of  the  process.  This  method  of  procedure  frankly  abandons  the 
attempt  to  obtain  the  whole  of  the  available  energy  of  combustion  as 
electrical  energy. 

Three  such  processes  have  been  suggested.  One  is  that  used  in  the 
Jungner  cell.1  Carbon  acts  on  strong  H2S04.  The  S02  formed  is 
conducted  to  the  cell,  and  there  combines  electromotively  with  atmo- 
spheric oxygen,  the  resulting  S03  dissolving  in  the  H2S04  electrolyte. 
The  latter  is  continually  drawn  off,  and  used  to  generate  more  S02.  The 
second  process  is  of  considerable  interest,  and  has  been  much  investi- 
gated.2 It  consists  in  principle  in  converting  the  fuel  into  producer  gas, 
and  causing  the  resulting  CO  to  enter  solution  anodically  in  a  suitable 
electrolyte. 

Generator-gas  Cells.—  The  decrease  of  available  energy  corresponding 
to  the  reaction  [C]  -f  [OJ  —  >  [C02]  is  96,635  cals.  at  room  temperature. 
When  carbon  is  burnt  to  CO,  much  heat  is  liberated—  29,650  cals.  for  the 
reaction 


Consequently  the  free  energy  decrease  during  the  reaction 

[C0]+J[02]--  >[COJ. 

is  much  less  than  the  decrease  during  the  reaction 
[C]+[OJ   -->[C02]. 

At  room  temperature  it  is  61,880  cals.     It  follows  that  if  carbon  is 
burnt  to  CO,  and  this  gas  combined  electrochemically  with  oxygen  at 

fil  Q 

room  temperaf  ure,  the  electrical  energy  obtained  will  only  be  —  ,  or 

966 

1  P.  215.  '•  Originally  by  Grove. 

P  2 


212    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

64  per  cent,  of  that  which  the  reaction  C  +  02  — *C02  would  furnish 
if  electrochemically  carried  out.  But  if  even  this  could  be  managed 
in  practice  it  would  be  a  great  advance.  To  the  64  per  cent,  must 
also  be  added  the  energy  which  can  be  generated  from  the  29,650  cals. 
liberated  in  the  gas  producer.  This  might  amount  to  another  5  per  cent. 
of  the  original  96,635  cals.,  making  the  maximum  possible  utilisation  of 
energy  69  per  cent.  The  CO  —  02  cell  would  have  one  advantage  over 
the  C  —  02  cell,  however.  Its  E.M.F.  would  be  higher.  The  combina- 
tion of  one  mol.  CO  with  half  a  mol.  02  necessitates  the  passage  of 
only  two  faradays,  and  we  have 

E  X  96540  X  2  =  4'19  X  61880 
E  =  1'34  volts, 

compared  with  1'05  volts,  the  E.M.F.  of  the  C  -  02  cell. 

All  attempts  to  construct  a  practicable  CO — 02  cell  working  at 
room  temperature  have  failed  (the  best  known,  that  of  Borchers,  we 
shall  presently  discuss).  The  difficulty  of  finding  a  suitable  electrolyte 
in  which  the  CO  can  ionise  has  been  insuperable.  Another  difficulty 
encountered  in  all  gas  cells  is  their  easy  polarisability.  This  is  due 
to  the  fact  that  the  concentration  of  the  gas  in  the  ionising  electrode 
material  is  nearly  always  very  low,  and  can  only  be  renewed  by  diffusion 
from  the  gas  atmosphere  surrounding  the  latter.  This  process  is 
invariably  slow  in  comparison  with  the  rate  of  ionisation,  and,  unless 
the  current  density  is  exceedingly  small,  polarisation  at  once  occurs. 
This  fact  has  led  to  experiments  with  generator-gas  cells  at  high  tem- 
peratures. Under  these  conditions  the  rate  of  diffusion  is  vastly  in- 
creased, and  polarisation  should  be  consequently  diminished.  There 
is  also  the  possibility  that  electrochemical  combination  of  CO  and  02 
may  be  accelerated  at  high  temperatures  just  as  their  chemical  com- 
bination is  accelerated,  i.e.  that  the  CO  may  ionise.  It  must,  however, 
not  be  forgotten  that  working  at  high  temperatures  still  further  limits 
the  electrical  energy  obtainable  by  the  combustion  of  the  fuel.  We 
have  seen  that  the  E.M.F.  of  the  C0-02  cell  at  17°  is  T34  volts.  But 
it  falls  to 

1-25  volts  at  200° 

1-18  volts  at  350° 

Ml  volts  at  500° 

0-88  volt  at  1000°. 

If  the  cell  were  worked  at  500°,  instead  of  getting  64  per  cent,  of 
the  maximum  value,  as  at  room  temperature,  only  53  per  cent,  would 
be  obtained,  a  fraction  further  diminished  by  the  heat  consumed 
in  keeping  the  cell  at  its  working  temperature. 

Bucherer l  worked  in  this  way,  employing  fused  alkaline  carbonates 

1  D.  R.  P.  88327. 


xv.]  PRIMARY  CELLS  213 

as  electrolytes,  and  using  as  electrodes  a  platinum  tube  through  which 
oxygen  was  led,  and  a  nickel  tube  through  which  CO  was  passed.  One 
might  suppose  the  cell  to  function  as  follows  : 

At  anode : 

CO  -f  C03" >  2C02  +  2  ©  (formation  of  C02). 

At  cathode  : 

|02 >  0"  +  2  ©  (formation  of  Na20). 

In  electrolyte : 

0"  +  C02 >  C03"  (reformation  of  Na2C03). 

Total  reaction  : 

CO-f  J02 >C02. 

But  in  practice  complications  entered,  reactions  taking  place  between 
electrodes  and  electrolyte,  and  the  rate  of  diffusion  of  the  gases  through 
the  metal  electrodes  being  too  slow.  The  cell  proved  unworkable. 
Later  experiments  were  undertaken  by  Haber  and  Moser.1  Using 
glass  as  electrolyte,  platinum  black  electrodes,  and  temperatures  of 
450°— 500°,  they  succeeded  in  showing  that  the  combinations  CO— 02 
and  C— 02  could  be  made  electromotively  active,  giving  the  calculated 
E.M.F.s.  But,  owing  to  the  extreme  ease  with  which  these  cells  became 
polarised,  their  results  are  without  practical  value. 

In  the  third  combined  chemical  and  electrical  process,  the  carbon 
reduces  hydrogen  from  water,  and  this  hydrogen  subsequently  combines 
electrochernically  with  oxygen.  The  hydrogen  is  prepared  in  the  form 
of  water-gas  in  a  low  temperature  gas  producer,  and  suitably  separated 
from  the  accompanying  oxides  of  carbon.  The  most  notable  attempt 
on  these  lines  was  made  by  L.  Mond  and  Langer,  and  will  presently  be 
described.  But  it  has  had  no  technical  success,  despite  the  time 
and  skill  spent  on  it.  Mond  and  Langer  worked  at  room  temperature, 
and  were  much  troubled  by  the  easy  polarisability  of  their  electrodes.2 
As  with  the  generator-gas  element,  the  idea  of  working  at  higher 
temperatures  was  obvious,  and  more  likely  of  success  in  this  case,  there 
being  no  difficulty  about  the  actual  ionisation  (qualitative)  of  the 
hydrogen  as  there  was  with  CO. 

But  no  suitable  electrolyte  has  so  far  been  found.  Haber  and 
Fleischmann 3  and  Haber  and  Foster 4  have  shown  that  at  high  tempera- 
tures glass  and  porcelain  allow  the  electrochemical  combination  of  the 
gases  to  proceed,  and  that  the  theoretical  voltage  results  on  open  circuit. 

1  Zeitsch.  Elektrochem.  11,  593  (1905). 

2  On  the  anodic  passivity  of  hydrogen,   see  Sackur,  Zeitsch.  Phys.  Chem. 
54,  641  (1906). 

3  Zeitsch.  Anarg.  Chem.  51,  245  (1906). 
^Zeitsch. _Anorg.  Chem.  51,  289  (1906). 


214     PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

But  the  cells  polarise  with  the  smallest  current.  The  experiments  of 
Beutner l  were  hardly  more  successful.  He  used  an  experimental  arrange- 
ment very  similar  to  Bucherer's,  employing  molten  mixtures  of  various 
halides  as  electrolytes.  On  open  circuit  he  obtained  rather  low  values 
for  the  E.M.F.  of  the  H2-02  cell,  owing  to  diffusion  of  the  gases  through 
the  electrolyte  to  the  opposite  electrode,  and  to  the  oxygen  electrode 
functioning  imperfectly.  And  a  current  density  of  0'8  amps./d.m.8 
lowered  the  cell  voltage  to  O25  volt.  Addition  of  substances  such  as  CuO 
to  the  melt  to  act  as  oxygen  carriers  had  no  success. 

The  four  best-known  attempts  to  utilise  electrochemically  the  free 
energy  of  carbon  combustion,  or  a  part  of  it,  are  due  to  Jacques,  to 
Jungner,  to  Borchers,  and  to  Mond  and  Langer. 

Jacques  Cell. — This  consisted  of  electrodes  of  battery  carbon  and 
iron  in  molten  NaOH.  Round  the  iron  air  was  blown  in.  An  E.M.F. 
was  obtained  of  about  one  volt,  the  iron  being  the  positive  electrode, 
and,  according  to  the  inventor,  represented  the  free  energy  of  combina- 
tion of  the  carbon  anode  and  atmospheric  oxygen  to  C02.  He  asserted 
moreover  that  the  C02  formed  did  nob  dissolve  in  the  electrolyte,  but 
was  evolved  as  gas.  Neglecting  this  statement,  it  is  assumed  that  the 
reaction  at  the  anode  is 

C  -f  60H'  +  4©  — >  CO/  +  3H20 
and  at  the  cathode 

2H20  -f  02  +  40  — >  40H' 
the  total  reaction  being 

C  +  Oa  +  20H'  — *  CO/  +  H20. 

It  was  shown  by  Haber  and  Bruner  2  that,  though  the  last  equation 
expresses  the  total  result,  the  mechanism  of  the  cell's  action  differs 
from  that  sketched  above.  The  iron  cathode,  which  at  first  dissolves 
chemically  in  the  alkali,  soon  becomes  passive,  and  then  indeed  acts 
as  an  air  electrode.  It  does  so  by  virtue  of  the  presence  of  some  man- 
ganese in  the  melt,  either  there  originally  or  dissolved  out  of  the  iron. 
This  manganese  exists  as  a  mixture  of  manganate  and  manganite 
(Mn04"  and  Mn03")  in  equilibrium  with  the  atmospheric  oxygen.  The 
manganate  preponderates.  When  the  cell  is  working,  the  cathodic 
process  consists  in  the  discharge  of  H'  ions  (from  the  water  present  in 
the  melt)  and  the  interaction  of  the  hydrogen  with  the  Mn04"  ions, 
reducing  them  to  Mn03"  ions.  The  latter  are  almost  instantaneously 
reoxidised  by  the  air,  so  rapidly  th.it  the  iron  electrode  behaves  almost 

1  Zeitsch.  EleJetrochem.  17,  91  (If HI). 

2  Zeitsch.    Elektrochem.  10,   697  (1901);    12,  78   (UMi).      Bechterew  [Zeitsch. 
Ekktrochem.  17,  80 1  (/.''//)]  has  recently  published  a  detailed  experimental  investi- 
gation of  the  Jacques  cell.     His  work  confirms  that  of  Haber  and  Bruner  in  essential 
particulars. 


xv.]  PRIMARY  CELLS  215 

perfectly  as  a  reversible  air  electrode.     The  successive  reactions  are 
therefore 

2IT  —  ,H2  +  2© 
H2  +  MnO/  --  >  Mn03"  +  H20 


or,  in  total, 

2H-+JO,  —  >H20+2©. 

At  the  carbon  electrode,  the  first  effect  is  the  production  of  CO  by 
local  combustion.  This  gas,  with  or  without  the  intermediate  forma- 
tion of  sodium  formate,  produces  Na2C03  and  liberates  hydrogen.  If 
CO  is  blown  in,  or  sodium  formate  or  oxalate  added,  the  same  result 
is  obtained.  And  it  is  this  hydrogen  dissolved  in  the  carbon  electrode 
which  is  electromotively  active.1  The  anodic  reactions  are  therefore 


CO  +  2NaOH  --  *  Na2C03  +  H2 

H2  +  2©  —  >  2H' 

or,  in  total, 

C  +  J02  +  2NaOH  +  2©  —  >  Na2C03  +  2H 
The  total  result  of  the  action  of  the  cell  is  therefore 

+  2NaOH  -  >  Na2C03  +  H20, 


and  the  E.M.F.  is  the  E.M.F.  of  a  H2-02  cell,  working  at  that  tempera- 
ture and  under  those  conditions.  Later  work  by  Taitelbaum  2  has 
shown  many  carbonaceous  substances  to  behave  thus  in  molten  NaOH. 
Sugar,  charcoal,  sawdust,  lignite,  coal,  and  coal  gas  all  give  hydrogen 
which  becomes  electromotively  active.  The  number  of  coulombs 
obtained  is  often  a  surprisingly  high  fraction  of  the  theoretical  number. 
But  the  cell  polarises  very  easily,  as  the  velocity  with  which  the  ionised 
hydrogen  is  replaced  by  chemical  action  is  very  low.  This  fact,  coupled 
with  the  conversion  of  expensive  NaOH  into  cheap  Na2C03  makes  the 
Jacques  cell  a  technical  impossibility. 

Jungner  Cell.  —  As  we  have  seen,  we  have  here  the  electromotive 
combination  of  oxygen  and  sulphur  dioxide,  the  latter  being  prepared 
by  the  action  of  the  fuel  on  strong  H.S04.  The  cell  contains  two 
chambers  separated  by  a  vertical  porous  diaphragm.  The  two  elec- 
trodes are  of  graphite.  The  cathode  compartment  is  packed  with  pieces 
of  graphite,  and  through  it  air  passes  ;  the  anode  compartment  is  packed 
with  porous  coke  or  gas  coal,  and  a  stream  of  moist  S02  passed  through. 
The  anode  chamber  contains  sulphuric  acid,  and  the  diaphragm  is  well 

1  Xobis  [Dissertation  (Dresden,  1909)]  showed  hydrogen  dissolved  in  carbon 
to  be  electromotively  inactive  at  room  temperature  arid  in  acid  electrolytes. 
-  ZeittcJi.  Elektrochem.  16,  286  (1910). 


216    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

saturated  with  the  same.  The  cathode  chamber  contains  nitrosyl- 
sulphuric  acid  of  high  concentration.  This  depolarises  the  H'  discharge, 
and  is  supposed  to  be  immediately  reoxidised  by  the  air,  thus  acting 
as  an  oxygen  carrier.  In  the  anode  compartment,  H2S04  is  continually 
formed,  drawn  off,  and  used  to  produce  more  S02.  The  E.M.F.  is 
dependent  on  the  concentration  of  the  acid,  and  is  stated  to  vary  from 
0'5  volt  at  medium  concentration  to  0*3  volt  or  less  with  strong  acid. 
Jungner  says  that  70  per  cent,  to  80  per  cent,  acid  is  the  most  convenient 
working  strength,  the  temperature  being  70°. 

Taitelbaum  has  studied  cells  of  this  type  also.1  He  showed  that 
the  anodic  process  is  due  to  the  electromotive  activity  of  the  S02. 
The  potentials  given  by  different  fuel  stuffs  placed  in  the  anode  chamber 
vary,  depending  on  the  rate  at  which  the  H2S04  attacks  them,  but  all 
approach  the  value  corresponding  to  S02.  He  also  investigated  several 
oxygen  carriers  for  the  catholyte.  Ferro-ferri  and  mercuro-mercuri 
sulphate  mixtures  came  too  slowly  into  equilibrium  with  atmospheric 
oxygen,  and  were  therefore  unsuitable.  But  mixtures  of  thallous  and 
thallic  sulphates  or  the  sulphates  of  tetra-  and  penta-valent  vanadium 
reacted  very  quickly,  soon  showing  the  air  potential  (  -}-  1*35  volts  at 
250°  under  the  conditions  prevailing).  He  finally  constructed  cells 
containing  the  vanadium  depolariser  in  the  catholyte  and  different 
fuels  in  the  anolyte.  At  250°  he  obtained  on  open  circuit  0'4-0'G  volt, 
depending  on  the  fuel  used.  The  figure  was  usually  higher,  the  more 
rapidly  fuel  and  acid  reacted.  But  all  these  elements  polarised  quickly, 
though  not  as  rapidly  as  the  Jacques  cell.  Thus,  whilst  a  small  Daniell 
cell  of  similar  dimensions  only  dropped  in  voltage  from  1*08  to  0'99  volts 
at  a  current  of  0*1  ampere,  a  fuel  cell  using  charcoal  was  polarised 
from  an  open  circuit  voltage  of  0'45  volt  to  0'25  volt.  Cells  with  the 
nitrosyl-sulphuric  acid  recommended  by  Jungner  behaved  far  worse, 
the  oxygen  carrier  evidently  acting  slowly  and  irreversibly. 

Considering  these  results,  one  must  conclude  that  the  Jungner 
element  leaves  the  problem  unsolved.  Though  less  easily  polarisable 
than  the  Jacques  element,  its  very  low  E.M.F. ,  corresponding  to  about 
a  30  per  cent,  utilisation  of  the  free  energy  -of  carbon  combustion, 
renders  its  further  development  highly  unlikely. 

Borchers  Cell.— This  was  a  very  ingenious  attempt  to  effect  the 
electrochemical  combination  of  CO  and  02,  but  as  it  has  been  conclu- 
sively shown  that  its  E.M.F.  did  not  depend  on  the  presence  of  the 
CO  we  need  only  very  briefly  consider  it.  CO  dissolves  in  a  hydro- 
chloric acid  solution  of  CuCl,  forming  a  loose  additive  compound.  The 
anode  chamber  in  Borchers'  cell  was  filled  with  this  solution,  and  CO 
gas  was  bubbled  through.  The  cathode  compartment  contained  the 
'uCl  solution,  but  with  oxygen  instead  of  carbon  monoxide 

*  Loc.  cit. 


xv.]  PRIMARY  CELLS  217 

bubbling  through  it.  Copper  electrodes  were  used.  Borchers  hoped 
that  changes  on  the  following  lines  would  occur  : 

At  cathode : 

(a)  2Cu" *2Cu'+2© 

(6)  J02+2Cu'+2H' — >2Cu"+H20. 
Total:  J02  +  2H* >H20+2©. 

At  anode : 

(a)  2Cr *Cl2  +  20 

(6)  C12  +  CO  (of  dissolved  complex)  +  H20 >  C02  -f-  2H* 

+  2Cr. 
Total:  C04-H20 — *C02  +  2H'+2Q, 

giving  a  total  cell  reaction 

C0+j02 >C02. 

But  the  small  E.M.F.s.  observed  cannot  be  ascribed  to  any  such 
reaction.  R.  Mond  showed  that  if  the  copper  electrodes  were  replaced 
by  carbon  the  E.M.F.  sank  almost  to  zero,  and  Barnes  and  Veesen- 
mayer x  failed  to  detect  any  C02  in  the  anode  gases.  As  Mond's  observa- 
tion indicates,  the  probability  is  that  the  observed  E.M.F.s.  depended 
essentially  on  the  presence  of  the  copper  electrodes. 

Mond-Langer  Cell.2— These  workers  were  guided  by  the  following 
principles : — 

(a)  The  current  density  must  be  exceedingly  low. 

(6)  The  gases  must  only  have  a  short  distance  to  diffuse  through 
before  reaching  the  surface  at  which  they  ionise. 

(c)  The  electrodes  must  be  kept  free  from  any  film  of  liquid,  other- 
wise the  solution  of  the  gas  is  hindered.  These  three  conditions  are 
essential  for  supplying  gas  to  the  electrodes  as  quickly  as  it  is  ionised, 
and  therefore  for  the  reversible  working  of  the  cell. 

One  of  the  constructions  they  used  is  shown  in  Fig.  55.  The  elec- 
trolyte—dilute H2S04— is  contained  in  the  porous  diaphragm  C  (stone, 
gypsum,  asbestos,  pasteboard),  just  as  it  is  in  dry  cells.  The  two  sides 
of  C  are  covered  with  very  thin  platinum  sheets,  pierced  with  a  large 
number  of  tiny  holes,  some  1500  per  cm.2  These  sheets,  which  con- 
stitute the  electrodes,  are  backed  with  leaden  strips  to  decrease  the 
internal  resistance,  provided  with  leads  of  copper  wire,  and  finally 
painted  on  the  back  with  platinum  black.3  The  diaphragm  and 
electrodes  are  then  set  in  an  ebonite  frame  A,  and  clamped  together 
between  two  other  ebonite  frames  B,  B',  the  frames  being  separated  by 

1  Zeitech.  Angew.  Chem.  8,  101  (1895). 
-  Proc.  Roy.  Soc.  A.  48,  296  (1889). 

3  Best  prepared  by  precipitating  boiling  HsPtCle  with  Na^COs,  and  reducing 
with  H.COONa. 


218    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

rubber  rings.  Through  the  shallow  chambers  behind  the  electrodes 
(of  the  depth  of  the  rubber  rings)  the  reacting  gases  are  led,  one  on  each 
side,  and  commence  to  ionise. 

The -results  given  depend  largely  on  the  quality  of  the  platinum 
black. 

On  open  circuit  about  0'97  volt  was  usually  got.  The  reversible 
value  is  T23  volts,  and  the  highest  figure  ever  got  with  platinum  elec- 


FIG.  65. — Mond-Langer  Cell. 

trodes  about  1*15  volts.  The  inner  resistance  depended  on  the  dia- 
phragm, its  nature,  thickness,  and  amount  of  electrolyte  present; 
but  it  was  generally  low.  A  gypsum  diaphragm  of  area  (one  side) 
350  c.m.2  and  thickness  8  m.m.  had  a  resistance  of  only  0*02  ohm. 
Such  an  element  furnished  currents  up  to  8  amperes,  but  its  E.M.F. 
rapidly  fell.  As  typical  of  the  final  results  obtained,  it  may  be  recorded 
that  a  cell  with  a  total  active  surface  of  700  c.m.2  and  containing  0*35 
gram  Pt  foil  and  1  gram  Pt  black  could  give  a  current  of  2  to  2 '5 
amperes  at  0'73  volt.  This  was  reckoned  to  correspond  to  an  energy 
efficiency  of  about  50  per  cent. 

Good  as  this  result  appears  compared  with  those  of  other  cells,  it 
was  only  achieved  by  using  an  impracticable  apparatus.  Technically, 
the  first  cost  would  be  too  high,  and  the  complications  impossible.  The 
platinum  black  is  also  very  sensitive  to  the  presence  of  traces  of  impuri- 
ties. Haber  l  found  that  2*5  per  cent,  of  CO  present  in  the  hydrogen 
seriously  affects  the  absorption  of  the  latter  by  the  platinum  electrode. 
Finally,  great  concentration  changes  go  on  during  the  electrolysis. 
If  sulphuric  acid  is  the  electrolyte,  this  will  concentrate  in  the  layer 
of  the  diaphragm  next  to  the  anode  (hydrogen  electrode)  owing  to  the 
migration  of  the  HSO/  and  SO/'  ions,  and  water  will  collect  in  the 
cathode  layer,  enormously  increasing  the  internal  resistance.  These 
changes  cannot  be  neutralised  by  circulation  of  the  electrolyte  or  by 
stirring.  The  only  possibility  would  be  to  frequently  interchange  the 

1  Qrundriaa  der  Technischen  Elektrochemie,  p.  199  (1898).. 


xv.]  PRIMARY  CELLS  219 

streams  of  gas,  and  thus  alter  the  direction  of  the  current.  All  these 
facts  help  to  render  the  Mond-Langer  cell  technically  impracticable. 

Hydrogen-chlorine  Cell.— For  completeness,  one  other  gas  cell 
should  be  discussed.  It  would  be  of  great  importance  for  the  alkali- 
chlorine  industry1  if  a  technical  cell  could  be  constructed  in  which 
hydrogen  and  chlorine  gases  could  be  caused  to  combine  electrochemi- 
cally  to  hydrochloric  acid.  The  hydrogen  in  most  alkali-chlorine  works 
is  allowed  to  escape,  the  chlorine  is  converted  into  bleach,  the  market 
for  which  is  very  depressed  at  present.  In  many  cases,  even  to  allow 
hydrogen  and  chlorine  to  combine  chemically  might  be  advantageous. 
There  is  no  doubt  that  their  electrochemical  combination  would  be. 
If  reversibly  carried  out,  giving  a  concentrated  acid,  the  cell  would 
have  an  E.M.F.  of  about  1'2  volts  at  room  temperature.2  Supposing 
the  brine  electrolysis  to  require  4*5  volts,  the  manufacturer  would  save 
25  per  cent,  to  30  per  cent,  of  his  power,  and  would  make  strong  HC1 
instead  of  bleach.  This  is  doubtless  a  difficult  problem,  but  never- 
theless far  easier  than  effecting  the  combination  of  hydrogen  and  oxygen, 
as  chlorine  ionises  readily,  very  differently  from  oxygen.  Nor  would 
there  be  serious  difficulties  due  to  concentration  changes,  as  in  the  Mond- 
Langer  cell. 

Nobis 3  has  studied  the  subject.  He  found  the  ionisation  of  chlorine 
to  take  place  easily  at  platinum,  graphite  or  carbon.  Such  electrodes 
were  not  readily  polarisable.  Hydrogen  was  electromotively  inactive 
at  carbon,  but  ionised  at  platinum  or  platinised  carbon.  These  elec- 
trodes were  very  polarisable.  This  is  essentially  due  to  the  low  solu- 
bility of  hydrogen  in  aqueous  solutions.  The  gas  must  pass  from 
electrolyte  into  electrode  before  it  can  ionise,  and  the  amount  dis- 
solved in  the  electrolyte  becomes  quickly  exhausted.  The  more 
rapidly  hydrogen  is  bubbled  through,  the  better  the  results  obtained. 
Nobis  constructed  a  small  diaphragm  cell,  about  6"  X  5"  X  4' ',  and 
was  able  to  take  continuously  from  this  at  room  temperature  a  current 
of  O63  ampere  at  0*73  volt.  By  working  at  60°,  the  power  furnished 
could  be  increased  by  50  per  cent.  These  results  however,  necessitated 
passing  hydrogen  gas  very  rapidly  through  the  cell,  less  than  1  per  cent, 
being  utilised.  This  would  introduce  great  difficulties  technically. 


Literature 
Bern.    Elemenle  und  AkJcumulatoren. 

P.  385.  -  P.  82.  3  Dissertation  (Dresden,  1909). 


CHAPTER  XVI 

SECONDARY  CELLS 

1.  General  Considerations 

ELECTRICAL  energy  can  only  be  conveniently  stored  in  quite  small 
amounls.  The  storage  of  large  quantities  is  possible  indirectly  by 
transforming  the  electrical  energy  into  some  kind  of  energy  which 
can  be  stored  and  retransformed  into  electrical  energy  at  will.  Such 
a  form  of  energy  is  chemical  energy,  and  a  system  in  which  these 
reciprocal  transformations  of  chemical  and  electrical  energy  can  take 
place  is  termed  a  secondary  cell  or  accumulator,  as  distinguished  from 
primary  cells,  which  are  only  concerned  with  the  transformation  of 
chemical  into  electrical  energy. 

We  have  discussed  at  length x  the  conditions  fulfilled  by  an  ideal 
primary  cell.  They  are 

(a)  High  capacity  of  energy  and  current  per  unit  weight  and  volume. 
A  high  voltage  is  thus  desirable. 

(6)  No  chemical  action. 

(c)  Reversibility. 

(d)  Low  resistance. 

(e)  Simplicity  and  strength  of  construction.     Conditions  (6),  (d),  and 
(e)  all  declare  disadvantageous  the  use  of  two  liquids  and  the  diaphragm 
thereby  rendered  necessary. 

(f)  Durability. 

(g)  Low  cost  of  materials. 

These  conditions  should  also  be  satisfied  by  secondary  cells,  and 
most  are  even  more  important  than  is  the  case  with  primary  cells. 

Thus  :— 

(a)  The  capacity  per  unit  weight  is  of  prime  importance,  as  they 
are  used  on  a  large  scale  compared  with  primary  cells,  and  form  a 
convenient  means  of  transporting  electrical  energy. 

(6)  As  they  are  meant  for  continued  use,  their  life  not  being  limited 
to  a  single  discharge,  and  as  they  may  have  to  stand  idle  for  long 

1  Pp.  195-199. 
220 


SECONDARY  CELLS  221 

periods,  it  is  essential  that  even  very  slight  chemical  effects  causing 
deterioration  or  loss  of  stored  electrical  energy  should  be  excluded. 

(c)  As  they  store  up  electrical  energy  in  order  to  give  it  out  later, 
it  is  desirable  that  the  losses  during  these  operations  be  minimal.  Hence, 
both  the  transformation  of  electrical  into  chemical  energy  (charging)  and 
that  of  chemical  into  electrical  energy  (discharging)  should  proceed 
nearly  reversibly. 

(d)  The  losses  are  lower,  the  lower  the  resistance  of  the  cell. 

(e)  A  general  principle  for  the  construction  of  all  chemical,  and 
particularly  electrochemical,  apparatus. 

(/)  Of  the  highest  importance,  secondary  cells  being  intended  for 
long  continued  use. 

(g)  Provided  (/)  is  fulfilled,  less  important  than  with  primary  cells. 

Successful  Cells. — Only  two  secondary  cells  need  to-day  be  con- 
sidered. The  one  is  the  universally  used  lead  accumulator  [Lead 
peroxide  |  sulphuric  acid  |  lead],  the  other  the  Edison  nickel-iron 
accumulator  [Nickelic  hydroxide  |  caustic  potash  |  iron],  which  will 
probably,  in  its  improved  form,  become  extensively  used.  The  former 
adequately  fulfils  conditions  (6),  (c),  (d),  and  (g),  and,  if  well  treated, 
also  (/).  Its  construction  is  very  simple,  but  the  mechanical  strength 
of  the  lead  plates  rather  low,  and  their  great  weight  a  drawback. 
The  improved  Edison  accumulator  fulfils  conditions  (a),  (b),  (d),  (f).  Its 
first  cost  is  somewhat  high,  owing  to  the  materials  and  to  complexities 
in  construction ;  but,  as  the  cell  is  mechanically  strong  and  durable,  that 
is  of  minor  importance.  Unfortunately,  the  charging  and  discharging 
losses  considerably  exceed  those  of  the  lead  accumulator. 

Unsuccessful  Attempts. —  Of  the  numerous  other  attempts  made  to 
construct  practicable  accumulators  a  few  may  be  mentioned.  With  an 
acid  electrolyte,  the  best  known  is  perhaps  the  combination  Pb02 1  acid 
ZnS04  |  Zn.  Its  E.M.P.  is  2' 47  volts,  but  local  action,  chemical 
solution  of  the  zinc,  and  the  impossibility  of  reversibly  depositing  the 
latter  during  charging,  render  its  use  impossible,  v.  Welsbach's  cell, 

Carbon  |  Ce2(S04)3  -f  Ce(S04)2  |  ZnS04|  amalgamated  zinc, 
infringed  several  principles  of  secondary  cell  design,  particularly  in 
using  two  fluids  and  a  diaphragm.  Many  cells  with  alkaline  electrolytes 
have  been  proposed,  with  oxides  of  copper,  silver,  nickel,  or  mercury 
at  the  positive  pole,  metallic  zinc,  cadmium,  cobalt,  or  iron  at  the 
negative  ;  but  none  have  proved  successful. 

At  one  time  hopes  were  entertained  that  the  Lalande  primary  cell l 
would  also  function  satisfactorily  as  an  accumulator,  and  extensive 
tests  were  carried  out.  But  its  behaviour  during  charging  was  very 
unsatisfactory  :  the  CuO  plate  did  not  behave  reversibly,  and  an  energy 
efficiency  of  only  35  per  cent,  to  50  per  cent,  usually  resulted  ; 2  whilst 

1  P.  203. 

-  Johnson,  Tram.  Amer.  Electrochem.  Soc.  1,  187  (1902) ;  and  Figure  54. 


222    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

the  zinc  deposited  with  difficulty  unless  the  electrolyte  was  previously 
saturated  with  zinc  oxide.  Local  action,  negligible  during  the  short 
life  of  the  primary  cell,  played  a  part,  and  it  was  found  necessary  to 
surround  the  CuO  plate  with  a  diaphragm  ;  whilst  the  gradual  absorp- 
tion of  atmospheric  C02  lowered  the  solvent  properties  of  the  electro- 
lyte for  ZnO,  and  led  to  the  latter  being  slowly  deposited  in  the  cell. 

2.  The  Lead  Accumulator 

The  chemical  changes  taking  place  in  this  cell  were  first  made 
clear  by  Gladstone  and  Tribe.  When  in  a  condition  to  furnish  electrical 
energy  it  consists  of  the  system  Pb02 1  H2S04 1  Pb.  During  discharge 
the  lead  peroxide  is  reduced  to  lead  sulphate,  and  at  the  other  elec- 
trode lead  is  oxidised  to  lead  sulphate.  The  discharged  system  is 
therefore  PbS04  |  H2S04  |  PbS04.  On  charging  the  process  is  reversed, 
and  the  system  Pb02  |  H2S04  |  Pb  regenerated.  The  chemical  action 
involved  is 

Pb  +  Pb02  -f  2H2S04  ;±  2PbS04  +  2H20 

taking  place  from  left  to  right  during  discharge,  and  from  right  to  left 
during  charge.  During  charge,  therefore,  the  acid  electrolyte  becomes 
more  concentrated,  during  discharge  more  dilute.  The  positive  pole 
is  the  PbOo  electrode.  Positive  electricity  passes  out  from  it  during 
discharge  and  enters  through  it  during  charge.. 

Construction.— Accumulators  are  made  in  all  sizes,  from  very  small 
elements  with  two  or  three  plates  for  experimental  work  up  to  cells 
furnishing  16,000  amp. -hours  on  discharge  and  weighing  some  tons.  The 
containing  vessels  for  the  smaller  units  are  of  glass,  and  for  the  larger 
ones  of  lead-lined  wood.  For  transportable  cells  they  are  constructed 
of  celluloid  or  hard  rubber.  The  plates  are  hung  vertically,  positives 
and  negatives  alternately,  similar  plates  being  connected  in  parallel. 
Sometimes  they  are  guided  by  grooves  in  the  sides  of  the  cell.  Their 
distance  apart  is  generally  8-15  mm.,  being  greater  the  larger  the  cell. 
For  transportable  cells,  where  the  utmost  lightness  is  of  importance, 
this  distance  may  fall  to  3-5  mm. 

To  guard  against  short -circuiting,  glass  tubes  or  rods  are  inserted 
between  adjacent  plates.  Between  the  lower  ends  of  the  plates  and 
the  bottom  of  the  container  is  a  free  space.  Besides  supplying  a  reser- 
voir of  acid,  short-circuiting  due  to  conducting  material  dropping 
from  the  plates  is  thus  avoided.1  The  two  end  plates  are  always 

1  There  is  an  accompanying  drawback.  The  extra  layer  of  liquid  decreases 
the  resistance  locally  in  comparison  with  the  resistance  between  other  parts  of  the 
plates.  The  current  density  thereby  proportionately  rises,  causing  concentration 
differences,  and  tending  to  make  the  plates  '  buckle.'  See  Schoop,  Electrochem. 
2nd.  4,  268,  307  (1906). 


XVI.] 


SECONDARY  CELLS 


223 


negatives,  as  positive  plates,  if  only  active  on  one  side,  become  deformed 
owing  to  volume  changes  during  working.  The  electrolyte  is  usually 
H2S04  of  S.G.  1-15,  a  21  per  cent,  (weight)  solution,  but  up  to  S.G. 
1*20  can  be  used. 

The  Plates. — Accumulator  plates  consist  of  (a)  the  supporting  and 
conducting  framework,  and  of  (6)  the  active  mass.  The  framework 
is  usually  of  hard  lead,  containing  4-6  per  cent,  antimony,  and  on  to 
it  the  active  mass  is  brought  by  one  of  two  distinct  methods.  The 
first  is  due  to  Plante,  and  consists  in  starting  with  a  solid  lead  plate 
of  one  piece  with  the  framework,  and  transforming  its  surface  layers 
either  chemically  or  electrochemically  into  the  Pb02  or  spongy  lead 
required.  In  the  second  method,  that  of  Faure,  the  framework  is 
of  somewhat  complicated  construction,  provided  with  projections, 
cavities,  pockets,  etc.  In  these  interstices  is  placed  a  paste  of  suitable 
materials,  and  this  paste  changed  electrochemically  into  Pb02  or  finely 
divided  lead.  The  process  of  producing  active  material  on  an  accumu- 
lator plate  is  (in  both  cases)  termed  forming,  and  the  layer  produced 
must  be  sufficiently  thick  and  mechanically  strong. 

Plante  Plates.— In  the  original  Plante  process  a  current  was 
passed  between  two  sheets  of  lead  in  dilute  H2S04,  its  direction  being 
frequently  reversed.  At  the  one  electrode  a  thin  layer  of  Pb02  was 
first  produced,  hydrogen  being  liberated  at  the  other  plate.  On 
reversing  the  current,  the  latter  became  superficially  oxidised,  the  Pb02 
on  the  first  plate  being  reduced  to  spongy  lead.  This  process,  many 
times  repeated,  finally  resulted  in  the  formation  on  one  plate  of  a 
layer  of  Pb02,  on  the  other  of  a  layer  of  spongy  lead.  By  thus  elec- 
trolysing for  a  year  a  thickness  of  about  1  mm.  could  be  reached. 
Such  an  active  deposit  is  naturally  very  strong  mechanically,  but  the 


FIQ.  56.— Types  of  Plante  Plates. 

process  has  otherwise  two  great  disadvantages.  The  capacity  of  the 
finished  plate  is  very  low,  compared  with  its  weight  and  volume,  and 
the  time  (and  electrical  energy)  needed  for  the  forming  is  excessive. 
As  the  capacity  depends  on  the  total  weight  (or  volume)  of  active 
material  present,  it  is  evident  that  it  can  be  increased  by  increasing 


224    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

either  the  area  of  lead  affected  or  the  thickness  of  the  layer.  To  increase 
the  latter  beyond  a  certain  limit  is  not  desirable.  Its  mechanical 
strength  is  lessened,  and  the  underlying  parts  are  not  in  good  contact 
with  the  mass  of  the  electrolyte. 

Accordingly  it  has  become  usual  to  employ  plates  of  highly- 
developed  surface  for  forming.  Fig.  56  (a)  shows  in  side  section  the 
ribbed  plate  with  a  solid  lead  core  introduced  by  the  Brothers  Tudor. 
In  Fig.  56  (6)  is  shown  a  plate  manufactured  by  the  A.F.A.G.  of 
Berlin-Hagen.  The  lead  core  is  dispensed  with,  supporting  ribs  being 
used.  The  weight  is  thereby  decreased,  and  free  circulation  allowed 
to  the  acicl.  Fig.  56  (c)  shows  the  D.P.  (Dujardin-Plante)  plate, 
consisting  of  parallel  lead  strips,  burnt  together  at  the  ends. 

Rapid  Forming. — The  second  improvement  introduced  allows  of  a 
far  more  rapid  forming  than  heretofore.  To  the  dilute  H2S04  is  added 
a  small  quantity  of  some  *  catalyst '  such  as  HC104,  HC103,  HN03, 
S02,  CHs.COOH.1  The  result  is  the  anodic  production  of  an  adherent 
layer  of  PbS04  instead  of  the  Pb02.  This  is  more  porous  than  the 
latter,  and  allows  further  action  to  proceed.  By  thus  electrolysing  with 
a  current  density  of  about  0'8  amp./d.m.2,  as  much  active  material 
can  be  produced  in  a  week  as  by  the  original  Plante  process  in  a  year.2 
Before  treating  the  plates  further,  they  must  be  very  carefully  washed 
free  from  every  trace  of  the  addition  agent  used.  Cathodic  reduction 
will  then  give  a  plate  of  spongy  lead,  and  anodic  oxidation  in  dilute 
H2S04  a  plate  of  Pb02. 

The  explanation  of  the  action  of  the  addition  agents  is  that  in 
solutions  containing  only  the  one  anion  SO/',  the  Pb"  ions,  on  entering 
solution,  are  immediately  precipitated  as  a  dense  non-conducting 
PbS04  crust  on  the  anode.  Current  can  only  penetrate  this  with  diffi- 
culty. At  those  points  where  it  does  so  the  current  density  is  very 
high,  and  the  electrode  potential  considerably  more  positive  than 
the  equilibrium  value.  Pb""  ions  are  produced,  the  sparingly  soluble 
Pb02  precipitated,  and  oxygen  evolved.  If,  however,  a  small  fraction 
of  the  anions  present  does  not  form  an  insoluble  compound  with  the 
Pb"  ions,  these  anions  will  accumulate  round  the  anode,  carried 
thither  by  ionic  migration.  And  though  the  Pb"  ions  dissolving  will 
be  ultimately,  as  before,  precipitated  as  PbS04,  this  precipitation  will 
not  occur  in  the  liquid  layers  immediately  bounding  the  electrode,  but 
at  some  small  distance  away.  This  distance  will  be  determined  by  the 
current  density,  temperature,  and  the  concentration  of  the  S04"  and 

SO  " 

added  ion  in  the  electrolyte.     If  the  ratio  -  -  be  very  great, 

added  ion 

no  appreciable  effect  will  be  produced.     If  it  be  rather  less,  then  a 

1  Fischer  has  suggested  phosphoric  acid.     Zeitsch.  Elektrochem.  16,  355  (W 10). 
-  Methods  have  been  recently  so  much  improved  that,  by  working  at  high 
current  densities,  the  whole  process  of  forming  can  be  completed  in  half  a  day. 


XVI.] 


SECONDARY  CELLS 


225 


layer  of  PbS04  will  be  formed  which,  though  adherent,  is  still  sufficiently 
porous  to  allow  electrolytic  conduction  to  take  place  through  it.  These 
are  the  conditions  aimed  at  during  forming.  If  the  ratio  be  still  further 
lowered,  a  loose  precipitate  of  PbS04  will  result,  which  will  stream 
away  continuously  from  the  electrode.1  For  an  investigation  of  the 
influence  of  current  density,  temperature,  and  the  concentrations  of 
the  electrolyte  constituents  when  using  HN03,  see  Just,  Askenasy,  and 
Mitrofanofi.2  Schleicher3  has  published  a  similar  investigation,  using 
chlorate  and  perchlorate  as  additions. 

In  practice,  negative  lead  sponge  Plante  plates  are  never  used,  as 
the  finely  divided  lead  originally  present  agglomerates  together,  and  the 
diminution  in  surface  leads  in  time  to  a  serious  diminution  in  capacity. 
Negative  plates  are  always  pasted,  and  of  about  twice  the  capacity  of 
a  positive  of  equal  size.  On  the  other  hand,  Plante  positives  are 
widely  used.  Owing  to  their  great  surface  they  are  particularly 
suitable  for  high  rates  of  discharge,  and  distortion  and  losses  due  to 
volume  changes  in  the  active  material  are  negligible.  For  portable 
accumulators  their  weight  renders  them  unsuitable. 

Fame  Plates.— In  Faure  plates  the  holders  for  the  active  material 
are  of  the  most  varied  description,  designed  to  secure  a  maximum 
weight  of  paste  of  sufficient  mechanical  strength,  making  maximum 
contact  with  electrolyte  and  supporting  frame.  The  construction  of 
the  positives  and  negatives  also  differs  because  the  positive  and  negative 
active  materials  show  opposite  volume  changes  in  course  of  time. 


(?    ft 


(a)  (b) 

FIG.  57.— Types  of  Faure  Plates. 

Thus,  whilst  the  conversion  of  PbS04  to  spongy  lead  or  Pb02  during 
charging  involves  an  increase  of  volume  in  both  cases,  the  negative 
active  mass  shows  a  decrease  in  volume  on  standing,  the  positive 
material  an  increase. 

Fig.  57  (a)  shows  the  positive  plate  of  the  Chloride  Co/s  accumu- 
lator, consisting  of  a  lead  frame  pierced  with  a  number  of  circular  holes, 


1  See  p.  388. 


2  Zeitsch.  Elektrochem.  15,  872  (1909). 
3  Ibid.  17,  554  (1911). 

Q 


226    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

each  of  which  contains  a  corrugated  lead  spiral,  projecting  out  for 
2-3  mm.  on  either  side  of  the  plate.  The  interstices  in  these  spirals 
contain  the  active  mass.  Fig.  57  (6)  shows  a  plate  consisting  of  a 
simple  lead  grating  or  lattice-work,  its  meshes  being  filled 'with  active 
material.  The  Tudor  negative  plates  are  of  this  type.  Some  plates 
consist  almost  entirely  of  active  material,  and  must  be  discharged  very 
slowly,  as  the  surface  of  contact  of  the  latter  with  the  lead  frame  is 
small,  and  the  resistance  consequently  high.  Further,  a  rapid  rate  of 
discharge,  in  consequence  of  the  large  and  rapid  volume  changes  it 
would  produce,  would  soon  lead  to  disintegration  of  the  plate. 

In  the  case  of  positive  plates  of  this  kind,  there  is  a  limit  which  it 
is  impossible  to  exceed  in  increasing  the  ratio  of  active  material  to 
inactive  lead  frame.  Gladstone  and  Tribe  showed  that  the  positives 
of  an  accumulator  undergo  slow  self-discharge,  the  Pb02  and  the  lead 
of  the  supporting  frame  acting  as  a  short-circuited  cell.  In  this  way 
some  of  the  lead  of  the  frame  is  converted  into  PbS04,  and,  on  recharging, 
into  Pb02.  Thus  a  constant  corrosion  of  the  lead  grating  proceeds, 
with  formation  of  sulphate  and  peroxide,  and  if  the  grating  is  initially 
too  weak,  the  whole  plate  may  later  collapse. 

The  paste  with  which  the  plates  are  treated  generally  consists  of 
PbO  (for  positives,  Pb304  can  be  used),  worked  up  with  H2S04  or.  an 
NaH>S04  solution.  Shortly  after  pasting  on,  it  sets  to  a  hard  mass  of 
basic  lead  sulphate.  The  plates  are  formed  electrochemically  in  a 
solution  of  magnesium  or  aluminium  sulphate,  kept  as  nearly  neutral 
as  possible.  When  the  reduction  to  spongy  lead  and  oxidation  to 
peroxide  is  complete,  the  plates  are  washed  free  from  all  traces  of  salts, 
and  alternately  discharged  and  charged  once  or  twice  in  H2S04.  Various 
organic  substances,  such  as  glycerine,  gelatine,  etc.,  are  often  added  to  -! 
the  electrolyte  during  forming.  It  is  claimed  that  capacity,  mechanical 
strength,  and  porosity  are  thereby  increased.  The  way  in  which 
these  substances  act,  if  at  all,  is  far  from  clear.  Mercury  salts  are 
also  added.  They  undoubtedly  do  eflect  an  increase  in  capacity.  The 
overvoltage  for  both  hydrogen  and  oxygen  discharge  is  higher  at 
amalgamated  than  at  pure  lead,  and  consequently,  when  charging,  the] 
useful  production  of  spongy  lead  and  Pb02  continues  for  a  longer  time,! 
gas  evolution  commencing  later. 

The   great  disadvantage    of    pasted    plates    is   their  mechanical 
weakness.       In  spite  of  great  care,  the  active  material  tends  to  fall] 
out  in  time,  and  particularly  so  at  high  current  densities,  with  con-< 
sequent  rapidly  succeeding  volume  changes. 

Eflect  of  Impurities.— The  purest  materials  are  necessary  in  con-! 
structing  and  setting  up  accumulators.     The  lead  must  be  free  from] 
and  copper.     The  H2S04  and  active  material  must  contain  no 
HN03  or  HC1.     Otherwise  local  chemical  solution  of  the  lead  will  be  i 
increased,  and  during  charge  a  certain  amount  of  forming  will  occur 


XVL]  SECONDARY  CELLS  227 

every  time  at  the  positive  electrode.  Iron  must  be  absent,  or  it  will 
cause  a  lowering  of  electrical  efficiency,  owing  to  its  being  alternately 
anodically  oxidised  to  the  ferric  state  and  cathodically  reduced  to  the 
ferrous  condition.1  Manganese  salts  behave  similarly.  And  all  traces 
of  copper,  silver,  gold,  platinum,  or  arsenic  must  be  absent.  If  any 
one  of  these  is  deposited  on  the  lead  (all  are  electronegative  to  it,  and 
have  lower  hydrogen  overvoltages)  local  action  will  be  rapidly  increased. 
It  must  be  remembered  that  lead  would  dissolve  chemically  in  H2S04, 
liberating  hydrogen,  only  for  the  high  overvoltage  necessary  for  H' 
discharge  at  that  metal  (even  at  the  spongy  metal).  A  normal  cell 
standing  charged  loses  1-2  per  cent,  of  its  energy  content  daily,  owing 
to  chemical  and  electrical  local  action.  With  a  bad  electrolyte  this 
can  rise  to  50  per  cent. 

Reversible  Voltage  Relations.— The  voltage  of  the  lead  accumu- 
lator depends  essentially  on  the  strength  of  the  acid,  and  also,  to  a 
less  extent,  on  the  temperature.  Dolezalek  found  the  following  values 
at  15° : 

TABLE   XXXI 

Density  of  acid  Percentage  of  acid  Voltage 

•05                                           7-37  1-906 

•15                                         20-91  2-010 

•20                                         27-32  2-051 

•30                                         39-19  2-142 

•40                                         50-11  2-233 

He  showed  that  these  and  other  figures  corresponded  very  closely  to 
values  calculated  thermodynamically  from  the  vapour  pressures  of  the 
acids  used.  In  practice,  acid  of  density  1'15-1'20  is  used.  The 
temperature  coefficient  of  E.M.F.  is  very  small,  and  also  depends  on 
the  acid  concentration,  becoming  more  positive  as  the  latter  increases. 
Our  most  reliable  measurements  are  due  to  Dolezalek,  and  a  selection 
is  here  given  in  Table  XXXII : 

TABLE   XXXII 

dE 
Per  cent.  HoS04  (by  weight)  Density  ^ 

volt 
1-57  1-01  -0-00045 


degree. 
6-53  1-044  ±  0-0000 

11-6  1-08  +  0-00025 

17-0  1-12  +  0-00038 

20-9  1-15  +  0-00037 

27-3  1-20  +  0-00032 

With  very  dilute  acid  the  temperature  coefficient  is  negative,  becomes 
zero  with  acid  of  S.G.  1'044  ([H2S04]  =  0'7),  increases  to  a  maximum 

1  Cf.  pp.  162,  417. 

Q  2 


228    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 


positive  value  of  0-00039 


volt 


with  1'13  S.G.  acid,  and  slowly  falls 


degree 

as  the  acid  concentration  further  increases.  Using  the  figure  for 
1*15  S.G.  acid,  we  calculate  that  the  E.M.F.  of  a  technical  accumulator 
will  fall  from  2'010  volts  to  2'004  volts  at  0°,  and  rise  to  2'023  volts 
at  50°. 

Charge  and  Discharge  Curves.— The  above  are  all  reversible  values, 
and  hold  only  for  cells  with  a  negligibly  small  current.  In  working 
cells  the  relations  are  rather  different.  Fig.  58  shows  charge  and  dis- 
charge *  curves  for  an  accumulator  working  at  normal  current  densities. 

Volts 
28 


2-6 
24 
2-2 
2-0 
1-8 
1-6 
1-4 
1-2 


\ 


Time 
FIG.  58. — Charge  and  Discharge  Curves  of  Lead  Accumulator. 

The  discharged  cell  has  the  normal  voltage  of  slightly  over  2'0  volts. 
On  leading  in  current  this  rises  rapidly  to  2*1  volts,  slowly  increases 
during  the  charge,  and  shortly  before  the  end  rises  quickly  to  about  2*6 
volts.  These  changes  are  accompanied  by  gas  evolution  at  the  plates. 
Oxygen  is  already  evolved  in  small  quantities  at  the  positive  when  2'2 
volts  are  reached,  and  the  liberation  of  hydrogen  at  the  negative  com- 
mences at  2'3  volts.  Charging  is  stopped  when  this  hydrogen  evolution 
has  become  considerable.  The  voltage  then  rapidly  falls  to  its  normal 
value.  When  discharged,  the  voltage  drops  quickly  at  the  commence- 
ment to  about  1*9  volts,  and  then  falls  slowly  to  1'8  volts.  At  this 
point,  when  it  again  commences  to  decrease  rapidly,  the  discharge  is 
stopped.  The  usual  rule  is  to  discharge  until  the  drop  in  voltage  is 
one-tenth  the  initial  voltage  of  the  cell.  Left  to  itself  after  discharge, 
the  accumulator  rapidly  recovers,  the  voltage  rising  to  its  normal 
value. 

Electrical  Losses.— It  is  clear  that  the  voltage  difference  between 
charge  and  discharge  curves  means  that  the  accumulator  stores  up 

1  The  discharge  curve  of  an  accumulator  can  t>e  regarded  as  a  particular  case 
of  the  polarisation  discharge  curves  discussed  on  p.  1 13,  only  differing  in  the  quantity 
of  products  present  on  the  electrodes. 


xvi.]  SECOND AKY  CELLS  229 

electrical  energy  and  gives  it  out  again  with  an  efficiency  falling  con- 
siderably below  100  per  cent.  One  could  suppose  the  voltage  loss  to 
result  from  two  causes  :  (a)  voltage  required  to  overcome  the  internal 
resistance  of  the  accumulator,  and  (6)  voltage  losses  from  irreversible 
electrode  effects.  Owing,  however,  to  the  very  high  conductivity 
of  the  electrolyte  (accumulator  acid  has  usually  a  resistance  of  only 

1'4-1'5  -        ),  the  resistance  of  technical  accumulators  is  very  small, 
cm. 

and  the  difference  thus  arising  between  the  two  curves  does  not 
exceed  0'05  volt.  The  remaining  voltage  difference,  amounting  to 
about  0'2  volt  in  the  above  case,  must  therefore  result  from  some 
irreversible  electrode  effect. 

This  point  also  has  been  investigated  by  Dolezalek,  who  has  shown 
the  polarisation  to  be  undoubtedly  due  to  concentration  changes  in  the 
electrolyte,  occurring  at  the  electrodes.  During  charge,  we  know  that 
at  the  positive  electrode  PbS04  is  changed  to  Pb02,  water  disappears 
from  the  electrolyte,  and  H2S04  is  formed,  whilst  at  the  negative 
electrode  PbS04  is  reduced  to  metallic  lead,  with  formation  of  H2S04. 
The  effect  of  charging  therefore  is  to  tend  to  increase  the  acid  concen- 
tration at  the  electrodes.  With  free  diffusion  possible  between  the  main 
bulk  of  the  electrolyte  and  that  in  immediate  contact  with  the  electrodes, 
very  little  effect  would  be  observed,  as  the  quantity  of  H2S04  thus  pro- 
duced is  very  small  compared  with  the  total  acid  in  the  electrolyte. 
But  when,  as  in  the  present  case,  the  concentration  increase  takes  place 
inside  the  porous  masses  of  lead  and  Pb02  which  cover  the  electrodes, 
diffusion  outwards  is  hindered,  and  consequently,  during  charge,  the 
electrodes  are  in  contact  with  H2S04  of  a  higher  concentration  than  that 
in  the  body  of  the  cell.  As  the  E.M.F.  rises  with  increase  in  strength 
of  acid,  it  follows  that  the  voltage  of  a  charging  accumulator  must 
exceed  that  of  the  same  accumulator  when  idle.  During  discharge 
the  reverse  changes  occur,  the  solution  in  the  electrode  pores  becomes 
diluted,  and  the  E.M.F.  falls  below  the  reversible  value. 

The  increase  and  decrease  in  acid  concentration  which  must  be 
assumed  are  quite  modest.  If  during  charge  the  concentration  in  the 
pores  rose  to  30  per  cent.,  and  during  discharge  dropped  to  10  per  cent., 
this  would  suffice  to  explain  the  voltage  differences  in  Fig.  58.  The 
gradual  rise  or  fall  in  voltage  during  working  results  from  increasing 
difficulty  of  diffusion  between  the  electrolyte  and  the  inner  pores,  as 
the  surface  layers  of  the  active  material  become  exhausted.  When 
standing,  diffusion  no  longer  being  counteracted  by  the  production  of 
further  concentration  changes,  equilibrium  must  quickly  be  established 
with  the  main  electrolyte. 

This  explains  the  rapid  rate  at  which  overcharged  and  discharged 
accumulators  assume  their  normal  voltage  on  standing.  The  polarisa- 
tion in  cells  with  weak  acid  exceeds  that  in  cells  with  strong  acid 


230    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP] 

discharged  at  the  same  current  density.  This  is  to  be  expected,  as 
the  percentage  difference  in  concentration  resulting  is  greater.  The 
structure  of  the  positive  plate  being  much  denser  than  that  of  the! 
negative,  and  the  concentration  changes  produced  there  being  greater! 
than  those  at  the  negative,  would  indicate  the  peroxide  plate  as  the] 
chief  contributor  towards  this  polarisation.  That  is  in  fact  so.  Streinta 
showed  that  the  difference  between  the  electrode  potentials  at  thel 
positive  electrode  during  charge  and  discharge  was  about  twice  as  great 
as  that  between  the  negative  electrode  potentials  under  the  same] 
conditions.  Finally,  with  decrease  of  current  density,  the  difference] 
between  charge  and  discharge  curves  continuously  diminishes  until  itj 
practically  disappears  at  low  current  densities.  This  simply  means 
that  diffusion  nullifies  concentration  changes  more  quickly  than  they 
can  be  produced  by  electrolysis.  All  facts  then  support  the  view  that, 
the  polarisation  in  working  accumulators  is  essentially  concentration! 
polarisation. 

Similarly  the  rapid  fall  in  voltage  at  the  end  of  the  discharge  must 
be  ascribed  to  the  same  cause.     It  certainly  only  depends  indirectly  on 
the  exhaustion  of  active  material,  as  some  of  the  latter  is  always  present 
in  a  discharged  plate.     The  rapid  rise  in  voltage  at  the  end  of  the  j 
charge  is  caused  by  the  high  overvoltage  necessary  for  the  discharge  ; 
of  hydrogen  and  oxygen  gases  at  lead  and  Pb02  respectively.     We 
have  no  exact  data  for  the  current  densities  used,  but  the  2'5  volts  1 
finally  required  probably  consist  of  1*2  volts  for  reversible  electrolysis 
plus  0*8  volt  for  hydrogen  overvoltage  plus  0'5  volt  for  oxygen  over- 1 
voltage. 

Capacity.— The  quantity  of  energy  an  accumulator  can  furnish 
depends  on  its  capacity  as  well  as  on  its  voltage.  An  element  of  high 
capacity  must  allow  of  a  good  utilisation  of  the  inner  layers  of  the 
active  material.  We  have  seen  how  this  utilisation  essentially  depends  ' 
on  the  possibility  of  rapid  diffusion  between  the  main  electrolyte  and 
that  contained  in  the  porous  mass.  Any  circumstance  favouring  this 
diffusion  will  increase  the  cell  capacity.  The  more  porous  the  active  ] 
material  is,  the  better  will  be  its  utilisation.  And  the  higher  the  tem- 
perature, the  greater  will  be  the  cell  capacity.  Similarly  the  capacity 
will  be  greater  the  lower  the  current  density,  as  then  the  concentra- 
tion changes  causing  polarisation  are  less  rapidly  produced,  whilst  the 
rate  of  diffusion  remains  unaffected.  The  capacity  also  depends  to 
some  extent  on  the  concentration  of  the  acid,  reaching  a  maximum 
figure  with  30  per  cent.  acid.  The  conductivity  of  H2S04  is  also  a  maxi- 
mum at  this  strength,  and  there  is  a  close  connection  between  these  two 
facts.  Stationary  cells  are  now  manufactured  to  give  up  to  10-15 
watt-hours  per  kilo,  (including  weight  of  acid).  Transportable  batteries 
furnish  up  to  30  watt-hours.  But  the  plates  contain  so  large  a  pro- 
portion of  paste  that  they  are  often  too  fragile. 


xvi.]  SECONDARY  CELLS  231 

In  course  of  time,  particularly  if  the  cell  stands  in  a  discharged 
state,  the  capacity  becomes  permanently  lower,  owing  to  the  agglomera- 
tion of  the  finely  divided  active  material.  This  phenomenon,  termed 
sulphating,  is  particularly  marked  in  the  negative  plate,  and,  as  we  have 
seen,  the  original  capacity  of  negatives  is  made  considerably  to 
exceed  that  of  positives  for  this  reason.  The  process  essentially 
consists  in  the  dissolving  of  the  finer  particles  of  PbS04  in  the 
electrolyte,  and  their  reprecipitation  as  larger  and  more  insoluble 
crystals,  possessing  relatively  much  less  surface. 

Efficiency. — The  ampere-hour  efficiency  of  the  lead  accumulator 
is  usually  about  95  per  cent.  The  slight  losses  are  due  to  self-dis- 
charge and  local  action,  and  to  gas  evolution  during  charging.1  The 
watt-hour  or  energy  efficiency  depends  on  the  rate  of  charge  and 
discharge,  decreasing  as  these  increase,  and  is  usually  75-85  per  cent. 
The  greater  part  of  the  loss,  as  we  have  seen,  is  essentially  due  to 
concentration  polarisation. 

Theory.— The  theory  of  the  lead  accumulator  has  been  fully 
developed  by  Dolezalek  in  his  well-known  monograph.  Some  points 
we  have  already  discussed,  and  we  will  here  consider  two  or  three  other 
aspects  of  the  question,  though  anything  like  a  full  treatment  is 
impossible.  We  have  seen  that  for  reversibly  working  galvanic 
cells  the  following  form  of  the  Gibbs-Helmholtz  equation  holds, 

ffW 

96540rcE  -  4-19  U  =  T  96540  n  — ,  where  U  is  the  heat  of  reaction 

aL 

of  the  corresponding  chemical  change,  n  the  number  of  faradays 
corresponding  to  the  same  amount  of  chemical  change  taking  place 

cZE 

electrochemically,  and  —  the  temperature  coefficient  of  E.M.F.     The 
(t  JL 

value  of  U  for  the  reaction 

Pb02  +  Pb  +  2H2S04 >  2PbS04  +  2H20 

has  been  determined  by  Streintz  and  by  Tscheltzow.  They  obtained 
values  of  86,800  and  88,800  cals.  respectively,  figures  however  which  only 
hold  good  for  very  dilute  solutions,  about  1[H2S04] :  400[H20].  We 
will  take  the  mean  of  87,800  cals.  The  acid  used  in  practice  has  S.G. 
1-15,  and  contains  1[H2S04]  :  21[H20].  The  heat  of  dilution  of  this 
acid  to  the  concentration  ratio  1 :  400  is  800  cals.  per  mol.  Hence  the 
value  of  U  for  M5  S.G.  acid  is  87,800  +  2  X  800  =  89400  cals.  E  as 
measured  at  15°  by  Dolezalek  is  2*01  volts,  n  is  two.  If  therefore,  using 

<ZE 
the  above  equation,  we  calculate  -=,  we  have 


1  On  the  influence  of  gas  evolution  on  the  capacity,  see  papers  by  Rumpf  and 
by  Streintz,  Zeitsch.  Elektrochem.  16,  163,  747  (1910). 


232    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

dE   96540  X  2  X  2*01  -  4-19  X  89400 
dT  =       288  X  96540  X  2 

=  +0-00024-^1. 
degree 

The  actual  value  is  4-  0-00037  --      —  .  the  agreement  being  excellent 

degree 

considering  the  nature  of  the  calculation. 

A  —  U  being  positive—  i.e.  more  electrical  energy  being  given  out  by 
the  accumulator  than  corresponds  to  the  heat  of  reaction  —  it  follows 
that,  joule  heat  apart,  heat  must  be  absorbed  by  the  accumulator 
during  discharge  and  evolved  during  charge.  Direct  temperature 
measurements  and  quantitative  experiments  carried  out  by  Streintz 
by  the  method  described  on  p.  81  have  confirmed  this. 

The  different  electrode  processes  are  best  considered  in  the  light  of 
the  theory  first  put  forward  by  Le  Blanc.  According  to  this  view, 
the  following  reactions  take  place  during  discharge  and  are  reversed 
during  charge  : 

Positive  Electrode  — 

(a)     Pb02  solid  -  >  Pb02  dissolved 


(6)     4IT+20"  -  >2H20. 

(c)  Pb""  -  »Pb"+2©. 

(d)  Pb"-fS04"  -  *PbS04  undissociated 

—  ».  PbS04  solid. 
Negative  Electrode— 

(a)  Pb+20  -  *Pb". 

(b)  Pb"  +S04"  -  *PbS04  undissociated 

—  >  PbS04  solid. 
The  total  reaction  at  the  positive  is  then 

Pb02  +  4H'  +  SO/  —  >  PbS04  +  2H20  +20; 
at  the  negative 

Pb  +  SO/  +  2  ©  -  >  PbS04  ; 
and  in  the  whole  cell 

Pb02  +  Pb  +  2H2S04  —  >  2PbS04  +  2H20. 

The  positive  electrode  potential  at  room  temperature  will  therefore  be 
,c_Ep  ,  0-058        [PV-] 

~          >' 
the  negative  electrode  potential 

(5,  =  E.P.Pb..^Pb  +  log  [Pb"], 

2 

and  the  E.M.F.  of  the  cell 

+  0-029  log 


xvi.]  SECONDARY  CELLS  233 

Gumming,1  from  experiments  with  HN03  solutions  of  Pb02  and 
Pb(N03)2,  has  determined  the  value  of  E.P.pb....  _^  Pb..  as  +  1-8  volts. 
The  value  of  E.P.pb.._>ph  is  —  O12  volt.  The  ionic  concentrations 
of  Pb""  and  Pb"  in  1-15  S.G.  H2S04  are  somewhat  difficult  to 
calculate.  To  calculate  [Pb""]  we  will  use  an  expression2  deduced 
by  Gumming  to  express  the  solubility  of  Pb02  in  HN03,  and  which 
will  probably  furnish  fairly  correct  results  for  H2S04  also.  It  is 

[Pb""]  =  (acid  normality)4  X  (active  mass  of  water)2 
X  0-00024  millimols. 

To  get  the  active  mass  of  the  water,  3'  6  per  cent,  must  be  subtracted 
from  unity  for  each  equivalent  of  acid  present.  T15  S.G.  H2S04  is  4'88 
equivalent  normal,  and  we  calculate 

[Pb""]  =  0-91  X  10-*'  mols. 

The  value  of  [Pb"]  can  only  be  estimated  at  this  concentration  by 
extrapolating  from  experimental  figures  for  more  dilute  acids.  The 

most  probable  value  is  about  5  X  10"6  -  —  *.  Substituting  in  the 
above  formula,  we  have 


E  =  1-8  +  0-12  +  0-029  log  r  _  =  2-11  volts 

2o  X  10 

(correct  figure  2*01  volts). 

The  discrepancy  of  O'l  volt  is  not  great,  considering  the  assumptions 

made. 

The  formula  can  also  be  somewhat  differently  written.     We  have 
[Pb""]  X  [O"]2  =  Klf 

where  K!  is   the    solubility   product    of   Pb02.3    As  also  [H']2  .  [0"] 
=  K.[H20]  we  obtain 


Similarly  " 


where  K3  is  the  solubility  product  of  PbS04.     We  have  then 


The  influence  exerted  by  the  acid  concentration  is  thus  very  clearly 
shown. 

For  other  important  points,  particularly  details  on  this  effect  of 
acid  concentration,  the  reader  is  referred  to  Dolezalek's  monograph. 

1  Zeitsch.  Elektrochem.  13,  19  (1907).  -  Really  only  empirical. 

3  P.  74. 


234    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

3.  The  Iron  Accumulator 

This  accumulator,  developed  to  some  extent  by  Jungner,  but  chiefly 
by  Edison  and  his  co-workers,  and  usually  known  as  the  Edison  cell,  is 
the  only  competitor  of  the  lead  accumulator  which  has  to-day  even  a 
limited  use.  It  is  probable  that,  as  the  result  of  certain  constructional 
improvements,  this  use  will  receive  a  great  extension  in  the  near  future. 
The  electrochemical  system  of  the  charged  Edison  cell  is  Ni(OH)3  | 
concentrated  KOH  |  Fe,  the  Ni(OH)3  forming  the  positive  pole.  On 
discharge  this  Ni(HO)3  is  reduced  to  Ni(OH)2,  the  iron  being  simulta- 
neously oxidised  to  Fe(HO)2.  The  state  of  the  discharged  cell  is 
therefore  Ni(HO)2  |  concentrated  KOH  |  Fe(OH)2.  On  charging,  the 
reverse  action  takes  place,  Ni(OH)3  and  iron  being  regenerated.  The 
equation  representing  these  changes  is 

2Ni(OH)3  -f  Fe  ^±  2Ni(OH)2  +  Fe(OH)2, 

taking  place  from  left  to  right  during  discharge,  from  right  to  left 
during  charge.  One  important  point  is  at  once  noticeable  in  which  the 
lead  and  iron  accumulators  differ.  Not  only  is  the  electrolyte  here  alka- 
line and  not  acid,  but  no  water  is  separated  from  or  taken  up  by  the 
plates  during  the  working  of  the  cell.1  The  electrolyte  concentration 
remains  practically  constant  during  charge  and  discharge,  and  the 
E.M.F.  is  almost  independent  of  the  same. 

The  Plates. — In  the  Edison  cell,2  the  active  mass  is  present  in  a 
condition  corresponding  to  the  Faure  pasted  plates  of  the  lead  accumu- 
lator. But  owing  to  its  small  cohesion,  to  the  large  volume  changes 
which  take  place  on  working,  particularly  in  the  nickel  hydroxide 
plates,  and  to  the  fact  that  the  light  Edison  cells  are  particularly  in- 
tended for  transport  purposes,  and  are  therefore  liable  to  be  subject  to 
joltings  and  rough  usage,  any  kind  of  open  grid  arrangement  is  im- 
possible. The  active  mass  is  instead  packed  into  closed  containers  of 
thin  nickel-plated  steel  called  '  pockets/  pierced  with  numbers  of  tiny 
holes  to  allow  of  access  of  the  electrolyte.  Plates  of  extraordinary 
rigidity  and  permanence  can  thus  be  made,  buckling  and  loss  of  active 
material  being  completely  avoided. 

Negative  Plates. — The  negative  plate  of  the  new  A  type  of  cell 
(Fig.  59)  consists  of  a  nickel-plated  steel  frame,  provided  above  with 
a  flange  for  electrical  connection  with  other  plates,  and  containing 
twenty-four  pockets  arranged  in  three  horizontal  rows.  These  pockets 
are  rectangular  in  shape,  3"  X  J"  X  J",  and  are  filled  with  finely  divided 
iron,  made  by  reducing  iron  oxide  with  hot  gases.  A  little  mercury  is 

*  For  a  slight  modification  of  this  statement,  see  p.  240. 

2  For  details  of  construction  and  working  of  the  latest  types  of  Edison  cells, 
tee  papers  by  Holland  and  others,  Kledr.  64,  741,  764 ;  65,  185 ;  66,  47,  83 
(1909-10). 


XVI.J 


SECONDARY  CELLS 


235 


also  added  (as  oxide,  subsequently  reduced).  This  increases  the  con- 
ductivity of  the  mass  when  discharged,  and  has  also  a  markedly  favour- 
able effect  on  the  capacity.  When  the  pockets  are  filled  and  in  position, 
the  whole  plate  is  so  compressed  that  the  surfaces  of  the  pockets  are 


FIG.  59. — Negative  Plates  of  Edison  Accumulator. 

no  longer  plane,  but  curved.  The  effect  of  expansion  and  contraction 
during  working  is  thus  neutralised,  and  the  active  material  kept  in 
continual  good  contact  with  itself  and  the  conducting  steel  containers. 


FIG.  CO. — Positive  Plates  of  Edison  Accumulator. 


Positive  Plates. — These  were  formerly  made  in  the  same  way,  but 
owing  to  the  exceptionally  large  volume  changes  to  be  dealt  with, 
their  mechanical  strength  proved  insufficient.  The  new  construction 
is  shown  in  Fig.  60.  A  steel  frame  contains  two  rows  of  fifteen  pencil- 
shaped  pockets.  These  pockets  are  of  nickel-plated  steel,  about  4" 


236    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

long  and  J"  in  diameter,  perforated,  as  before,  and  provided  with  a 
double-lapped  spiral  seam.  They  are  further  strengthened  by  eight 
steel  rings,  closely  fitting  on  the  outside.  The  active  filling  consists 
of  Ni(OH)2.  To  increase  its  conductivity  graphite  was  formerly 
added  (as  also  in  the  negative  plate).  It  was  however  found  that,  on 
charging,  this  graphite  became  slowly  oxidised,  and  the  capacity  of  the 
plate  consequently  fell.  The  addition  now  used  consists  of  very  thin 
flakes1  of  electrochemically  prepared  nickel.  These  flakes  alternate 
with  horizontal  layers  of  Ni(OH)2  in  the  vertical  pencil-shaped  tube.  A 
single  tube  will  contain  about  350  double  layers  of  nickel  and  nickel 
hydroxide,  the  hydroxide  having  an  average  thickness  of  about  O'Ol". 
The  nickel  makes  contact  all  around  its  circumference  with  the  nickel 
container. 

By  means  of  these  improvements  the  capacity  of  the  positive 
plate  has  been  increased  and  made  much  more  permanent,  and  its 
mechanical  strength  and  durability  leave  now  nothing  to  be  desired. 
The  capacity  of  the  positive  is  nevertheless  far  less  than  that  of  the 
negative,  and  eventually  determines  the  capacity  of  the  cell  itself.  It 
has  been  estimated  2  (for  the  older  type  of  cell)  that,  during  an  average 
discharge,  75  per  cent,  of  the  active  material  of  the  positive,  but  only  10 
per  cent,  of  that  of  the  negative,  plate  is  utilised. 

General  Construction. — As  large  low- voltage  currents  are  not  re- 
quired for  traction  work,  the  Edison  accumulator  units  are  of  moderate 
size.  As  made  now,  they  contain  either  four,  six,  or  eight  positive 
plates,  the  number  of  negatives  being  one  greater.  These  plates  are 
arranged  alternately,  a  few  mm.  apart,  in  suitable  grooves  in  a  hard- 
rubber  framework,  rods  of  hard  rubber  being  also  inserted  between 
them.  All  like  plates  are  connected  together  by  nickelled  steel  rods 
passing  through  holes  provided  in  their  flanges,  and  these  are  in  their 
turn  connected  with  vertical  leads  of  the  same  material.  The  whole 
frame,  with  its  system  of  plates,  is  placed  inside  a  container  of  nickelled 
sheet  steel.  The  two  leads  pass  up  through  bushes  of  hard  rubber  in  the 
lid  of  this  container,  which  is  also  provided  with  a  valve  permitting  the 
escape  of  gases  during  charge,  but  preventing  entrance  of  atmospheric 
C02,  and  with  a  suitable  opening  for  filling.  The  electrolyte  is  21  per  cent. 
KOH  solution  (S.G.  1-21).  Lately  it  has  been  usual  to  add  a  little  LiOH, 
which  is  said  to  improve  the  behaviour  of  the  positive  electrode.  It 
is  difficult  to  see  how  this  addition  is  effective. 

The  electrolyte  must  be  replaced  about  every  twelve  months.  As 
there  is  a  continual  loss  of  water  due  to  gassing  during  charging,  and  as 
the  quantity  of  electrolyte  in  the  cell  is  small  in  comparison  with  a 
lead  cell  of  similar  capacity,  it  is  necessary  regularly  and  frequently  to 
add  water  during  working. 

1  Cf.  p.  294.  2  Grafenberg,  Zeitsch.  Eleklrochem.  11,  730  (1905). 


XVI.] 


SECONDARY  CELLS 


237 


Reversible  Voltage  Relations.— The  E.M.F.  of  the  Edison  cell  is 
1* 33-1  "35  volts.  It  is  only  very  slightly  dependent  on  the  con- 
centration of  the  potash,  as  Foerster  has  shown  between  the  limits  of 
0-2  N-6  N.  The  following  are  some  of  his  figures  for  25°  l : 


Electrolyte 
5-3    N  .  KOH 
2-82  N  .  KOH 
1-0    N  .  KOH 


Voltage 
1-3349  volts 
1-3377 
1-351 


The  temperature  coefficient  of  E.M.F.,  as  measured  by  Thompson  and 
Richardson,2  is  positive,  and  increases  as  the  alkali  concentration 
diminishes.  They  obtained  the  following  values  : 


50  per  cent.  KOH   +0*00008  -=-, 

degree 

23-8  per  cent.  KOH  +  0'00026-0'00022, 
6-25  per  cent.  KOH  +0-00069. 

The  Edison  cell  will  therefore  cool  slightly  during  discharge.  The 
resistance  of  a  cell  depends  somewhat  on  its  size  and  on  the  stage  of 
discharge  reached,  but  varies  between  0'0015-0'006  ohm. 

Charge  and  Discharge.—  The  above  reversible  values  for  E.M.F. 
hold  good  of  course  only  for  cells  standing  idle.  When  current  passes 
irreversible  effects  come  into  play,  and  these  are  unfortunately  of 


Volts. 

1-8 


1-6 


1-4 


1-2 


1-0 


0-8 


0-6 


04 


Time 


FIG.  61. — Charge  and  Discharge  Curves  of  Edison  Accumulator. 

considerable  magnitude  in  the  present  case.  Fig.  61  contains  typical 
charge  and  discharge  curves,  taken  at  a  normal  rate.  If  discharged 
immediately  after  charging,  the  initial  voltage  is  about  1*45  volts. 

1  Zeitsch.  Elektrochem.  14,  285  (1908). 

2  Trans.  Amer.  Electrochem.  Soc.  7,  95  (1905). 


238    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

This  rapidly  falls  to  1*3  volts,  and  then  drops  very  slowly  during  the 
course  of  the  discharge  to  1*15  volts.  At  this  point  it  begins  to  fall 
more  rapidly,  and  drops  to  zero,  showing  a  second  short  halt  at  about 
0*7  volt.  Under  normal  working  conditions  this  stage  is  not  reached, 
the  discharge  being  interrupted  when  the  voltage  has  fallen  to  O9  volt. 
On  charging,  the  curve  shows  a  small  maximum  shortly  after  the  start, 
and  a  subsequent  continuous  rise  from  1*65  to  1*8  volt.  From  the 
beginning,  an  evolution  of  hydrogen,  which  finally  becomes  consider- 
able, is  noticed  at  the  negative  plate.  Oxygen  evolution  at  the 
positive  does  not  commence  immediately,  but  is  very  brisk  before  the 
end  of  the  charge. 

Efficiency. — Owing  to  these  facts,  the  ampere-hour  efficiency  of  the 
iron  accumulator  is  low— 82  per  cent,  under  normal  conditions  com- 
pared with  the  95  per  cent,  of  the  lead  accumulator.  It  depends 
largely,  however,  on  the  rate  of  discharge  and  charge.  If  these  are  low, 
or  if  a  small  fraction  only  of  the  possible  capacity  of  the  cell  is  utilised, 
then  the  ampere-hour  efficiency  can  be  much  higher.  The  voltage 

1*2 
efficiency  is  about  72  per  cent,  (roughly  - — ),  and  consequently  the 

watt-hour  efficiency  59  per  cent.,  whereas  that  of  the  lead  accumu- 
lator averages  80  per  cent.  The  voltage  losses  are  not  in  this  case  due 
to  concentration  polarisation,  but,  as  we  shall  see,  are  essentially  con- 
nected with  the  current  losses. 

As  at  present  manufactured,  iron  accumulators  will  furnish  150-300 
amp.-hours  or  180-360  watt-hours  at  normal  rates  of  discharge.  The 

energy  capacity  per  unit  weight-lS-lS™*^0"1'8  (28-33  ^^Ls)_ 

pound  kilo 

is  high,  but  nevertheless  shows  no  overwhelming  advantage  in  that 
respect  over  the  portable  forms  of  the  lead  accumulator.  The  advan- 
tage of  the  iron  cell  lies  more  particularly  in  its  indifference  to  violent 
mechanical  treatment,  to  overcharging,  to  discharging  above  the  normal 
rate,  and  in  its  freedom  from  deterioration  on  standing  or  after  long 
use.  In  fact,  both  capacity  and  efficiency  appear  gradually  to  increase 
during  use.  Holland  records  a  case  in  which  three  cells,  after  helping 
to  drive  a  wagon  for  some  17,000  miles,  had  improved  with  respect  to 
overcharge  capacity,  watt-hour  output  under  normal  conditions  (an 
increase  of  16  per  cent.),  and  efficiency. 

Self-discharge. — The  self -discharge  of  an  iron  accumulator  is  con- 
siderable at  first,  even  8-10  per  cent,  in  the  first  twenty-four  hours. 
After  that,  however,  it  becomes  very  much  less,  and  only  reaches  15  per 
cent,  after  weeks.  This  initial  drop  in  capacity,  which  is  very  sensitive 
to  temperature,  is  associated  with  evolution  of  oxygen  from  the  Ni  (OH)3 
plate,  and  is  accompanied  by  the  drop  in  voltage  already  mentioned. 
The  iron  electrode  also  slowly  decreases  in  capacity,  owing  to  chemical 
solution  of  the  finely  divided  metal  in  the  alkali,  with  evolution  of 


xvi.]  SECONDARY  CELLS  239 

hydrogen.     But  as  the  capacity  of  the  negative  plate  is  so  great,  this 
self-discharge  never  becomes  noticeable. 

Theory.—  Our  knowledge  of  the  theory  of  the  iron  accumulator 
has  in  recent  years  become  very  complete,  chiefly  owing  to  the  compre- 
hensive researches  of  Foerster,1  though  contributions  have  also  been 
made  by  others,  notably  Schoop,2  Zedner,  and  Faust.  In  its  simplest 
form,  we  can  write  the  equation  representing  the  iron  accumulator 
processes  thus  — 

Fe  +  Ni203  ;±  FeO  +  2NiO. 
In  that  case,  the  potential  of  the  positive  electrode  at  18°  is  given  by 


&[0"];K2 
+  0-058  log -^- 


where  Kx  and  K2  are  the  solubility  products  of  nickelic  and  nickelous 
oxides  respectively,  and  [0"]  the  molar  0"  concentration  in  the  electro- 
lyte. Similarly  the  potential  of  the  negative  electrode  at  18°  is 

&  =  E.P.Fe.._^Fe  +0-029  log  [Fe'1 

KS. 

[O'T 

K3  being  the  solubility  product  of  ferrous  oxide.  Combining  these 
values  we  obtain 


=  E.P.Fe,_Fe  +0-029  log  p^, 


=  KP^...^..  -  E.P.^.^  +0-058  log 

a  value  for  the  E.M.F.  which  is  independent  of  the  alkali  concentration, 
and  determined  only  by  the  electrolytic  potentials  of  the  two  electrode 
processes  concerned  and  by  the  solubility  products  of  the  different 
oxides.  We  arrive  at  the  same  conclusion  if,  instead  of  writing  the 
equation  as  above,  we  write  it  as  is  usually  done  —  viz.  : 

Fe  +  2Ni(OH)3  ^±  Fe(OH)2  +  2Ni(OH)2. 

Effect  of  Alkali  Concentration.—  But  we  have  seen  that  the  E.M.F. 
really  is  slightly  affected  by  the  alkali  concentration,  and  Foerster  has 
shown  the  explanation  to  be  that  the  colloidal  hydroxides  of  nickel  and 
iron  are,  as  Van  Bemmelen  showed,  not  definite  compounds,  but  rather 

1  Zeitsch.  Elektrochem.  13,  414  (1907)  ;  14,  285  (1908)  ;   and  with  Herold,  16,  461 
(1910). 

2  Electrochem.  Ind.  2,  272,  310  (1904). 


240    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

oxides  containing  more  or  less  adsorbed  or  occluded  water.     A  very 
nearly  correct  equation  is  the  following  : 

Fe  +  Ni203,  1'2H20  +  2«9H20  ±^  FeO,  zH20  -f  2(NiO,  y  H20) 

where  x  +  2y  =  4'1. 

The  exact  values  of  x  and  y  are  unknown.  If  we  arbitrarily  assume 
that  x  =  1*5  and  y  —  1*3  (very  possible  values),  we  obtain  for  the  total 
equation 

Fe  H-  Ni,0,,  1'2H20  +  2-9  H20  ±^  FeO,  1-5H20  +  2(NiO,  1-3H20) ; 
for  the  equation  representing  the  reaction  at  the  negative  plate 

Fe  +  20H'  +  0'5H20  ^±  FeO,l'5H20  +  20; 
and  for  the  equation  at  the  positive 

Ni203,l'2H20  +  2-4H20  ^±  2(NiO,l'3H20)  +  20H'  +  20. 

The  electrolyte  as  a  whole  therefore  becomes  more  concentrated  on 
discharge,  owing  to  the  absorption  of  water  by  the  active  material ; 
the  electrolyte  in  the  neighbourhood  of  the  positive  becomes  considerably 
more  concentrated,  water  being  taken  up  and  OH'  ions  entering  solu- 
tion ;  and  the  solution  around  the  negative  becomes  somewhat  diluter, 
the  absorption  of  water  not  compensating  for  the  disappearance  of  OH' 
ions.  In  fact,  as  Foerster  and  Schoop  have  shown,  these  specific 
gravity  changes  can  be  followed  experimentally.  The  phenomena  are 
reversed  on  charging.  But,  compared  with  the  concentration  differ- 
ences produced  in  the  pores  of  the  lead  accumulator  plates,  these  effects 
are  small,  and  do  not  account  for  the  low  efficiency  of  the  iron  cell,  the 
explanation  for  which  is  furnished  by  Foerster Js  studies  of  the  pro- 
cesses taking  place  at  the  two  electrodes. 

Nickel  Oxide  Plate.— He  worked  chiefly  with  2-8  n.  KOH  and  at 
room  temperature.  Under  these  conditions  a  freshly  charged  nickel 
oxide  electrode  shows  an  initial  potential  £h  =  -f-  O60  volt.  This  falls 
rapidly  at  first,  then  more  slowly,  till  after  six  weeks  it  finally  reaches  the 
value  -f  0'4:7  volt.  At  this  figure  it  will  remain  constant  indefinitely, 
and  Foerster  showed  that  the  value  represents  the  equilibrium  potential 
of  the  hydrated  nickel  sesquioxide.  The  initial  potential  fall  is  accom- 
panied by  an  oxygen  evolution,  and  analysis  showed  that  the  freshly 
charged  plate  contains  considerably  more  oxygen  than  corresponds  to 
Ni20a.  That  this  oxygen  is  not  present  as  adsorbed  gas  is  shown  by  the 
fact  that  it  will  react  with  H202. 

If  the  freshly  charged  positive  plate  be  discharged,  the  potential 
follows  a  course  corresponding  with  the  changes  shown  by  the  plate 
on  standing.  There  is  an  initial  rapid  fall  of  potential  (unaccompanied 
by  oxygen  evolution),  followed  by  a  long  discharge  during  which  the 
potential,  though  slowly  dropping,  is  almost  constant,  and  about  0'04 


xvi.]  SECONDAKY  CELLS  241 

volt  lower  (at  a  normal  rate  of  discharge)  than  the  equilibrium  value  for 
the  hydrated  Ni203.  Finally  it  drops  rapidly,  and  there  is  a  second 
short  halt  at  about  —  O'l  volt.  The  different  stages  thus  correspond 
exactly  to  the  stages  in  the  discharge  of  the  accumulator  as  a  whole. 
This  is  because  a  small  fraction  only  of  the  total  capacity  of  the  iron 
electrode  is  utilised,  the  conditions  there  remaining  practically  constant 
throughout.  On  charging,  the  potential  of  the  positive  plate  rises 
rapidly  and  then  more  slowly  to  about  -f-  0*65  volt.  Towards  the 
end  it  becomes  practically  constant.  Oxygen  evolution  begins  after 
some  time,  and  then  rapidly  increases  until  it  corresponds  fully  to  the 
current  passing. 

To  explain  these  phenomena,  Foerster  assumes  that  the  first  process 
taking  place  at  the  positive  plate  during  charging  is  OH"  discharge  to 
oxygen,  which  at  once  combines  with  the  depolariser  present — viz.  NiO. 
Arguing  from  certain  known  analogies  the  oxide  supposed  to  be  thus 
formed  is  not  Ni203,  but  Ni02.  This  peroxide  then  reacts  chemically 
according  to  the  equation 

Ni02  +  NiO  — >  Ni203, 

the  sesquioxide  resulting  from  a  secondary  chemical  reaction.  Anodic 
polarisation  therefore  produces  a  solid  solution  of  NiO,  Ni203,  and 
Ni02,  the  proportions  of  the  two  latter  oxides  gradually  increasing 
during  the  charge.  Of  these  oxides,  Ni02  is  unstable,  and  rapidly 
evolves  oxygen  in  the  pure  condition,  forming  Ni203.  Only  the  fact 
that  it  is  dissolved  in  the  sesquioxide  (and  monoxide)  lends  it  a  certain 
stability  and  lowers  its  rate  of  decomposition.  But  finally,  as  the 
quantity  of  monoxide  decreases,  the  combination  of  the  two  to  form 
sesquioxide  is  unable  to  take  place  so  rapidly,  the  concentration  of 
dioxide  in  the  solid  solution  grows,  and  the  point  is  reached  at  which  it 
begins  to  decompose  appreciably,  evolving  oxygen.  At  the  same  time, 
also  owing  to  the  decreasing  quantity  of  NiO  present,  the  depolarisa- 
tion  of  discharged  oxygen  with  formation  of  Ni02  takes  place  less 
quickly,  and  some  of  the  gas  is  liberated  in  the  free  state.  (Ni203 
depolarises  the  oxygen  discharge,  only  less  readily.) 

A  state  is  finally  reached  in  which  the  active  mass  consists  of 
a  solid  solution  of  Ni02  in  Ni203.  The  concentration  of  Ni02  is 
an  equilibrium  one,  determined  by  equal  rates  of  (a)  spontaneous 
chemical  decomposition  of  Ni02  (2Ni02  — >  Ni203  +  J  02),  and 
(6)  reformation  by  reaction  between  the  Ni203  and  anodic  oxygen. 
The  total  oxygen  evolved  corresponds  to  the  total  current  passing — 
a  small  fraction  results  chemically  from  the  decomposition  of  the 
dioxide,  the  greater  part  comes  from  direct  anodic  discharge.  The 
greater  the  current  density,  the  quicker  the  rate  of  production  of 
Ni02  from  Ni203 — the  higher  therefore  the  concentration  of  Ni02  in 
the  solid  solution. 


242    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAI 

On  interrupting  the  current,  oxygen  will  be  evolved  by  the  activ 
mass  owing  to  the  decomposition  of  the  dioxide.  This  evolution  wi 
be  rapid  at  first,  corresponding  to  the  initial  high  concentration  c 
Ni02;  later,  as  this  concentration  sinks,  the  gas  will  be  given  o: 
more  and  more  slowly,  ceasing  of  course  when  the  active  mass  contain 
all  its  nickel  as  Ni203.  If,  immediately  after  charging,  the  plate  b 
discharged,  there  will  be  an  initial  high  potential  value  correspondin 
to  the  high  Ni02  concentration,  which,  as  this  concentration  become 
less,  will  continually  fall  until  the  value  corresponding  to  Ni203  i 
reached. 

The  short  halt  at  —  O'l  volt  Foerster  attributes  to  some  other  oxid 
produced  during  the  decomposition  of  the  Ni02. 

The  whole  explanation,  which  is  amply  supported  by  experiments 
evidence,  and  which  accounts  satisfactorily  for  all  the  phenomen 
observed,  is  summarised  below. 

Charging. 

(i)  20H'— >H20+0+20) 

(ii)        NiO  +  0 >  Ni02  [  with  much  NiO  present. 

(iii)    Ni02+NiO — >  Ni203 

(i)  2Ni02 — ,Ni203+i02  ) 

(ii)       Ni203  +  0  —  2Ni02  (with  difficulty)       W1lttle  °r  »° 

(iii)  20H' 

Standing. 

2Ni02 

Discharging. 

(i)  2Ni02  +  H20  — >  Ni203  +  20H'  +  2  0  (if  soon  after  charge) 
(ii)    Ni203  +  H20  — >  2NiO  +  20H'  +  2  ©. 

Iron  Plate. — We  have  already1  mentioned  Foerster's  views  on  th 
passivity  of  iron  and  similar  metals,  and  how  he  supposes  that  whe: 
pure  these  metals  are  passive,  and  only  become  active  when  they  hav 
received  a  charge  of  nascent  hydrogen,  the  degree  of  activity  corre 
spending  to  the  concentration  of  the  hydrogen.  In  the  present  cast 
it  is  well  known  that  finely  divided  iron  is,  under  ordinary  conditions 
passive  in  alkaline  solutions.  But  if  cathodically  charged  wit 
hydrogen  it  becomes  active,  showing  a  potential  in  2*85  n.  KOH  c 
-0-87  volt.  It  will  now  readily  dissolve  anodically,  and  Foerste 
and  Herolri  have  conclusively  shown  the  electrode  process  to  be 

Fe  -|-  2  ©  — >  Fe"  (i) 

the  product  being  ferrous  hydroxide. 

1  P.  142. 


xvi.]  SECONDARY  CELLS  243 

At  the  same  time,  this  anodic  polarisation  tends  to  cause  the  dis- 
solved hydrogen  to  ionise,  and  thus  diminishes  the  hydrogen  charge 
in  the  original  iron.  Consequently  the  potential  becomes  more  and 
more  noble,  and  the  metal  dissolves  less  and  less  easily.  Finally,  when 
the  limit  of  —  O75  volt  is  reached,  passivity  sets  in  and  the  iron  no 
longer  dissolves  as  Fe"  ions.  Another  process  instead  commences — viz. 

Fe"  +0  — >  Fe-  (ii) 

involving  the  transformation  of  ferrous  into  ferric  hydroxide.  (Some 
ferrous  hydroxide  will  be  present,  formed  electrochemically  in  the  first 
stage  :  more  will  result  from  chemical  interaction  between  the  ferric 
hydroxide  now  produced  and  the  finely  divided  iron.  So  the  total 
result  of  this  second  stage  will  be  the  transformation,  partly  chemical 
and  partly  electrochemical,  of  iron  into  ferric  hydroxide.) 

But  we  know  that  in  an  iron  accumulator  under  ordinary  con- 
ditions the  capacity  of  the  iron  electrode  greatly  exceeds  that  of  the 
nickel  hydroxide  electrode.  Consequently,  before  the  potential  of  the 
iron  electrode  has  fallen  to  —  0*75  volt,  the  discharge  is  finished,  and 
the  electrode  receives  a  fresh  treatment  with  nascent  hydrogen  during 
charging  which  once  more  renders  it  active.1  The  second  discharge 
stage  is  never  reached  under  normal  technical  conditions.  Foerster  and 
Herold  showed,  however,  that  small  quantities  of  iron  oxides  in  the 
iron  powder  considerably  decrease  the  capacity  of  the  first  stage  of  the 
discharge,  and  accelerate  the  commencement  of  the  second  stage.  This 
is,  however,  neutralised  in  the  technical  cell  by  the  mercury  present. 
The  nature  of  its  action  is  far  from  clear,  but  it  certainly  keeps  the 
capacity  of  the  iron  electrode  corresponding  to  stage  (i)  high  and 
constant,  and  prevents  any  production  of  ferric  hydroxide.2 

On  charging,  stage  (i)  is  by  no  means  reversed.  Hydrogen  is  evolved 
very  soon  after  the  start  and  in  gradually  increasing  quantities  until 
it  corresponds  to  the  total  current  passing.  The  potential  rises  sharply 
from  the  equilibrium  value  of  —  0*87  volt  through  a  slight  maximum  to 
about  —  1'05  volts,  and  then  gradually  during  the  charge  approaches 
its  final  value  of  about  —  1*15  volts.3  We  have  seen  that  an  increase  of 
hydrogen  concentration  in  the  iron  makes  it  less  passive — i.e.  makes  its 
potential  less  noble.  It  follows  therefore  that,  as  the  hydrogen  evolu- 
tion during  charge  increases,  and  with  it  the  hydrogen  concentration 
in  the  iron,  the  polarisation  necessary  for  the  iron  deposition  must 
become  greater  and  greater. 

1  It  is  observed  in  practice  that,  if  the  charging  be  carried  out  at  too  low  a 
current  density,  the  capacity  of  the  negative  during  the  succeeding  discharge  is 
small.     The  reason  is  that  the  potential  of  the  iron  depends  on  the  concentration 
of  dissolved  hydrogen,  and  this  in  its  turn  on  the  current  density  and  potential 
prevailing  during  charge. 

2  Cf.  perhaps  its  action  in  the  lead  accumulator  (p.  226). 

3  These  figures  are  for  2-8  n.  KOH. 

B  2 


244       PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY 

If  we  tabulate  the  potentials  and  voltages  in  the  cell  during  charge 
and  discharge,  we  obtain  the  fallowing  approximate  figures  : 

TABLE  XXXIII 


Charge 

At  rest 

Discharge 

Cell 

+  Electrode 
—  Electrode 

1-65  >1-80 
(average  1-67) 
+  0-60  >  +  0-65 
_1-05  *-M5 

1-34 

+  0-47 
-0-87 

1-3  »0-9 

(average  1-2) 
_|_  0-45  >  +  0-10 
-0-85  >  —  0-80 

It  is  clear  that  much  the  greater  part  of  the  irreversible  voltage  losses 
is  due  to  the  charging,  and  that  the  positive  electrode  behaves  rather 
better  than  the  negative  in  this  respect.  During  discharge,  on  the 
other  hand,  the  losses  are  very  evenly  divided  between  the  two  plates, 
it  only  being  towards  the  end  that  the  potential  of  the  positive  quickly 
falls  off.  It  is  interesting  to  compare  these  data  with  those  for  the 
lead  accumulator,  where  the  losses  are  mostly  of  an  entirely  different 
nature  (concentration  polarisation),  are  distributed  pretty  evenly 
between  charge  and  discharge,  and  are  mostly  due  to  the  positive  plate. 


Literature 

Dolezalek.     Theory  of  the  Lead  Accumulator. 
Bein.     Elemente  und  Akkumulatoren. 
Holland.     The  1910  Edison  Storage  Battery. 


CHAPTER    XVII 
COPPER— SILVER— GOLD 

IN  this  chapter  we  shall  deal  with  the  electrometallurgy  of  copper, 
silver,  and  gold.  The  very  important  subject  of  electrolytic  copper 
refining  will  be  first  treated.  We  shall  then  discuss  in  turn  the  various 
attempts  made  to  win  copper  by  electrochemical  methods  from  its 
ores,  the  electrolytic  refining  of  silver  and  gold,  and  the  electrolytic 
modification  of  the  cyanide  process  for  gold  extraction. 

1.  Copper  Refining.    Theory 

We  are  here  concerned  with  the  anodic  solution  of  crude  copper  in 
aqueous  CuS04  and  its  cathodic  deposition  in  the  pure  state.  When 
copper  electrodes  are  dipped  into  a  CuS04  solution,  the  reaction 

Cu"  +  Cu — >2Cu'  (a) 

sets  in,  and,  disturbing  influences  apart,  continues  until  equilibrium 
prevails  at  the  surface  of  copper  and  electrolyte.  The  equilibrium 

constant    of    this    reaction     ( K  = ;,        )   is    O5xlO~4  at    room 

V          [On**]/ 

temperature.  A  molar  normal  CuS04  solution  may  have  [Cu"]  =  0*5, 
and  we  calculate  [Cu']  =  0*005.  At  higher  temperatures,  the  equilibrium 
moves  strongly  over  in  favour  of  the  Cu'  ions,  and  K  consequently 
increases.  Thus  if  acid  CuS04  solution  be  boiled  with  finely  divided 
copper  powder  and  filtered,  crystals  of  metallic  copper  will  be  deposited 
on  cooling. 

Let  us  now  particularly  consider  a  cathode  at  which  the  above 
equilibrium  has  been  set  up  and  persists  while  current  is  flowing.  Three 
electrochemical  reactions  are  possible — 

Cu" >  Cu  +  2  ©  (6) 

Cu' >  Cu  +  ©  (c) 

Cu"-— >Cu'  +  ©  (d) 

and  it  is  clear  that  all  must  proceed  equally  easily — otherwise  one 
particular  ion  would  be  produced  or  removed  preferentially,  and  the 
equilibrium  thus  become  disturbed.  The  relative  magnitudes  of  the 

245 


246    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

different  reactions,  determined  by  the  condition  that  equilibrium 
persists,  will  depend  on  the  concentrations  of  the  ions  involved.  Hence 
reaction  (b)  will  preponderate  because  of  the  far  greater  concentration  of 
the  Cu"  ions.  Further  if  [Cu"]  is  only  varied  slightly,  reactions  (c) 
and  (d)  will  take  place  to  an  equal  extent,  otherwise  the  equilibrium 
will  be  disturbed.  We  see  then  that  the  result  of  the  passage  of  two 
faradays  is  the  conversion  of  one  [Cu"]  to  metallic  copper,  as  the  sum 
of  the  effects  of  (c)  and  (d)  equal  the  effect  of  (b).  Copper  is  therefore 
deposited  as  a  di-valent  metal  in  accordance  with  Faraday's  Law. 

But  if  the  equilibrium  point  of  the  reaction  (a)  is  not  reached,  or  is 
passed,  things  are  different.  That  electrochemical  action  will  proceed 
most  easily  which  tends  to  restore  equilibrium.  Thus  if  it  were  possible 
to  produce  near  the  cathode  an  excess  of  Cu'  ions  beyond  the  amount 
demanded  by  the  Cu"  concentration  and  the  constant  K,  reaction  (c) 
would  preponderate,  and  an  excess  of  Cu'  ions  deposit  until  equili- 
brium were  reached.  The  yield  would  exceed  that  demanded  by 
Faraday's  Law  if  copper  be  taken  as  a  di-valent  metal.  If,  on  the 
contrary,  the  Cu'  concentration  were  below  the  equilibrium  value, 
reaction  (d)  would  commence,  tending  to  restore  equilibrium.  A 
lower  yield  of  copper  than  the  theoretical  would  result.  And  this  last 
case  is  important  in  the  theory  and  practice  of  copper  refining.  For 
efficient  working,  the  Cu'  concentration  at  the  cathode  must  not  become 
low.  Now  it  may  fall  below  the  equilibrium  value  for  two  reasons. 
Firstly,  the  Cu'  concentration  necessary  for  equilibrium  may  be  raised 
as  the  result  of  an  increase  in  the  constant  K,  due  to  a  rise  in  tempera- 
ture. Secondly,  the  Cu'  ions  may  be  continually  removed  by  some 
subsidiary  chemical  reaction.  Two  such  reactions  must  be  taken 
into  account.  They  are  oxidation  by  atmospheric  oxygen, 

2Cu'  +  2H'  +  J02 >  2Cu"  +  H20  (e) 

and  hydrolysis  of  the  cuprous  sulphate, 

Cu'  +  OH' >  J(Cu20  +  H20)  (/) 

In  the  one  case,  Cu"  ions  are  formed,  in  the  other,  Cu20  precipitated. 
The  effect  of  different  conditions  on  the  cathodic  yield  of  metal  will 
now  be  discussed  in  the  light  of  the  above  considerations. 

(1)  Presence  of  dissolved  oxygen  from  air.     To  be  avoided  as  it 
directly  leads  to  reaction  (e),  and  hence  to  a  low  yield,  owing  to  the 
preference  thereby  given  to  the  electrochemical  process  (d). 

(2)  Influence  of  temperature.     A  high  temperature  is  disadvan- 
tageous.    It  increases  the  velocity  of  (e),  whereby  Cu*  ions  are  used  up. 
It  favours  the  right-hand  side  of  the  equation  (/).     And  lastly,  as  we 
have  seen,  it  causes  an  increase  in  the  constant  K.     More  Cu'  ions  must 
be  electrochemically  produced  to  satisfy  the  equilibrium  conditions. 

(3)  H'  concentration.     A  high  value  increases  the  rate  of  (e),  in 


XVII.] 


COPPER 


247 


accordance  with  the  laws  governing  reaction  velocity.  On  the  other 
hand,  too  low  a  value  allows  the  hydrolysis  (/)  to  take  place,  with 
consequent  removal  of  Cu'  ions  as  insoluble  Cu20.  In  practice,  an 
electrolyte  containing  free  acid  is  always  used. 

(4)  Cu"  concentration.     This  should  not  be  too  high,  as,  in  accord- 

rOn']2 

ance  with  the   equation   K  =  ;•_-  -  ,   it  would   necessitate  a  corre- 

[Cu"] 

spondingly  high  Cu*  concentration  to  satisfy  the  equilibrium  conditions. 

(5)  Current  density.    This  must  not  be  too  low,  as  it  gives  a  relatively 
long  time  for  the  reactions  (e)  and  (/ )  to  take  place  in.     On  the  other 
hand,  it  must  not  become  too  high.1    If  it  does,  the  copper  is  produced 
as  a  dark-coloured  spongy  mass  which  does  not  adhere  to  the  cathode. 
Hydrogen  is  also  evolved,  owing  to  exhaustion  of  the  Cu"  ions,  and 
the  resulting  lowering  of  the  H'  concentration  may  cause  precipitation 
of  Cu.,0.     By  using  a  hot  strong  CuS04  solution,  and  a  low  current 
density,  it  is  easily  possible  for  practically  all  the  current  passing  to 
be  concerned  with  the  reaction  Cu"  — >  Cu*  -f  0,  and  thus  for  no 
copper  to  be  deposited.     If  the  electrolyte  is  sufficiently  acid,  the  Cu" 
ions  are  oxidised  as  in  (e).    In  the  absence  of  acid,  the  hydrolysis  (f) 
takes  place,  and  almost  pure  Cu20  results.     Table  XXXIV,  containing 
figures  from  the  papers  of  Foerster  and  Seidel 2  and  Schwab  and  Baum,3 
shows  how  the  current   efficiency  of  copper  deposition  varies  with 
temperature  and  current  density. 

TABLE  XXXIV 


Temperature 

Current  density  in  amps,  /metre" 

0-3 

1-0 

13-5 

30 

37 

100 

120 

200 

300 

400 

Per 

Per 

Per 

Per 

Per 

Per 

Per   Per 

Per 

Per 

cent,  cent. 

cent. 

cent. 

cent. 

cent. 

cent.  cent. 

cent.  cent. 

20° 

80   90 

100 

100 

100 

100 

50° 

100 

100 

100 

100 

100 

70° 

96 

98 

100 

100 

100 

90° 

91 

96 

99 

99 

100° 

3 

47 

60 

83 

83 

Owing  to  differences  in  the  electrolytes  used,  the  figures  are  not 
all  strictly  comparable,  but  only  inessential  alterations  are  thereby 
produced. 

From  the  above  discussion,  we  gather  that  the  best  cathodic 
conditions  for  copper  refining  are — 

(1)  Absence  of  dissolved  oxygen  or  other  oxidising  agent. 

(2)  Low  temperature. 

1  P.  124.  -  Zeitsch.  Anorg.  Chem.  14, 106  (1897). 

a  Jour.  Phys.  Chem.  7,  493  (1903). 


248    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

(3)  Acid  solution. 

(4)  Moderate  Cu"  concentration. 

(5)  Fairly  high  current  density. 

The  successful  functioning  of  a  copper  voltmeter  depends  of  course 
on  the  same  conditions  being  observed. 

At  the  anode  we  have  a  very  similar  state  of  things.  The  same 
equilibrium  as  at  the  cathode  Cu"  +  Cu  ^±  2Cu'  strives  to  set 
itself  up.  The  participating  electrochemical  reactions  are  : 

Cu  +2© >Cu"  (g) 

Cu  +   © *Cu'  (h) 

Cu'  +   © "Cu"  (k). 

The  disturbing  chemical  influences  are  the  same,  and  the  resulting 
continual  removal  of  Cu'  ions  tends  to  be  neutralised  by  the  reaction  (h). 
Copper  therefore  dissolves  anodically  in  quantity  exceeding  that 
demanded  by  Faraday's  Law,  assuming  the  metal  to  be  di-valent. 
We  have  already  seen  that  it  deposits  cathodically  in  amounts  less  than 
correspond  to  Faraday's  Law.  Combining  these  two  facts,  it  is  obvious 
that  the  electrolyte  must  gradually  become  richer  and  richer  in  copper. 
At  the  same  time  it  becomes  poorer  in  acid,  as  at  both  electrodes  Cu' 
ions  are  produced  and  subsequently  oxidised  throughout  the  bath 
according  to  the  equation  (e).  Wohlwill  states  that  the  acid  consump- 
tion is  also  caused  by  solution  of  the  finely  divided  copper l  in  presence 
of  air — thus  : " 

Cu  -f  2H'  -f  J02 >  Cu"  +  H20. 

One  other  interesting  point  must  be  noticed.  The  cuprous  copper 
is  practically  entirely  present  as  the  complex  anion  CuSO/.  We  have 
the  equilibria— 

[Cu']2  =  K[Cu"]  and 
[Cu']  .  [SO/]  =  K, .  [CuSO/], 

whence  [CuSO/]2  =  ^2 .  [SO/']2 .  [Cu"], 

Kx2 

or,  as  [SO/]  =  [Cu"]  (approximately) 

[Cu2S04]2  =  K2[CuS04]3. 

We  thus  see  that  the  equilibrium  quantity  of  cuprous  salt  decreases 
more  quickly  on  dilution  than  does  that  of  the  cupric  salt.  Consequently 
when  the  concentrated  equilibrium  solution  produced  at  the  anode  has 
diffused  away  a  little  into  the  bulk  of  the  electrolyte,  it  will  have 
become  supersaturated  with  respect  to  cuprous  copper,  and  the  re- 
action 2Cu* *  Cu"  -f-  Cu  will  take  place.  Finely  divided  crystals  of 

copper  are  precipitated  and  afterwards  found  in  the  anode  slimes. 

1  See  below. 


XVII.] 


COPPER 


249 


This  phenomenon  becomes  less  and  less  marked  as  the  current  density 
increases,  and  it  has  been  suggested  that  the  degree  of  supersaturation 
is  so  great  that  copper  precipitation  sets  in  whilst  the  cuprous  salt 
is  still  in  the  immediate  neighbourhood  of  the  anode,  and  the  metal  is 
practically  redeposited  chemically  on  the  electrode. 

For  another  reason  a  high  anodic  current  density  is  desirable — less 
time  comparatively  being  available  for  the  oxidation  and  hydrolysis  of 
the  Cu*  ions.  There  is  a  limit,  however,  to  its  increase,  as,  if  the  CuS04 
solution  becomes  too  concentrated  in  the  neighbourhood  of  the  anode, 
it  will  crystallise  out  with  formation  of  a  badly  conducting  solid 
layer.  A  consideration  of  the  anode  processes  in  the  electrolysis  of 
a  CuS04  solution  between  copper  electrodes  confirms,  therefore, 
the  conclusions  already  reached  as  to  the  most  favourable  working 
conditions. 

In  actual  copper  refining,  with  the  crude  metal  forming  the  anode, 
the  conditions  are  somewhat  different,  owing  to  the  impurities  dissolved 
anodically,  some  of  which  accumulate  in  the  electrolyte.  For  several 
reasons  it  is  generally  impracticable  to  electrolytically  refine  a  metal 
containing  less  than  97  per  cent,  copper.  The  electrolyte  quickly 
becomes  foul  and  needs  frequent  renewal.  And  the  percentage  of  un- 
attacked  copper  remaining  in  the  anode  residue  is  too  high.  Typical 
analyses  of  anode  copper  are  given  in  Table  XXXV : 


TABLE  XXXV 


American  Converter  Copper 


I 

II 

Cu 

99-25 

99-35 

Ag 

0-34 

0-24 

Au 

0-001 

0-02 

Pb 

0-01 

Bi 

0-002 

As 

0-03 

0-02 

Sb 

0-05 

0-00' 

Fe 

Trace 

0-01 

Ni 

0-002 

Se  +  Te 

0-01 

Insoluble 

Oxygen 

0-30 

Pres 

S 

III 

98-60 
0-05 

0-10 
0-05 
0-80 
0-10 
0-10 
0-10 


0-10 


English 


1 

German 

IV 

V 

98-24 

98-87 

0-10 

0-11 

0-0003 

0-0007 

0-02 

0-08 

0-04 

0-09 

0-94 

0-39 

0-40 

0-34 

Trace 

0-05 

0-28 

0-02 

0-05 
0-03 


0-04 


Several  points  may  be  noticed.  The  two  samples  of  American  metal 
have  more  silver  and  gold  than  the  raw  European  copper.  This  is 
simply  because  of  the  nature  of  the  ore.  On  the  other  hand,  the  low 
percentages  of  arsenic,  antimony,  and  bismuth  are  due  to  the  treatment 
with  air  in  the  converter,  whereby  these  substances  are  driven  off  as 
volatile  oxides.  The  impurities  in  anode  copper  that  must  be  con- 
sidered are  therefore  nickel,  iron,  lead,  gold,  silver,  selenium,  tellurium, 


250    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

arsenic,  antimony,  bismuth,  and  sulphur  and  oxygen  present  as  Cu20  and 
Cu2S.  Traces  of  platinum,  zinc,  cobalt,  and  tin  sometimes  occur. 

Behaviour  of  Anodic  Impurities. — The  behaviour  of  these  impurities 
during  the  electrolysis  was  carefully  investigated  by  Kiliani,1  and  the 
great  majority  of  his  observations  and  conclusions  have  been  repeatedly 
confirmed.  Nickel,  iron,  and  lead,  together  with  any  zinc,  cobalt, 
and  tin  which  may  happen  to  be  present,  are  electropositive  with 
respect  to  copper  and  completely  dissolve.  As  the  current  is  simul- 
taneously removing  copper  from  the  electrolyte  cathodically,  their 
presence  makes  the  electrolyte  poorer  in  copper.  The  lead  of  course  is 
at  once  precipitated  as  PbS04,  and  is  found  in  the  anode  slimes.  Any 
tin  will  slowly  come  down  as  insoluble  basic  sulphate.  The  other 
four  metals  mentioned  will  collect  in  the  electrolyte.  Being  so  much 
more  electropositive  than  copper,  they  will  require  a  far  higher  cathodic 
polarisation  than  does  the  latter  before  being  deposited  on  the  cathode, 
and  can  therefore  be  allowed  to  accumulate.  Thus,  for  electro- 
deposition  from  a  solution  of  the  same  normality,  nickel  requires  a 
cathodic  polarisation  0'7  volt  greater  than  does  copper.  This  is  partly 
due  to  a  reaction  resistance.  A  limit  to  the  accumulation  of  these  salts 
is  set,  however,  by  their  own  solubility,  the  corresponding  depletion  of 
the  Cu"  ions,  and  the  alterations  in  the  nature  of  the  cathodic  deposit 
thereby  produced. 

The  following  substances  less  electropositive  than  copper  remain 
undissolved  at  the  current  densities  used,  and  are  afterwards  found 
in  the  anode  slimes — Au,  Ag,  Pt,  Se,  Te.  With  high  current  densities 
and  high  percentages  of  silver,  a  little  of  that  metal  may  dissolve, 
when,  if  remaining  in  solution,  it  would  deposit  on  the  cathode.  As 
we  shall  see,  a  little  HC1  or  other  soluble  chloride  is  usually  added  to 
the  electrolyte,  and  the  Ag'  concentration  thus  kept  exceedingly  low.2 
Cu2S  is  unattacked  and  is  found  in  the  slimes,  whilst  Cu20  partially 
dissolves.  This  reaction  is  probably  merely  chemical,  depending  on 
the  acid  content  of  the  electrolyte. 

The  most  important  impurities,  because  of  their  deleterious  effect 
on  the  properties  of  the  refined  copper  and  the  ease  with  which  they 
enter  solution  and  are  cathodically  precipitated,  are  arsenic,  antimony, 
and  bismuth.  Small  quantities — up  to  0'8  per  cent. — of  antimony  or 
arsenic  improve  the  tensile  properties  of  copper  ;  bismuth  on  the 
other  hand  is  harmful.  But  all  three  metals,  even  when  only  a  few 

1  Berg-  und  Hutien-Zeit.  1885,  249,  260,  273. 

2  It  should  be  noted  that  from  Fig.  36  we  should  expect  the  anodic  silver  to 
dissolve  as  readily  as  the  copper.     We  are  here,  however,  dealing  with  a  complex 
system  containing  other  substances  than  silver  and  copper,  and  a  huge  excess  of  the 
latter  metal.    It  is  partly  due  to  the  presence  of  these  other  metals,  which  lower 
the  electrolytic  solution  pressure  of  the  silver,  and  partly  because  the  anodic 
polarisation  is  always  kept  very  low,  that  the  proportion  of  silver  actually  dissolving 
is  so  small. 


xvii.]  COPPER  251 

thousandths  per  cent,  are  present,  xjause  a  very  considerable  fall  in 
the  electrolytic  conductivity  of  the  copper,  its  most  valuable  property, 
the  increasing  of  which  is  the  very  purpose  of  electrolytic  refining. 
The  possibility  of  the  precipitation  of  these  three  impurities  is  clear 
when  we  consider  the  data  available  on  the  single  potential  differences 
which  they  show  against  solutions  of  the  corresponding  ion.  Neumann  l 
using  normal  solutions  of  SbCl3,  Bi2(S04)3,  and  AsCl3,  to  a  great  extent 
hydrolysed  and  with  indefinite  and  very  low  ionic  concentrations, 
found 

As  |  solution     +  0-27  volt, 

Bi  |  solution     +  0'21  volt, 
Sb  |  solution     +  0-10  volt, 

whilst  for  Cu|n.  CuS04,  £h=  +  0'308  volt.  As  the  potentials,  even 
for  very  small  ionic  concentrations  and  with  electrodes  of  the  pure 
metal,  approach  so  nearly  the  value  for  Cu  |  n.  CuS04,  one  can  well 
imagine  that,  with  the  discharge  somewhat  depolarised  by  the  copper 
cathode,  with  increased  amounts  of  ionic  impurity  in  solution,  or  with 
a  low  Cu"  concentration  at  the  cathode,  or  an  increased  cathodic 
polarisation,  produced  by  using  a  high  current  density,  a  potential  may 
be  reached  at  which  arsenic  or  bismuth  will  deposit. 

The  best  remedy  is  to  use  anodes  containing  as  little  as  possible  of 
these  constituents.  We  have  seen  that  converter  copper  is  better  than 
reverberatory  furnace  copper  in  this  connection.  When  that  is  im- 
possible, the  ionic  concentration  of  these  impurities  in  the  electrolyte 
must  by  other  means  be  kept  very  low. 

As"*  ions  on  entering  solution  are  immediately  almost  completely 
hydrolysed  as  follows  : 

2As"'  +  60H'  —  >  As203  +  3H20, 

the  free  As'"  concentration  being  exceedingly  small.  As  OH'  ions 
are  used  up,  and  as  copper  is  simultaneously  deposited  cathodically, 
the  effect  of  the  presence  of  arsenic  in  the  anode  is  to  decrease  the 
CuS04  content  and  to  increase  the  acidity  of  the  electrolyte.  As  the 
concentration  of  the  As203  increases,  it  can  begin  to  act  as  an  anodic 
depolariser,  and  be  oxidised  to  A^Oo.  The  antimony  dissolves  as 
Sb2(S04)3.  This  salt  again  is  very  strongly  hydrolysed,  this  time  giving  a 
sparingly  soluble  basic  sulphate  or  oxide.  Its  effect  on  the  electrolyte 
is  similar  to  that  of  arsenic.  Bismuth  behaves  exactly  like  antimony. 
The  equations  controlling  the  ionic  concentrations  of  these  metals  are 


Sb2(S04)3;±2Sb-+3S04" 
Bi2(SO)  ^ 


Zeitech.  Phys.  Chem.  14,  193  (1894). 


252    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

To  keep  the  As'"  content  low,  the  OH'  concentration  should  be  high — 
i.e.  the  H'  concentration  should  be  low.  On  the  other  hand,  the  Sb'" 
and  Bi'"  concentrations  are  essentially  determined  by  that  of  the  SO/" ; 
and  this  again,  owing  to  the  far  greater  dissociation  of  H2S04  than  of 
CuS04,  is  chiefly  a  matter  of  the  H2S04  content.  If  this  is  kept  high,  it 
favours  the  non-production  of  Sb'"  and  Bi'"  ions,  but  also  favours 
the  production  of  As'"  ions. 

As  a  matter  of  fact,  owing  to  arsenic  being  a  much  more  weakly 
basic  metal  than  antimony  or  bismuth,  with  a  smaller  ionising  tendency, 
its  effect  can  be  neglected  in  comparison  with  theirs,  and  it  is  found 
that  the  bath  is  best  made  pretty  strongly  acid.  The  concentrations 
of  Sb'"  and  Bi"'  are  also  kept  low  by  the  presence  of  the  small  quantity 
of  soluble  chloride  already  mentioned,  which  causes  the  formation  of  the 
very  insoluble  oxychlorides  SbOCl  and  BiOCl.  Foerster  gives  the 
following  figures  showing  how  the  last  three  impurities  divide  them- 
selves between  slimes  and  electrolyte  : 

f.  .  .     ,         ,                         Percentage  in  Percentage  in 

electrolyte  slimes 

As    0-059                           77  per  cent.  23  per  cent. 

Sb    0-065                           39  per  cent.  61  per  cent. 

Bi    0-032                            22  per  cent.  78  per  cent. 

The  products  of  electrolysis  are  : 

(1)  Cathode  copper. 

(2)  Anode  residues  and  slimes. 

(3)  Impure  electrolyte. 

The  following  table  contains  the  extreme  analysis  figures  for  ten 
different  samples  of  electrolytically  refined  copper.  The  figures  in  brackets 
denote  the  number  of  samples  containing  the  particular  constituents. 

Cu  99-85-99-99  Bi  (1)  0-0002 

Ag  (5)  0-0001-0-004  Fe  (4)  0-004-0-01 

Sb  (7)  0-0008-0-003  Pb  (4)  0-001-0-01 

As  (4)  0-001  0    (9)  0-002-0-07 

The  anode  slimes  will  contain  first  finely  divided  copper  from  the 

reaction  2Cu* *  Cu"  +  Cu.1    Then  gold,  silver,  and  platinum,  with 

which  will  be  alloyed  more  copper.  Then  sulphur,  selenium,  and 
tellurium,  in  all  cases  probably  combined  with  copper.  Finally,  basic 
bismuth  and  antimony  salts,  PbS04,  some  AgCl,  Cu20,  and  oxides  of 
arsenic.  The  actual  percentage  composition  of  the  slimes  can  naturally 
vary  within  wide  limits.  The  nature  of  the  original  ore,  the  previous 
metallurgical  treatment  of  the  anode  metal,  the  composition  of  the 
electrolyte,  and  the  current  density,  are  all  essential  factors  in  deter- 

1  See  p.  248. 


xvii.]  COPPER  253 

mining  their  composition.     The  following  Table  XXXVI  contains  the 
extreme  percentage  figures  of  analysis  of  fourteen  different  samples  : 

TABLE  XXXVI 

Cu  11 -01 -57 -00  Bi  0-34-5-70 

Ag  12-90-55-15  Pb  Trace-5-26 

Au  0-034-5-1  Te  1-0-3-97 

Sb  2-00-7-86  Se  0-39-1-72 

As  1-09-3-80  SO4  5-27-10-68 

The  electrolyte  becomes  gradually  richer  in  CuS04  during  the  electro- 
lysis, and  its  H2S04  content  decreases.  The  causes  have  already 
been  discussed.1  At  the  same  time,  nickel  and  iron  sulphates  accumu- 
late, as  also  do  Sb,  Bi,  and  As.  An  analysis  of  a  typical  foul  solution 
gave— 

Cu          51 '8  grams  per  litre 

Fe          13-2        „ 

As          14-06      „ 

Sb  0-62      „ 

H2S04     48 

The  treatment  of  both  slimes  and  electrolyte  will  be  shortly  discussed. 


2.  Copper  Refining.    Practice 

Two  distinct  arrangements  of  baths  and  electrodes  are  used  in 
technical  practice.  The  multiple  system  is  employed  in  Europe  and 
also  in  the  majority  of  the  great  American  refineries.  The  series  or 
Hayden  system  is  worked,  however,  in  two  of  the  largest  American  plants 
—the  Nichols  Refinery,  Brooklyn,  and  that  at  Baltimore.  In  the 
multiple  system,  all  the  anodes  in  each  tank  are  connected  in  parallel, 
also  all  the  cathodes  (just  as  in  Fig.  38),  and  a  large  number  of  such 
units  is  connected  in  series.  Through  each  tank  therefore  flows  a 
heavy  current  at  a  low  voltage.  In  the  series  system  the  arrangement 
is  as  in  Fig.  39,  the  tank  being  filled  with  bi-polar  electrodes,  from  one 
side  of  which  crude  metal  dissolves,  on  the  other  side  of  which  the 
refined  metal  is  deposited.  Each  unit  carries  a  much  smaller  current, 
but  absorbs  a  far  greater  voltage  than  a  unit  of  the  multiple  system 
containing  the  same  weight  of  copper. 

Multiple  System. — In  the  multiple  system  the  electrolysis  tanks 
are  generally  of  wood — lead-lined.  They  vary  considerably  in  size. 
An  average  American  vat  is  about  9'  X  3'  x  3'.  Those  used  in  Europe 
are  smaller.  They  are  very  carefully  insulated  on  glass  or  porcelain, 
and  are  placed  so  that  any  leak  of  electrolyte  can  be  at  once  detected. 
The  electrodes  can  be  suspended  inside  the  tank  either  by  providing 

1  Pp.  248,  251. 


254    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

them  with  lugs,  and  allowing  these  to  bear  directly  on  the  corresponding 
busbar  and  (carefully  insulated)  on  the  other  side  of  the  tank,  or, 
better,  by  laying  crossbars  over  the  tank,  resting  on  the  busbar  and  on 
the  side  ot  the  tank,  and  from  these  suspending  the  electrodes  by  hooks. 
This  second  method  has  the  advantage  that,  owing  to  the  absence  of 
lugs,  a  much  smaller  fraction  of  the  anode  remains  unattacked  after 
the  electrolysis.  It  averages  only  7  per  cent.,  whereas  using  anodes 
with  lugs,  it  may  easily  average  20  per  cent,  of  the  original  weight. 
This  represents  a  considerable  saving  of  interest  charges,  besides 
meaning  less  subsequent  melting  up  to  new  electrodes. 

The  anodes  are  of  cast  copper,  and  in  all  large  refineries  are  pre- 
pared by  a  special  casting  machine.  Their  size  corresponds  with  that 
of  the  vat.  In  the  American  refineries,  an  average  anode  is  35"  X  22" 
X  1*5".  The  time  they  remain  in  the  tanks  depends  on  their  thickness 
and  on  the  current  density.  To  keep  down  interest  charges  it  is  advan- 
tageous to  make  them  thin,  but  the  increased  labour  charges  for  more 
frequent  renewal  must  then  be  reckoned  in.  An  upper  limit  to  the 
possible  current  density  is  set  by  the  percentage  of  impurities  present. 
Only  with  comparatively  pure  anodes  are  high  current  densities  and 
therefore  quick  working  possible.  The  average  duration  of  treatment 
in  American  practice  has  been  recently  much  reduced,  and  is  now 
20-24  days. 

The  cathodes  are  prepared  electrolytically  in  separate  tanks,  often 
in  the  '  regenerating  tanks/  l  The  metal  is  deposited  on  a  sheet  of  pure 
rolled  copper,  which  has  been  lightly  coated  with  graphite  or  grease  to 
facilitate  subsequent  removal.  The  length  and  breadth  of  the  electrode 
are  the  same  as  those  of  the  anodes.  The  thickness  is  far  less,  generally 
1-2  mm.  and  never  exceeding  3  mm.  They  require  about  two  days 
to  make,  and  are  in  the  refining  tank  for  one  to  two  weeks.  A  unit  of 
the  size  described  will  contain  about  23  cathodes  and  22  anodes  arranged 
alternately,  the  distance  between  adjacent  electrodes  being  1|— 2". 
It  will  carry  about  4000  amperes  at  0'3  volt.  Some  hundreds  of  such 
units  will  be  arranged  in  series,  and  driven  off  one  generator. 

The  circulation  of  the  electrolyte  is  an  important  matter.  Methods 
have  been  proposed  and  indeed  employed  involving  the  blowing  of  air 
into  the  electrolyte.  It  must  be  emphasised  that  such  methods,  as  they 
bring  about  a  continual  oxidation  of  the  cuprous  copper  in  the  solution, 
are  essentially  irrational.  The  current  efficiency  of  the  process  is  lowered 
and  the  increase  of  CuS04  concentration,  and  consequent  deterioration 
of  the  electrolyte,  accelerated,  as  has  already  been  discussed.  The 
system  generally  adopted  is  gravity  circulation.  The  electrolyte  flows 
out  of  the  lowest  set  of  baths  into  a  supply  tank,  where  it  is  reheated  to 
the  working  temperature  by  leaden  steam  coils,  and  thence  pumped 

1  See  p.  267. 


xvn.]  COPPER  255 

up  to  the  highest  level  again.  The  velocity  of  circulation  depends  on 
the  working  conditions.  With  large  current  densities,  associated 
perhaps  with  the  use  of  high-quality  anodes,  or  at  low  working  tempera- 
tures, or  with  low  copper  concentrations,  the  rate  of  flow  must  be 
greater.  If  again  the  electrolyte  traverses  a  large  number  of  vats  before 
again  reaching  the  heating  and  supply  tank,  the  rate  of  circulation  should 
be  greater,  owing  to  the  cooling  which  otherwise  would  take  place. 
The  temperature  is  of  course  partly  kept  up  by  the  current. 

Electrolyte. — The  composition  of  the  electrolyte  is  determined  by 
several  factors.  A  high  CuS04  concentration  is  favourable,  as  there  is 
then  less  chance  of  cathodic  deposition  of  impurities,  or  of  a  non- 
adherent  copper  deposit  being  produced.  And  higher  current  densities 
and  a  lower  rate  of  circulation  of  the  electrolyte  may  be  used.  But  it 
must  not  be  too  high,  as  a  correspondingly  high  Cu'  concentration  is 
necessary  for  equilibrium,  to  reach  which  entails  current  losses.  Further 
there  is  a  danger  of  crystallisation  of  copper  sulphate  at  the  anode. 
The  electrolyte  almost  universally  used  contains  about  16  per  cent,  of 
CuS04,  5H20  crystals. 

A  high  H2S04  concentration  is  advantageous  because  (i)  of  the 
increased  conductivity  thus  obtained  ;  (ii)  it  lowers  the  Bi'"  and  Sb'" 
concentrations  ;  (iii)  it  prevents  precipitation  of  Cu20  and  consequent 
current  losses.  A  limit  to  its  increase  is  set  by  its  effect  in  (a)  diminish- 
ing the  solubility  of  CuS04 ;  (6)  increasing  the  As'"  concentration ; 
(c)  rendering  cathodic  H*  discharge  more  easy.  In  practice  6-10  per 
cent.  H2S04  is  present.  We  have  already  mentioned  the  customary 
addition  of  a  little  NaCl,  MgCl2,  or  HC1.  In  one  particular  refinery, 

the  chlorine  concentration  is  kept  at  O04  ~. .     Besides   keepinsr 

litre 

the  Sb'",  Bi"*,  arft  Ag*  concentrations  low,  it  seems  to  stop  the 
formation  of  TioAiles  on  the  cathode.  Finally,  the  addition  of 
(NH4).,S04  is  stated  to  work  well  if  much  arsenic  be  present.  The 
composition  of  ^he  electrolyte  is  constantly  controlled,  determinations 
being  made  of  conductivity,  acidity,  and  copper. 

Temperature.— A  »Mgh  working  temperature  is  advantageous,  on 
account  of  the  highet  conductivity  of  the  electrolyte  and  the  increased 
rates  of  diffusion,  which  aUow  of  higher  current  densities  being  used. 
On  the  other  hand,  more  cuprous  copper  is  needed  for  equilibrium  at 
the  electrodes,  which  means  current  losses  and  a  larger  amount  of 
copper  in  the  anode  slimes.  And  the  rate  of  oxidation  of  the  Cu'  ions 
and  the  deterioration  of  the  electrolyte  thereby  caused  are  increased. 
In  America  it  is  customary  to  work  at  40°-50°,  in  many  European 
refineries  at  about  35°. 

Current  Density.— This  all-important  question  is  decided  by  a 
consideration  of  the  following  points.  If  it  is  high,  the  rate  of  working 
is  rapid,  and  there  is  a  consequent  saving  in  space  and  in  interest 


256    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

charges.  It  is  favourable  to  the  electrolysis  in  that  it  brings  about  the 
establishment  of  a  smaller  Cu"  concentration  at  the  cathode,  a  smaller 
Cu'  concentration  being  thus  needed  ;  also  because  it  allows  proportion- 
ately less  time  for  the  oxidation  of  Cu'  to  Cu"  ions,  and  gives  an  anode 
slime  containing  a  smaller  copper  percentage.  But  too  high  a  current 
density  may  lead  to  hydrogen  evolution  and  to  the  production  of 
spongy  copper  and  Cu20  at  the  cathode,  also  to  the  deposition  of  such 
impurities  as  Bi,  As,  and  Sb.  With  impure  anodes  it  may  cause 
material  to  dissolve  which  otherwise  would  enter  the  slimes  (e.g.  silver). 
It  may  further  cause  crystallisation  of  copper  sulphate  on  the  anodes, 
and  in  any  case  necessitates  a  more  rapid  circulation  of  the  electrolyte. 
It  also,  of  course,  increases  the  voltage  and  the  necessary  energy 
expenditure. 

In  view  of  these  conflicting  influences  it  is  not  surprising  to  find  that 
current  densities  used  in  practice  in  the  multiple  system  vary  very 
considerably.  In  the  Norddeutsche  Affinerie  in  Hamburg,  the  current 
density  is  0*4-0'5  amp./ dm.2.  In  certain  American  refineries,  where 
very  pure  anodes  are  used  and  power  is  cheap,  current  densities  up  to 
4'3  amp.  /dm.2  have  been  employed.  If  still  further  increased,  spongy 
copper  begins  to  be  deposited.  The  average  American  practice  is 
1-2  amps./  dm.2.  In  England  and  on  the  Continent  it  is  rather  less. 

Voltage. — The  voltage  difference  across  an  average  multiple  system 
vat  is  O2-O25  volt,  and  rarely  exceeds  O3  volt.  Of  this,  the  polarisa- 
tion voltage,  due  to  the  difference  between  composition  of  anode  and 
cathode,  amounts  to  about  0*02  volt.  The  greater  part  of  the  remainder 
is  spent  in  overcoming  the  ohmic  resistance  of  the  electrolyte,  but  a 
considerable  fraction  is  nevertheless  due  to  defective  contacts.  Accord- 
ing to  Magnus,1  this  loss  may  even  amount  to  20  per  cent,  of  the  total 
potential  drop  across  the  tank. 

An  elaborate  investigation  was  carried  out  by  Schwab  and  Baum 2 
on  the  effect  of  changes  of  current  density  and  temperature  on  voltage 
and  current  efficiency  in  copper  refining.  The  electrolytes  used  were 
acid  CuS04  solutions  of  concentrations  employed  in  practice,  and  the 
copper  electrodes  were  placed  1  cm.  apart.  Fig.  62  expresses  the 
relationships  existing  between  voltage,  current  density,  and  tempera- 
ture. During  the  current  efficiency  tests  it  was  found  that  even  at  70°, 
working  with  technical  current  densities,  excellent  current  efficiencies  of 
practically  100  per  cent,  resulted.  The  contrary  results  of  Foerster  and 
Seidel 3  are  easily  explained  by  their  using  much  lower  current  densities. 

With  Schwab  and  Baum,  the  ratio  which  determines 

cathode  gain 

the  rate  of  deterioration  of  the  electrolyte  was  only  TOl-1'02  at  70° 

1  Electrochem.  2nd.  1,  561  (1WX).  "  Jour.  Phys.  Chem.  7,  493  (1903). 

3  Zeitach.  Anorg.  Chem.  14,  106  ( 1HU7). 


xvii.] 


COPPER 


257 


and  with  high  current  densities.  Fig.  63  gives  the  relations  between 
the  K.W.H.  necessary  for  the  production  of  one  kilo,  of  copper, 
current  density,  and  temperature.  They  finally  deduced  that,  working 
with  the  series  (Hayden)  system,  with  a  current  density  of  3*5-3*75 
amps./ dm.2,  and  with  covered  tanks  to  avoid  loss  of  heat,  after  taking 


0-36 1 


1-0    1-5      2-0     2-5     3-0      3-5      4-0 
FIG.  62. 


20°     30° '    40°      60°     60°     70a     80°     90* 
Temperature, 

FIG.  63. 


all  charges  into  account,  70°  is  the  best  economic  working  temperature. 
This  conclusion  has  however  been  received  with  criticism  in  technical 
circles. 

During  the  process  of  refining,  the  electrolyte  steadily  deteriorates. 
The  H2S04  is  gradually  neutralised,  and  impurities  accumulate  which  will 
finally  deposit  on  the  cathode.  Regeneration  is  necessary.  To  correct 
the  acid  concentration,  the  whole  electrolyte  is  often  circulated  through 
a  certain  number  of  baths  which  contain  copper  cathodes  and  insoluble 
lead  anodes.  At  the  latter,  oxygen  and  H2S04  are  liberated,  whilst  the 
cathodic  reaction  is  utilised  for  the  production  of  copper  cathodes  for 
the  refining  tanks.  Thus  both  the  H2S04  and  CuS04  concentrations  are 
corrected.  About  2-2*5  volts  are  absorbed  in  these  tanks,  which  may 
amount  to  1  per  cent,  of  the  total  number.  To  avoid  undue  accumula- 
tion of  impurities  such  as  As,  Sb,  Bi,  Fe,  Ni,  etc.,  some  20-30  per  cent, 
is  periodically  withdrawn  and  treated  in  various  ways  according  to 
local  circumstances.1  The  copper  may  be  electrolysed  out.  Or  the  acid 
may  be  neutralised  by  passing  the  hot  liquors  together  with  air  over  shot 
copper,  and  the  copper  content  of  the  solution  removed  by  crystallisation 
as  CuS04,  5H20,  and  finally  by  treatment  with  iron.  Having  removed 
1  See,  for  example,  Hetatt.  Chem.  Engin.  9,  154  (1911). 

3 


258    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

the  copper,  the  residue  may  be  discarded  or  worked  up  for  As,  Sb  or 
Bi  by  blowing  in  air  in  the  hot ;  and  nickel  sulphate  may  be  recovered. 

The  anode  slimes  are  generally  first  treated  with  hot  H2S04  and 
air  blown  in.  Cu,  As,  Sb  and  Bi  are  thus  largely  dissolved.  The 
undissolved  portion  is  washed  and  cupelled  with  the  addition  of  some 
lead.  Se  and  Te  are  removed  from  the  residue  by  fluxing  with  Na-2C03 
or  NaN03,  and  the  silver  and  gold  remaining  are  fused,  cast  into  anodes, 
and  parted  electrolytically  by  the  Moebius  process.1  Of  the  other 
constituents,  the  copper  is  obtained  as  sulphate,  and  the  arsenic  (as 
oxide),  the  tellurium,  and  the  selenium  (sometimes)  recovered  and 
sent  to  the  glass-maker. 

Series  System. — Much  of  the  above  description  of  the  working  of 
the  multiple  system  also  holds  good  for  the  Hayden  or  series  system. 
Some  essential  differences  must  now  be  considered.  The  tanks  used 
are  larger  than  in  the  multiple  system.  In  one  plant  they  are  16'  X  5' 
X  5' ;  in  another  plant  the  depth  is  considerably  less.  They  are 
constructed  of  slate,  instead  of  lead-lined  wood.  The  reason  is  that 
the  voltage  fall  along  a  tank  is  far  greater  than  in  the  multiple  system — 
instead  of  0*3  it  amounts  to  some  17  volts.  If  the  lining  of  the  tank 
were  conducting,  the  greater  part  of  the  current  would  not  flow  from 
end  to  end  of  the  tank  through  the  electrolyte,  but  from  one  end  elec- 
trode through  the  solution  to  the  wall,  along  it,  and  back  through  the 
solution  to  the  other  end  electrode.  Thus  copper  would  only  be 
deposited  on  the  end  cathode,  on  none  of  the  intermediate  ones.  As 
it  is,  there  is  always  a  certain  current  loss  through  part  of  the  current 
flowing  directly  down  the  tank  from  end  anode  to  end  cathode,  and 
thus  doing  the  minimum  amount  of  chemical  work.  The  gain  in 
weight  of  the  end  cathode  always  exceeds  the  gain  in  weight  of  any  inter- 
mediate cathode. 

The  series  system  requires  no  special  cathodes. 

On  the  other  hand,  the  anodes  need  careful  preliminary  treatment. 
They  must,  to  begin  with,  consist  of  good  quality  copper  which  easily 
dissolves  anodically,  leaving  only  a  small  residue.*  Otherwise  portions 
remain  unattacked,  and  the  cathodic  deposit  which  forms  on  the  other 
side  of  the  disappearing  anode  is  liable  to  be  dissolved.  For  the  same 
reason  all  surface  and  structural  irregularities  must  be  avoided,  and 
it  is  found  necessary  to  employ  copper  plates  which  have  been  rolled  and 
finally  hammered.  This  treatment  still  further  restricts  the  nature  of 
the  anode  material,  as  certain  kinds  of  copper,  otherwise  suitable  for 
refining,  cannot  be  worked  mechanically. 

A  bath  of  the  dimensions  given  will  contain  100-200  bi-polar  elec- 
trodes, placed  1-2  cm.  apart.  They  are  generally  6-8  mm.  in  thickness, 
and  about  60  cm.  square.  They  fit  into  slots,  cut  into  suitable 

1  Seep.2«0. 

2  This  condition,  as  we  have  seen,  can  be  fulfilled  in  America. 


xvii.]  COPPER  259 

wooden  uprights,  and  are  thus  kept  vertical.  ,  Their  edges  are 
smeared  beforehand  with  some  grease,  which  assists  the  subsequent 
detaching  of  the  finished  cathodes  from  the  anode  residue,  the  great 
bulk  of  which  consists  of  the  unattacked  portions  protected  by  the  slots. 
The  end  anode  is  often  made  of  lead  for  the  purpose  of  correcting  the 
acid  concentration.  Owing  to  the  proximity  of  adjacent  electrodes, 
the  voltage  drop  between  each  pair  is  low— 0'13  volt.  With  130  in 
series  the  voltage  along  the  bath  is  17  volts,  and  assuming  a  current 
density  of  T9  amps. /dm.2  the  current  carried  by  such  a  series  will  be 
36  X  1*9  =  68  amperes.  In  some  cases,  several  such  series  of  electrodes 
are  connected  in  parallel  in  the  one  bath,  thus  increasing  the  current 
taken  by  a  single  unit.  Owing  to  the  smaller  thickness  of  the  anodes, 
the  time  during  which  they  remain  in  the  bath  is  less  than  in  the 
multiple  system.  It  averages  about  twelve  days. 

The  electrolyte  used  is  of  much  the  same  composition  as  in  the 
multiple  process.  Less  of  it  is  needed  for  the  same  weight  of  electrode 
copper,  and  it  must  be  circulated  more  rapidly  and  regenerated  more 
often.  The  current  density  employed  is  seldom  below  1/6  amps./dm.2 
and  is  generally  nearer  2  amps./dm.2.  Owing  to  the  compactness  of 
the  plant,  close  proximity  of  the  electrodes,  etc.,  the  heat  produced  by 
the  current  is  more  effective  in  keeping  the  temperature  up  than  in 
the  multiple  system,  and  the  working  temperature  is  a  little  higher, 
averaging  about  50°. 

Comparative. — The  first  advantage  which  the  Hayden  process 
possesses  over  the  multiple  process  is  an  economic  one.  For  the 
same  copper  output,  a  smaller  plant  is  needed  with  less  electrolyte,  the 
weight  of  anodes  locked  up  is  far  less,  as  is  also  the  percentage  of 
unattacked  anode  at  the  end  of  the  electrolysis,  and  most  of  the  copper 
leads  and  busbars  are  avoided.  No  special  cathodes  need  be  prepared. 
On  the  other  hand,  the  process  is  less  adaptable  and  needs  careful  super- 
vision. Copper  anodes  of  moderate  or  poor  quality  cannot  be  used, 
and,  owing  to  the  need  for  their  corrosion  to  be  absolutely  uniform, 
very  high  current  densities  cannot  be  employed.  The  electrodes  require 
more  frequent  changing,  and  the  electrolyte  more  rapid  circulation  and 
more  frequent  regeneration.  Above  all,  the  preliminary  preparation 
of  the  anodes  is  tedious  and  costly. 

The  current  efficiency  of  the  multiple  system  is  decidedly  better 
than  that  of  the  series  system,  being  generally  96  per  cent.,  whereas 
the  latter  furnishes  only  90  per  cent.  The  deficiency  in  both  cases  is 
chiefly  due  to  short  circuit  and  shunt  current  losses,  and  these  are  far 
greater  in  the  series  process.  But,  on  the  other  hand,  the  voltage  drop  per 
pair  of  electrodes  is  about  twice  as  great  with  the  multiple  as  with  the 
series  system— 0*25  volt  as  against  0*13  volt.  Hence  the  multiple 

0*9       0*25 

process  requires   -— --  X  r— r  =  1/8  times  as  much  electrical  energy 
*  U'-Lo 

8  2 


260    PKINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

as  does  the  Hayden  process  for  the  refining  of  the  same  weight  of  copper. 
When  all  allowances  have  been  made  for  losses  in  leads  and  in  the 
dynamos  it  is  found  that  about  150  K.W.H.  are  needed  with  the  series 
system  and  about  300  K.W.H.  with  the  multiple  system  for  the  pro- 
duction of  one  ton  of  refined  copper.  Against  this  must  be  set  the  cost 
of  preparing  the  anodes  and  the  small  adaptability  of  the  Hayden 
system.  This  last  reason  makes  it  exceedingly  improbable  that  it  will 
ever  be  introduced  into  Europe. 


3.  Copper  Extraction 

Marchese  Process.  —  In  striking  contrast  to  the  present  position  of 
copper  refining  stands  the  problem  of  electrolytic  production  of  pure 
copper  from  its  ores  without  the  intermediate  formation  of  crude  metal. 
We  have  seen  that  a  chief  essential  to  the  success  of  an  electrolytic 
process  is  that  the  electrolyte  shall  be  pure  and  of  constant  composition, 
and  this  condition  is  the  main  obstacle  which  has  prevented  the  success 
of  the  hitherto  proposed  processes.  The  Marchese  process  is  a  striking 
example.  The  proposal  was  to  use  anodes  cast  from  copper-iron 
matte,  in  a  solution  containing  CuS04,  FeS04  and  free  H2S04,  copper 
being  deposited  cathodically.  It  was  hoped  that  the  anodes  would 
dissolve  easily,  partly  by  direct  oxidation  and  partly  by  the  action  of 
the  ferric  salt  present,  and  that  sulphur  would  be  deposited.  The 
process  was  tested  on  a  large  scale,1  but  proved  a  complete  failure.  As 
electrolyte  was  used  the  liquor  obtained  by  extracting  the  matte  with 
dilute  H2S04,  and  containing  free  acid,  CuS04,  FeS04  and  Fe2(S04)3— 
about  27-28  grams  Cu  and  15  grams  Fe  per  litre.  The  anodes,  which 
were  cast  with  difficulty,  had  the  composition  :  Cu  15-16  per  cent.  ; 
Pb  14  per  cent.  ;  Fe  41-42  per  cent.  ;  S  25  per  cent,  (on  another 
occasion  Cu  30  per  cent.  ;  Fe  40  per  cent.  ;  S  30  per  cent.).  On 
the  passage  of  a  current  the  phenomena  occurring  are  rather  com- 
plex. As  Egli2  has  shown,  Cu2S  dissolves  anodically  as  follows  :  — 


(a)  Cu2S+2©  —  >Cu 

(6)    CuS+2©  -  >Cu"+S. 

This  behaviour  appears  to  be  general  3  for  metallic  sulphide  anodes 
in  acid  solution.     We  therefore  also  have 


FeS+2©  -  >Fe 

PbS  +  2©  -  >Pb"+S. 

Further,  Fe2(S04)3  is  present  in  the  electrolyte,  and  more  is  formed  by 

1  Cohen,  Zeitsch.  Elektrochem.  1,50  (18f)4). 

~  Zeitech.  Anorg.  Chem.  30,  18  (/.W). 

:i  Bernfeld,  Zeitech.  Phys.  Chem.  26.  46  (1898). 


xvii.]  COPPER  261 

anodic  oxidation  of  FeS04.  This  will  act  chemically,1  dissolving  both 
Cu2S  and  CuS.  Finally,  the  FeS  will  be  attacked  by  the  H2S04. 

Experience  showed  that  the  sulphur  produced  anodically  adhered 
very  closely  to  the  electrode.  The  resistance  thus  introduced  caused 
increased  voltages— sometimes  a  rise  from  one  volt  to  five  volts.  The 
anodes  dissolved  unequally,  and  fragments  continually  broke  off.  Low 
current  densities — 0'3  amp./ dm.2 — had  to  be  used.  Finally,  the 
electrolyte  rapidly  deteriorated.  Acid  was  consumed,  the  iron 
content  increased,  and  various  impurities  (Bi,  Sb,  etc.)  dissolved  and 
contaminated  the  copper  deposit. 

Mansfeld  Experiments.— In  view  of  the  ill  success  of  the  above 
process,  a  recent  communication 2  dealing  with  large  scale  experimental 
trials  carried  out  at  Mansfeld  is  of  interest.  This  process  is  based  on  a 
patent 3  of  Borchers,  Franke,  and  E.  Giinther.  The  copper  ore  (sulphide) 
is  concentrated  by  blowing  in  a  Bessemer  converter  (the  S02  being 
worked  up  to  HJS04)  until  it  contains  >  72  per  cent.  Cu,  and  as 
nearly  as  possible  78-80  per  cent.,  which  about  corresponds  to  pure 
Cu2S.  Concentration  beyond  this  limit  results  in  a  loss  of  silver. 
This  rich  matte  is  cast  into  anodes  and  refined  in  an  acid  CuS04  solution. 
In  practice  the  best  results  are  obtained  with  anode  material  containing 
72-76  per  cent.  Cu.  As,  Sb,  Bi  are  almost  entirely  absent.  The  chief 
difficulty  is  the  deposition  of  sulphur ;  but  using  a  current  density  of  0*5 
amp./ dm.2  and  agitating  the  electrolyte,  the  voltage  could  be  kept 
steadily  below  one  volt  even  with  a  tolerably  thick  sulphur  layer.  The 
best  electrolytic  copper  is  produced.  These  results  were  given  by  a 
plant  capable  of  producing  twelve  tons  of  copper  per  week.  If  one 
compares  the  composition  of  the  anodes  employed  with  those  used  in 
the  Marchese  process,  one  can  well  suppose  that  a  pure  copper  will 
result  and  that  most  of  the  disturbances  of  the  Marchese  process  will 
not  be  felt.  It  is  not  clear,  however,  in  view  of  the  trouble  caused 
before  by  the  layer  of  sulphur,  how  this  difficulty  has  been  so  com- 
pletely overcome.  Probably  the  anodes,  which  can  be  readily  cast, 
are  much  thinner. 

Processes  Employing  Insoluble  Anodes.— To  avoid  using  an  impure 
anode,  with  the  consequent  fouling  of  bath  and  of  cathode  deposit,  the 
ore  or  matte  can  be  lixiviated  separately  for  the  preparation  of  the 
electrolyte,  and  an  insoluble  anode  used  for  the  electrolysis.  A  higher 
voltage  is  naturally  required  than  when  using  a  soluble  copper-rich 
anode.  This  treatment  has  also  been  applied  to  ores  too  poor  in 
copper  to  be  smelted.  The  processes4  of  Keith,  Carmichael,  and 
Laszczynski  make  use  of  electrolytes  obtained  by  lixiviation  of  cal- 

1  Thompson,  Electrochem.  Ind.  2,  225  (1904). 

-'  lletall.  5,  27  (1908).  :!  D.R.P.  160,046  (1904). 

4  Electrochem.  Ind.  1,  274  (1S03) ;  Metall.  3,  820  (1906) ;  Electrochem.  Ind.  5, 
421  (1907). 


262    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

cined  copper  ores  containing  up  to  5  per  cent,  copper  with  dilute 
(5  per  cent.)  H2S04.  Some  iron  usually  dissolves,  however  carefully 
the  ore  has  been  roasted.  The  filtered  electrolyte  may  contain 
2-5  per  cent.  Cu,  and  some  free  acid  (perhaps  1  per  cent.).  The 
anodes  are  of  lead,1  which  soon  become  coated  with  Pb02 ;  the 
cathodes  of  graphitised  lead  or  thin  copper  sheet.  A  cathodic 
current  density  of  O5-1  amp./  dm.2  can  be  used  ;  a  higher  one 
gives  a  spongy  deposit.  The  current  efficiency  is  90-100  per  cent. 
To  keep  it  to  this  figure,  the  copper  content  of  the  electrolyte  must  not 
fall  below  1  per  cent.2  When  it  reaches  this  value,  the  liquors,  now 
containing  up  to  5  per  cent,  free  H2S04,  are  withdrawn  and  used  for 
lixiviating  fresh  ore.  The  voltage  needed  depends  on  the  current 
density,  composition  of  electrolyte,  arrangement  of  electrodes,  etc.  It 
is  generally  2'2-2'5  volts. 

In  the  Carmichael  process  the  electrolyte  is  impregnated  with 
S02.  The  sulphite  ion  HS03'  is  formed  (OH'  +  S02),  and  depolarises, 
although  not  rapidly,3  the  OH'  discharge  at  the  anode,  the  reaction 

HS03'  +  OH'  — >  H-  -f  HSO/  +20 

taking  place.  In  this  way  the  voltage  can  be  kept  at  1'5  volts.  The 
S02  also  has  the  advantages  of  reducing  any  ferric  iron  present,  thus 
raising  the  current  efficiency,  and  of  regenerating  H2S04  in  the  bath 
(according  to  the  above  equation).  This  is  necessary,  as  some  is  always 
lost  in  the  lixiviation  because  of  the  presence  of  CaO,  etc. 

Assuming  for  these  processes  an  average  current  efficiency  of  95  per 
cent,  and  a  voltage  of  2'4  volts, 

1  ton  of  copper  requires 

96540  X  100  X  1000  X  1000  X  2'4 
95  X  31-8  X  1000  X  3600 

=  2130  K.W.H. 

And  one  H.P.  year  would  produce  3'1  tons  copper.  Economically 
these  processes  must  compete  with  wet  chemical  methods  (precipitation 
by  iron,  etc.),  followed  by  subsequent  refining.  We  cannot  further 
discuss  here  the  conditions  for  their  success  or  otherwise.  The  Lasz- 
czynski  process  is  at  present  operated  in  Poland  and  in  Siberia.  The 
Keith  and  Carmichael  processes  are  no  longer  worked. 

Siemens-Halske  Process.— Two  other  distinct  processes  of  consider- 
able interest  have  been  proposed  for  the  electrochemical  extraction  of 
copper,  have  received  exhaustive  trials,  but  have  been  abandoned.  In 

1  In  the  Laszczynski  process  surrounded  by  closely  fitting  diaphragms  of 
flannel  or  thick  cotton  cloth,  which  practically  prevent  the  oxidation  of  the  ferrous 
iron  present  in  the  liquors. 

2  See  also  Thompson,  Eleclrochem.  Ind.  2,  225  (1004). 

'A  According  to  Reinartz  [Melall.  5,  202  ( IMS)]  some  65  per  cent,  of  the  total 
current  is  thus  employed  in  oxidising  the  S02- 


xvii.]  COPPER  263 

the  Siemens  and  Halske  process  the  calcined  ore  was  lixiviated  with 
(essentially)  a  Fe2(S04)3  solution  containing  free  acid.  The  main 
reaction  occurring  is 

Cu2S  +  2Fe2(S04)3 — >  2CuS04  +  4FeS04  +  S. 

The  resulting  liquor  free  from  Fe'"  ions  entered  the  cathodic  compart- 
ment of  the  cells.  There  the  copper  was  deposited.  It  then  flowed 
through  a  diaphragm  into  the  anode  compartment.  There,  though 
using  an  insoluble  anode,  no  gas  is  evolved,  but  the  reaction  Fe"  -f-  © 
— >  Fe'"  takes  place.  Fe2(S04)3  is  regenerated,  and  the  liquor  is 
thus  ready  again  for  the  lixiviation  tanks.  The  total  reaction  in  the 
cell  for  the  passage  of  two  faradays  is 

CuS04  +  2FeS04  — >  Fe2(S04)3  +  Cu  or 

Cu"  -f-  2Fe"  — >  2Fe*"  +  Cu. 

Electrochemically  the  process  is  simple  and  ingenious,  and  the  actual 
electrolysis  went  very  smoothly.  Copper  cathodes  were  used,  taking 
0*16  amp. /dm.2  ;  the  anodes  were  of  lead  or  of  carbon.  The  electro- 
lyte when  entering  the  cathode  chamber  contained  3-5  per  cent.  Cu. 
About  two-thirds  were  removed  before  entering  the  anode  compartment. 
The  voltage  was  0'7  volt.  The  decomposition  voltage  can  be  calculated 
thus : — 

At  cathode  E.P.Cu.._^Cu  +0'33  volt. 

At  anode     E.P.^...^^..  +0-71  volt. 

The  copper  concentration  we  will  assume  3  per  cent.,  which  corresponds 
to  [Cu]  =  0-5  and  [Cu"]  about  O'l.  Hence 

<Scu"->cu= +0-33 +0-029  log  0-1 
=  0-30  volt. 

Similarly  we  may  assume  [Fe"]  and  [Fe"']to  be  approximately  equal. 
In  that  case  £Ff... ._>¥e..  =  E.P.^...^^..  =  0'71  volt,  and  the  decom- 
position voltage  is  0'71  —  0'30  =  0*41  volt.  The  difference  between 
this  value  and  the  working  value  is  due  to  the  electrolyte  and 
diaphragm. 

The  difficulties  which  led  to  the  abandonment  of  this  process  were 
two.  No  suitable  diaphragm  could  be  found— they  were  not  sufficiently 
durable  and  offered  too  great  a  resistance  to  the  current.  Then  came 
the  great  difficulty  of  regulating  and  co-ordinating  the  calcination  and 
lixiviation  of  the  ore  with  the  electrolysis.  The  latter  demands  a 
regular  supply  of  electrolyte  of  constant  composition,  which,  with  ores 
of  varying  composition,  undergoing  the  ill-defined  and  uncertain 
operation  of  roasting,  is  hard  to  guarantee.  In  any  case,  lixiviation  is 
very  slow,  and  the  ore  must  be  exceedingly  finely  ground  and  the 
liquor  sometimes  filtered.  Neither  difficulty  is  perhaps  unsurmount- 
able.  The  question  of  diaphragms  has  received  much  attention  during 
the  last  few  years,  whilst,  provided  that  the  supply  of  ore  available  were 


264    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

of  pretty  constant  composition,  one  could  expect  the  difficulty  of  the 
electrolyte  to  be  overcome.  It  should  be  mentioned  that  the  anodes 
sometimes  gave  trouble  :  lead  became  oxidised  with  formation  of  Pb02 
and  an  increase  in  voltage,  whilst  carbon  disintegrated  and  oxidised 
away.  This  is  closely  connected  with  variations  in  the  electrolyte 
composition.  If  the  Fe"  concentration  becomes  too  low,  then  oxygen 
will  be  evolved  and  the  disturbances  will  commence. 

Thompson  x  has  investigated  certain  features  of  this  process.  He 
studied  the  solvent  powers  of  Fe2(S04)3  and  H2S04  on  various  possible 
products  of  calcination,  and  the  relations  existing  during  the  electrolysis 
between  current  density,  current  efficiency,  and  concentration. 

Hoepfner  Process.— This  process 2  resembled  in  some  respects  the  one 
just  described.  Finely-ground  ore,  sometimes  unroasted,  was  lixiviated 
with  a  solution  containing  CuCl2  and  an  excess  of  CaCl2  or  NaCl.  The 
main  reaction  occurring  is  the  reduction  of  the  Cu"  ions  by  the  Cu2S 
in  the  ore 

2Cu"  -f  Cu2S — >  4Cu'  +  S. 

Owing  to  the  excess  of  01'  ions,  the  CuCl  formed  exists  in  solution  as 
complex  anion  CuCl./.  When  electrolysed,  copper  is  deposited  at  the 
cathode,  and  at  the  insoluble  anode  the  reaction  Cu*  -f-  ©  — *•  Cu" 
takes  place.  Thus  no  gas  is  evolved,  the  voltage  is  kept  down,  and 
the  extraction  liquor  regenerated.  The  total  cell  reaction  is  2  Cu*  — > 
Cu  -f-  Cu"  for  the  passage  of  one  faraday.  Hoepfner  claimed  as  advan- 
tages over  the  Siemens-Halske  process  that  (1)  the  Ag,  Au,  etc.,  in  the 
ore  were  extracted ;  (2)  far  more  copper  could  be  dissolved  for  the  same 
volume  of  electrolyte  ;  (3)  at  the  cathode  twice  as  much  copper  was 
precipitated  for  the  same  number  of  coulombs  passing. 

In  actual  practice  the  Very  finely -ground  ore  was  extracted  in  large 
wooden  drums  at  about  70°  with  a  liquor  containing  some  60  grams/litre 
cupric  copper.  PbCl2  crystallised  on  cooling,  and  silver  was  removed 
by  passing  over  copper.  To  remove  As,  Sb,  and  Bi,  the  product  was 
treated  with  lime,  chalk,  or  CuO.  Iron,  the  solution  of  which  cannot 
be  entirely  avoided,  was  precipitated  at  the  same  time.  (If  allowed  to 
accumulate,  it  would  lower  the  amount  of  cuprous  copper  which  could 
dissolve :  thus  Fe'"  -f  Cu'  ^  Cu"  -f  Fe".)  Cu20  was  not  precipitated 
because  of  the  extremely  low  cupio-ionic  concentration.  The  resulting 
purified  liquors,  containing  about  120  grams/litre  cuprous  copper, 
were  separated  into  two  streams.  One  flowed  through  the  cathode 
system  of  a  number  of  electrolysis  tanks  connected  in  parallel ;  the 
other  through  the  anode  system.  In  each  tank  the  carbon  anodes  and 
copper  cathodes  were  separated  by  a  diaphragm.  The  cathodic  and 
anodic  effluents  (the  former  almost  copper-free)  were  mixed  on  leaving 

1  Eledrochem.  Ind.  2,225  (I'.xil). 

2  Electrochem.  Ind.  1,  540  (1003).      Zeitech.  Angew.  Chem.  4, 160  (1801). 


xvii.]  SILVER  265 

the  tanks,  and  used  to  extract  fresh  ore.  0*8  volt  was  used,  and  a  current 
efficiency  of  about  90  per  cent,  obtained.  The  deficiency  is  due  to  the 
process  Cu"  — >  Cu*  -f-  0,  which  takes  place  very  easily  if  any  Cu" 
ions  are  present,  owing  to  the  exceedingly  low  concentration  of  Cu' 
ions  in  a  chloride  solution.  The  cathodic  current  density  was  probably 
about  2  amps. /dm.2.1 

Assuming  the  above  figures,  one  ton  of  copper  requires 

96540-xXf  X  WOO  X  1000  X  0'8 


X  6S-6X-3SQO-X  1000 
=  374  K.W.H. 


K.W.H. 


And  1  H.P.  year  will  produce  17'5  tons  copper,  nearly  six  times  the 
quantity2  furnished  by  a  catholyte  containing  Cu"  copper  and  an 
anolyte  from  which  oxygen  is  evolved.  In  spite  of  this,  the  Hoepfner 
process,  like  the  Siemens  and  Halske,  is  not  in  use.  And  the  reasons 
are  the  same  :  difficulties  in  rapidly  and  completely  extracting  the 
ore  and  in  keeping  the  composition  of  the  electrolyte  constant,  the 
conditions  changing  every  time  a  fresh  ore  is  treated,  and  difficulties 
with  regard  to  the  diaphragms. 


4.  Silver  Refining3 

The,  raw  material  of  the  silver  refiner  is  of  several  different  kinds. 
It  may  be  crude  gold  bullion,  containing  some  30  per  cent.  Au  and 
60  per  cent.  Ag  ;  or  the  anode  slimes  from  copper  refining  (freed  from 
copper)  or  the  silver  concentrates  from  the  desilverisation  of  lead 
(freed  from  zinc  or  lead),  when  it  will  have  94-98  per  cent,  silver,  and 
up  to  5  per  cent.  gold.  Or  it  may  consist  of  scrap  jewellery,  gold  and 
silver  plate,  etc.,  containing  perhaps  50  per  cent.  Cu,  and  large  quantities 
of  base  metals.  To  '  part '  gold-silver  bullion  or  to  extract  gold  values 
from  crude  silver,  the  old  chemical  methods  consisted  in  treating  with 
HN03  or  H2S04,  the  silver  dissolving  and  leaving  behind  gold  and 
platinum.  These  methods  have  now  been  almost  entirely  replaced 
by  electrochemical  processes.  The  advantages  gained  are  considerable. 
They  include  :  (1)  more  complete  recovery  of  valuable  constituents 
(Au,  Pt),  and  elimination  of  harmful  ones  (Te) ;  (2)  far  smaller  acid 
consumption  ;  (3)  greater  cleanliness  and  ease  of  working. 

As  in  copper  refining,  the  crude  metal  is  cast  into  anodes,  and  the 
pure  product  cathodically  deposited,  the  electrolyte  being  an  aqueous 
silver  salt.  This  procedure  is  only  altered  when  the  anodes  contain 
too  great  a  percentage  of  metals  less  noble  than  silver,  which  would  dis- 
solve, accumulate  in  the  electrolyte,  and  tend  to  deposit  on  the  cathode. 

1  Coehn  and  Lenz,  Zeitsch.  Elektrochem.  2,  25  (1895). 

-  See  p.  262.  Metall.  Chem.  Engin.  9,  443  (1911). 


266    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

The  best  salt  for  the  electrolyte,  easily  obtainable  and  readily  soluble, 
is  AgN03.  Its  behaviour  on  electrolysis  between  pure  silver  electrodes 
is  simple.1  At  low  current  densities  adherent  crystalline  silver  is 
quantitatively  deposited  cathodically.  At  higher  current  densities 
the  metal  no  longer  adheres,  but  forms  long  loose  crystals  stretch- 
ing towards  the  anode  and  readily  detached.  Finally,  hydrogen  is 
evolved.  At  the  anode  silver  is  dissolved,  again  quantitatively. 
The  most  accurate  coulometer  consists  of  silver  electrodes  in  AgN03 
solution.2 

The  impurities  which  may  occur  in  crude  silver  are  gold,  copper, 
platinum,  lead,  zinc,  bismuth,  cadmium,  iron,  tellurium,  tin,  nickel. 
The  Au,  Pt,  and  Te  remain  unattacked  anodically,  entering  the  slimes, 
which  may  also  contain  lead  as  Pb02,  and  Sn  and  Bi  as  basic  salts. 
Copper,  nickel,  iron,  zinc,  cadmium,  and  some  of  the  lead,  bismuth,  and 
tin  will  dissolve.  Of  these  copper  is  the  most  electro-negative,  and  hence 
most  likely  to  deposit  cathodically  with  the  silver.  E.P.Xp-^.AK 
is,  however,  O47  volt  more  positive  than  E.P.Cu-._>ru  (O80  —  0*33 
volt).  Copper  deposition,  therefore,  though  assisted  by  the  depolarising 
action  of  the  silver  cathode,  requires  a  low  Ag'  concentration,  and  a 
large  accumulation  of  Cu"  ions,  helped,  moreover,  by  a  fairly  high 
current  density  with  its  accompanying  increase  of  the  cathodic 
polarisation  or  by  insufficient  mixing  of  the  electrolyte.  The  chief 
effect  of  the  soluble  anodic  impurities  is  the  gradual  lowering  of  the 
silver  content  of  the  electrolyte,  as,  whatever  metal  is  dissolved 
anodically,  its  equivalent  of  silver  is  always  deposited  cathodically. 

Moebius  Process.3— For  the  treatment  of  silver  residues  from  the 
copper  or  lead  refinery,  the  Moebius  process  is  usually  employed.  The 
electrodes  are  hung  vertically  in  a  tank  of  earthenware  or  tarred  wood 
(about  12'  X  2'  X  2'),  divided  into  seven  compartments.  The  anodes 
are  suspended  four  or  five  together  from  a  common  bus-bar,  each  set 
being  surrounded  by  a  stout  bag  of  filter-cloth,  stretched  over  a  wooden 
frame.  They  have  the  following  average  composition  :  (a)  from 
copper  refining  95  per  cent.  Ag,  3  per  cent.  Au,  2  per  cent.  Cu,  Bi,  Pb, 
Te,  Pt  ;  (b)  from  lead  refining  98  per  cent.  Ag,  O5  per  cent.  Au,  T5  per 
cent.  Cu,  Bi,  Pb.  The  cathodes,  suspended  in  the  cell  alternately 
with  the  anodes,  are  of  thin  silver-foil,  and  of  area  almost  equal  to  the 
cross-section  of  the  bath.  In  each  compartment  will  be  perhaps  four 
cathodes  and  three  sets  of  anodes,  some  two  to  three  inches  apart. 
Like  electrodes  are  connected  in  parallel,  and  the  different  compart- 
ments in  series. 

1  Under  certain  circumstances,  other  reactions  can  take  place,  but  only  to  an 
insignificant  extent,  or  else  under  exceptional  conditions.  \Ye  need  not  further 
consider  them. 

•  See  p.  32. 

3  Trans.  Amer.  Electrochem.  Soc.  8,  125  (lw:>). 


xvn.]  '  SILVEK  267 

The  electrolyte,  because  of  interest  charges,  is  dilute— containing 
only  0-5-2-0  grams/litre  silver.  It  is  kept  slightly  acid  (0 1-0*5  gram/litre 
HN03),  the  reasons  being  to  increase  the  conductivity  and  to  avoid 
the  precipitation  of  basic  salts  or  hydroxides.  Owing  to  chemical  re- 
action with  the  silver  of  both  electrodes,  H'  ions  slowly  but  steadily  dis- 
appear from  the  electrolyte  during  electrolysis,  and  a  neutral  solution 
wouJd  soon  become  alkaline.  Although  the  silver  content  of  the 
solution  is  so  low,  a  high  current  density  is  used,  again  to  economise 
interest  charges.  Because  of  these  two  reasons  (low  Ag*  content  and 
high  current  density),  it  is  impossible  to  allow  copper  to  accumulate 
in  the  electrolyte  above  a  certain  limiting  concentration,  as  there  would 
be  a  risk  of  its  cathodic  deposition.  Under  normal  conditions  this 
limiting  value  is  about  4  per  cent.  A  normally  working  plant  will  have 
then  an  electrolyte  containing  per  litre— 

1  gram  silver. 
40  grams  copper. 
0-12  gram  free  HN03. 

To  keep  this  composition  constant,  a  fraction  of  the  liquors  must  be 
periodically  drawn  off,  and  replaced  by  a  solution  rich  in  HN03  and 
AgX03.  It  is,  of  course,  possible  to  work  with  a  higher  Cu"  concentra- 
tion, but  only  by  using  a  lower  current  density  and  adding  more 
acid. 

The  actual  current  density  employed  depends  on  the  Ag*  and  Cu" 
concentrations  in  the  bath,  on  interest  charges,  etc.  With  a  fresh  bath, 
free  from  copper,  it  will  be  3'5  amps,  /dm.2,  but  under  normal  conditions 
nearer  2-5  amps.  /dm.2.  At  this  high  figure,  with  such  a  low  Ag'  con- 
centration, the  silver  is  deposited  in  a  loose  crystalline  form  which 
tends  to  grow  out  towards  the  anode.  To  prevent  this,  each  cathode 
is  provided  with  a  wooden  scraper,  worked  by  an  eccentric,  the 
deposited  metal  falling  into  a  tray  at  the  bottom  of  the  tank,  which  is 
periodically  withdrawn  and  emptied.  The  movement  of  these  scrapers 
also  serves  to  agitate  the  electrolyte.  No  other  means  is  used— the 
liquors  are  not  even  circulated.  The  voltage  per  compartment  is 
I' 4-1/7  volts,  according  to  the  current  density.  For  a  refining  opera- 
tion this  figure  is  high,  the  cause  being  the  comparatively  poor  con- 
ductivity of  the  electrolyte.  The  cell  polarisation  is  only  0*15  volt. 
A  higher  working  temperature  would  decrease  the  voltage  and  allow 
of  heavier  current  densities,  but  the  chemical  action  of  the  acid  on  the 
electrodes  renders  this  impossible. 

The  current  efficiency  in  the  Moebius  process  is  94-95  per  cent.  A 
mere  trace  of  copper  is  deposited  (0*02  per  cent.),  and  the  deficiency 
must  be  ascribed  to  cathodic  reduction  of  Cu"  to  Cu"  ions  (afterwards 
re-oxidised  by  air  and  agitation),  and  partly  to  chemical  solution  of  the 


268    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

silver  cathode  by  the  HN03.  Further,  a  little  HN03  is  cathodically 
reduced  to  ammonia.  One  kilo  of  silver  requires  then 

1000  X  96540  X  100  X  1-6  _ 
108  X  95  X  3600  X  1000 

The  cathodic  deposit  is  999  fine— very  seldom  less.  The  anodes 
(J"  thick)  remain  in  the  bath  for  36-48  hours.  The  slimes  contain  Au, 
Pt,  a  little  Ag  and  Cu,  Te,  and  Pb02.  After  washing  and  extracting 
with  HN03,  they  are  cast  into  anodes  and  sent  to  the  gold-refining 
tanks. 

To  render  his  process  continuous,  Moebius  later  suggested  a  modi- 
fication. The  bath  was  a  long  shallow  tank,  through  which  the  cathode, 
in  the  form  of  an  endless  silver  band,  almost  the  width  of  the  bath, 
constantly  moved.  At  the  point  where  it  emerged  from  the  tank,  the 
silver  crystals  were  brushed  off.  The  composition  of  the  electrolyte, 
cathodic  current  density,  etc.,  were  essentially  as  before.  In  practice 
this  process  seems  to  have  proved  too  complicated.  The  silver  crystals 
did  not  detach  easily,  and  the  band  tended  to  break.  It  is  now  stated 
to  be  working  in  one  Mexican  refinery  only. 

Balbach-Thum  Process.1— A  further  modification  of  the  Moebius 
process,  the  Balbach-Thum  process,  is  in  operation  in  two  of  the  great 
North  American  refineries,_the  Raritan  and  the  Balbach  works.  The 

cell  is  particularly  simple.  It 
consists  of  a  shallow  glazed 
porcelain  trough  (Fig.  64),  the 
bottom  of  which  may  be  hori- 
zontal or  may  partly  or  wholly 
slope  gently  upwards.  This 
FIG.  64.— Balbach  Silver  Refining  Cell.  surface  is  lined  with  Acheson 

graphite  slabs,  which  form  the 

cathode,  of  area  about  eight  square  feet.  The  anodes  are  arranged 
horizontally  on  shelves  above  the  cathode,  and  enclosed  in  canvas 
diaphragms.  They  are  kept  in  the  bath  until  completely  dissolved. 
The  electrolyte  is  the  same  as  already  described. 

The  cathodic  current  density  is  2-2'6  amps. /dm.2,  at  the  anode 
twice  as  great.  The  voltage  is  3'2-3'8  volts.  This  high  value  is  due 
to  the  resistance  of  the  slimes  which  settle  in  and  on  the  diaphragm, 
and  also  to  the  distance  between  the  electrodes,  which  is  considerably 
greater  than  in  the  Moebius  process.  This  is  necessary,  as  there  is  no 
mechanical  arrangement  for  brushing  down  the  cathodic  silver  which 
rapidly  grows  out  towards  the  anode.  The  loose  deposit  must  be 
periodically  pressed  down  and  removed.  This  operation  at  the  same 
time  mixes  the  electrolyte,  the  only  way  in  which  this  happens,  as 

1  Electrochem.  Ind.  0,  277  (H:08). 


xvii.]  SILVER  269 

there  is  no  circulation.  If  often  done,  the  Cu"  concentration  can 
probably  be  allowed  to  rise  as  high  as  in  the  Moebius  process  ;  otherwise 
it  must  be  kept  lower,  which  necessitates  more  frequent  renewal  of  the 
electrolyte.  The  current  efficiency  is  about  93  per  cent.  Reckoning 
the  voltage  at  3' 5  volts,  we  see  that  the  energy  consumption  per  kilo 
is  twice  as  great  in  the  Balbach  as  in  the  Moebius  process.  And  there 
is  a  proportionately  larger  amount  of  silver  permanently  tied  up  in  the 
bath.  Power,  however,  is  a  small  item  in  the  refining  of  such  a  valuable 
product.  And  there  are  the  advantages  of  simplicity  and  a  more 
complete  separation  of  the  silver  than  the  Moebius  process  allows. 

Parting  of  Bullion.— The  electrolytic  parting  of  bullion  rich  in  gold 
is  effected  by  the  same  methods.  At  the  Philadelphia  mint x  a  modified 
Moebius  process  is  used.  The  anodes  (30  per  cent.  Au,  60  per  cent.  Ag, 
10  per  cent.  Cu,  Pb,  Bi,  Zn)  are  encased  in  cloth  bags  and  hung  alter- 
nately with  rolled  fine  silver  cathodes  in  an  earthenware  trough.  The 
electrolyte  contains  3-4  per  cent.  AgN03  and  1*5  per  cent.  HN03,  with 
certain  quantities  of  dissolved  anodic  impurities.  The  current  density 
used  is  only  0*75  amp. /dm.2,  and  the  voltage  is  low — one  volt.  A  trace 
of  gelatine  (1 : 8,000-10,000)  is  added  to  the  electrolyte.  This  causes  the 
silver  (or  nearly  all  of  it)  to  deposit  in  a  coherent  crystalline  form,  a 
behaviour  rendered  easier  by  the  low  current  density.  The  metal  is 
exceedingly  pure,  and  the  current  efficiency  approaches  100  per  cent. 
The  anodes,  after  parting,  retain  their  original  form  owing  to  the  high 
content  of  gold.  Owing  to  the  large  percentage  of  base  metal  dis- 
solving, the  electrolyte  needs  frequent  renewal. 

In  the  San  Francisco  mint,2  the  bullion  is  parted  by  the  Balbach- 
Thum  process,  no  attempt  being  made  to  deposit  coherent  metal.  The 
anodes  have  much  the  same  composition  as  at  Philadelphia.  The 
electrolyte  contains  about  4  per  cent,  silver  and  1-2  per  cent,  free 
HX03,  with  copper,  zinc,  bismuth,  and  lead.  The  anodic  current 
density  is  high—  5-5*5  amps. /dm.2,  and  3*5  volts  are  used.  The  silver 
is  produced  as  a  coarse  sugary  deposit  99*95  per  cent.  pure.  5  per  cent, 
of  the  electrolyte  is  daily  withdrawn  and  replaced  by  liquor  very  rich 
in  silver.  Although  the  percentage  of  base  metal  is  considerable  and 
the  current  density  high,  the  deposits  produced  are  pure  by  reason  of 
the  high  Ag*  concentration.  The  advantage  of  working  rapidly 
probably  far  outweighs  in  practice  the  drawbacks  of  the  high  voltage 
and  the  non-coherent  deposit. 

Dietzel  Process.3— When  the  percentage  of  copper  in  the  anode 
material  is  very  high,  the  procedure  for  the  recovery  of  the  different 
values  alters  essentially.  In  the  Dietzel  process  the  alloy  is  made  anode 
in  a  solution  containing  N03'  ions.  Silver,  copper  and  base  metals 

1  Electrochem.  Ind.  4,  306  (1906). 
-  Electrochem.  Ind.  6,  355  (1908). 
Zeitsch.  Elektrochem.  6,  81  (1899). 


270    PKINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

dissolve,  gold  and  platinum  are  untouched.  The  anode  liquors  are 
freed  from  silver  chemically  outside  the  electrolysis  cell,  and  then 
pass  to  the  cathode  compartment,  where  copper  is  deposited. 

Fig.  65  illustrates  the  electrolysis  cell,  which  is  divided  into  anode 
and  cathode  compartments  by  the  canvas  diaphragm  B.  The  entering 
liquor  deposits  copper  on  the  slowly  rotating  cylinders  AA.  It  passes 

through  B  and  arrives  at  the 
Electrolyte  anodes  CC.       These  consist  of 

plates  (3-5  mm.  thick)  of  very 
variable  composition,  contain- 
ing perhaps  5-7  per  cent.  Au, 
22-50  per  cent.  Ag,  40-65  per 
cent.  Cu,  5  per  cent.  Sn,  Zn, 
Pb,  and  a  little  Pt,  Cd,  Ni,  and 
Fe.  They  rest  on  glass  or  cellu- 
loid plates,  and  are  connected 
with  the  positive  lead  by  a 
FIG.  65.— Dietzel_Refining  Process.  network  of  platinum  wire.  The 

anode    liquors,    charged    with 

silver,  copper,  and  various  soluble  impurities,  leave  the  cell  at  D 
and  flow  through  a  number  of  jars  containing  scrap  copper.  There 
the  silver  is  deposited  and  an  equivalent  amount  of  copper  dis- 
solved. The  desilverised  liquors  are  pumped  up  into  a  supply  tank, 
from  which  they  re-enter  the  cathode  chamber. 

When  the  dissolved  impurities  reach  an  inconveniently  high  figure, 
part  of  the  electrolyte  is  withdrawn  and  replaced.  Further  HN03, 
which  is  being  constantly  consumed  by  the  copper  in  the  desilverising 
vessels,  is  regularly  added.  The  anodic  current  density  is  about  1'5 
amps./ dm.2,  and  the  average  voltage  2'5-3  volts.  The  anode  slimes 
contain  gold  with  a  little  silver,  platinum,  Pb02,  etc.  The  cathode 
deposit  contains  a  trace  of  silver  and  some  oxygen.  The  electrolyte 
used  is  only  very  faintly  acid,  and  we  have  seen1  that  this  favours  the 
cathodic  precipitation  of  Cu20.  The  current  efficiency  is  some- 
what under  100  per  cent. :  the  loss  is  probably  due  to  the  process 
Cu"  — >  Cu'  +  ©,  as  the  liquors  are  well  saturated  with  air  during 
their  journey  outside  the  cell,  and  any  Cu*  ions  would  be  oxidised. 
At  the  copper  cathode,  moreover,  an  appreciable  electrolytic  reduction 
of  N03'  to  NH3  takes  place. 

Electrolysis  as  a  factor  in  the  extraction  of  silver  from  its 
ores  is  beginning  to  make  an  appearance.  In  Mexico,  KCN  treat- 
ment of  silver  ores  has  been  introduced  ;  and  from  the  resulting 
liquors,  as  in  the  case  of  gold,2  the  silver  is  sometimes  precipitated 
electrolytically. 

1  P.  247.  2  P.  276. 


xvii.]  GOLD  271 

5.  Gold  Refining1 

The  advantages  of  electrochemical  methods  for  crude  bullion 
refining  have  been  already  discussed.  In  the  present  case,  using  a 
material  containing  90-95  per  cent.  Au,  they  comprise  recovery  of 
platinum  and  greater  ease  and  cleanliness  in  working.  The  actual 
cost  is  also  less,  due  to  the  very  low  acid  consumption.  This,  however, 
signifies  little,  owing  to  the  value  of  both  raw  material  and  product. 
Interest  charges  are  of  primary  importance.  The  process  employed 
is  the  Wohlwill  process,  in  which  the  crude  anodes  are  dissolved  in  an 
electrolyte  consisting  of  a  solution  of  HAuCl4  with  excess  of  free  HC1 
(KC1  or  NaCl  could  equally  be  used). 

Anodic  Behaviour  of  Gold. — The  behaviour  of  pure  gold  electrodes 
in  a  solution  containing  auric  chloride  is  rather  complex.  If  gold  be 
anodically  polarised  in  a  neutral  AuCl3  solution,  it  is  found  to  dissolve 


M        1-2        1-3        1-4       1-5        1-6       1-7        1-8        1-9 
Ep-  Volts. 

FIG.  66. 

nearly  quantitatively  at  an  anodic  potential  £}l  =  -J-  11  to  1-2  volts, 
according  to  the  equation  Au  +  3  0  — *  Au'".2  If  the  potential 
be  raised  to  1*4  volts,  the  anode  quickly  becomes  passive.  Finally,  at 
1'73  volts,  chlorine  is  evolved.  These  relations  are  clearly  shown  in 
the  accompanying  current  anode  potential  curve  (Fig.  66).  If  free 
HC1  be  added,  the  gold  begins  to  dissolve  at  the  same  potential,  but 
does  not  become  passive  so  easily,  the  potential  range  over  which  no 
electrode  action  takes  place  thus  becoming  smaller.  This  effect  is 
greater  he  greater  the  acid  concentration.  An  electrolyte  of  pure 
HC1  acts  similarly,  there  being  a  short  potential  range  over  which  no 
gold  dissolves  and  no  chlorine  is  evolved.  NaCl  acts  like  HC1— it 
is  the  Cr  which  counteracts  the  passivity,  as  it  does  in  many  similar 
cases.3  A  rise  of  temperature  has  a  similar  effect.  For  a  given  HC1 
content  and  for  a  given  temperature,  then,  there  will  exist  a  limiting 
anodic  potential,  and  consequently  a  limiting  current  density,  which 
must  not  be  exceeded  if  no  free  chlorine  is  to  be  evolved.  Thus  at 

1  Metall.  Chem.  Engin.  9,  443  (1911)  ;    Electrochem.  Ind.  1,  157  (1903) ;  8,  355 
(1908). 

-  Coehn  and  Jacobsen,  Zeitsch.  Anorg.  Chem.  55,  321  (1907). 
:i  See  pp.  140,  290,  292,  388. 


272    PKINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

70°,  with  3  per  cent,  free  HC1  present,  up  to  0*3  amp.  /cm.2  can  be 
used. 

The  gold  does  not  dissolve  quite  quantitatively  as  Au'"  ions,  but 
there  tends  to  be  set  up  at  the  anode  the  equilibrium  Au'"  -f-  2  Au  ~^ 
3  Au*.1  The  equilibrium  constant  of  this  reaction  is  unknown.  For 
our  present  purpose  this  makes  no  difference,  as  almost  all  the  gold, 
whatever  its  valence,  is  present  as  complex  anions  AuCl2'  and  AuCl/, 
and  the  equilibrium  constants  of  the  reactions  Au'  -f  2C1'  ~<~*  AuCl2' 
and  Au'"  -f-  ^Cl'y^AuCl/  would  also  need  to  be  known  before 
we  could  calculate  in  what  proportions  of  aurous  and  auric  salts  a  gold 
anode  would  dissolve  in  an  electrolyte  with  a  given  Cl'  content.  At  all 
events,  a  certain  amount  of  gold  dissolves  in  the  aurous  condition,  and 
the  loss  in  weight  of  the  anode  will  exceed  that  calculated  from  Fara- 
day's Law  on  the  assumption  that  the  reaction  Au  +  30 >  Au"* 

is  the  only  one  taking  place.  Further,  the  concentrated  solution 
leaving  the  anode  will  become  supersaturated  with  respect  to  Au'  ions 
when  it  has  mingled  with  the  more  dilute  main  electrolyte.  The 
reaction  3Au'  — >  Au'"  -f-  2Au  will  take  place,  and  metallic  gold 
will  be  precipitated. 

In  this  way  is  explained  the  production  of  finely-divided  gold 
crystals  observed  in  the  neighbourhood  of  a  gold  anode.2  The  higher 
the  current  density,  the  smaller  are  these  effects,  both  the  excess  loss 
in  weight  of  the  anode,  and  the  precipitation  of  finely-divided  gold. 
This  is  probably  caused  by  the  gold  becoming  somewhat  passive 
with  respect  to  the  production  of  Au*  ions  at  the  higher  current 
densities.  Table  XXXVII  gives  some  results  of  Wohlwill.  One 
ampere-hour  should  dissolve  2'45  grams  Au  if  the  sole  reaction  is 
Au  +  3©  — >  Au'". 

TABLE  XXXVII 

Current  density  Weight  of  gold  dissolved  per  ampere-hour 
0-049  amp.  /cm.-  3  -35  grams 

0-059  3-1 7-3-2(5 

0-074  2-77-2-84 

0-098  2-64 

0-15  2-53 

Cathodic  Behaviour  of  Gold  Solutions.— The  cathodic  phenomena 
during  the  electrolysis  of  HAuCl4  solutions  are  no  less  complex.3  At 
room  temperature,  using  a.  fresh  solution  containing  no  aurous  gold,  the 
first  electrode  reaction  begins  at  +  1*1  volt.  No  gold  is  deposited, 
but  we  have  the  reaction  Au'"  — >  Au'  +20  (i).  With  increasing 

1  Compare  p.  245. 

2  Of.  p.  248.     A  moderate  degree  of  super-saturation  of  Au*  ions  will  persist 
for  a  l«mir  tim«-  unless  the  solution  be  brought  into  contact  \vitli  indallic  gold. 

<  <><  hn  and  Jacobsen,  loc.  cit. 


xvii.]  GOLD  273 

cathodic  polarisation  there  is  another  discontinuity  at  +  O96  volt,  and 
gold  is  deposited  (ii).  Finally,  at  -j-  O90  volt  there  is  a  third  bend  (iii), 
although  the  deposition  of  gold  still  continues.  As  the  solution  is 
used  (i)  and  (ii)  approach  one  another,  until  finally,  when  the  equilibrium 
2Au  -f-  Au'"  ^H!l  3Au*  prevails,  there  are  only  two  decomposition 
points,  at  TO  and  O9  volt.  At  higher  temperatures  the  first  decom- 
position point  comes  earlier,  and  the  second  weakens  and  finally  dis- 
appears. Thus,  with  a  fresh  solution  at  85°,  there  were  only  two  bends 
at  1-2  and  0'9  volt. 

The  reason  for  the  smaller  cathodic  polarisation  required  by  the  pro- 
cess Au'"  — >  Au'  +  2  ©  in  the  hot  is  that  at  higher  temperatures  the 
equilibrium  Au'"  +  2  Au  ~  *  3  Au'  moves  over  in  favour  of 'the  Au' 
ions,1  and,  with  a  given  Au'  concentration,  the  driving  force  of  the 
reaction  is  thereby  increased.  The  causes  of  the  other  phenomena 
are  less  clear. 

At  ordinary  temperatures,  with  a  cathode  potential  between  TO  (ii) 
and  0'9  (iii)  volt,  gold  is  deposited  from  auric  chloride  solutions  with 
an  apparent  valency  of  2'6  to  2'9,  the  higher  the  potential  the  larger 
being  the  valency.  Whilst  with  a  cathode  potential  below  -f  0'9  volt 
(iii),  it  is  always  precipitated  quantitatively  according  to  the  equation 

Au'" >  Au  -j-  3  ©.    As  we  shall  shortly  see,  gold  refining  is  carried 

out  at  about  70°.  If  we  suppose  the  cathode  to  be  in  equilibrium  with 
the  electrolyte,  the  first  decomposition  point  will  have  disappeared 
and  the  second  will  hardly  be  noticeable. 

Hence  the  third  only  need  be  considered,  and  this  corresponds  to  a 
tri-valent  deposition  of  gold.  But,  under  working  conditions,  the  electro- 
lyte leaving  the  anode  is,  as  we  have  seen,  supersaturated  with  respect 
to  Au'  ions.  This  super  saturation  only  slowly  disappears  unless  the 
liquid  is  actually  in  contact  with  metallic  gold,  and  further,  the  Au'  ions 
are  not  oxidised  by  oxygen  to  Au'"  ions  as  is  the  case  with  copper.2 
When  the  electrolyte  arrives  at  the  cathode,  this  excess  is,  of  course, 
deposited,  and  hence,  in  practice,  the  increase  in  weight  of  the  cathode 
always  exceeds  the  amount  calculated  from  Faraday's  Law  on  the 
assumption  that  the  sole  reaction  is  Au'" >  Au  -j-  3  0.  According 

to  the  current  density  it  amounts   to  S'02-2'49  -  -   (theory 

amp.  hour 

2-45  grams). 

The  condition  of  the  deposited  metal  depends  on  the  gold  content 
of  the  solution,  the  temperature  and  the  current  density,  being  more 
compact  the  lower  the  last,  the  higher  the  first  two  factors.  Under 
unfavourable  conditions  a  loose  powder  results,  as  deposited  in  the  gold 
refinery  a  coarsely  crystalline  coherent  mass. 

The  gold  anodes  used  in  the  refinery  contain  generally  about  94  per 

1  Bose,  Zeitsch.  Elektrochem.  14.  85  (1908). 
-  P.  246. 


274    PKINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

cent.  Au,  5  per  cent.  Ag,1  and  1  per  cent.  (Cu,  Pb,  Pd,  Pt),  with  traces 
of  other  platinum  metals.  Of  the  impurities,  the  silver  gives  AgCl,  after- 
wards found  in  the  slimes.  Under  ordinary  conditions  the  formation  of 
this  insoluble  salt  limits  the  permissible  percentage  of  silver  in  the 
anodes.  If  6  per  cent,  is  exceeded,  the  layer  of  AgCl  becomes  so 
coherent  that  it  must  be  mechanically  brushed  away,  or  else  the  gold 
will  not  dissolve,  chlorine  being  instead  evolved.  To  avoid  this 
difficulty,  Wohlwill  has  devised  a  method  employing  an  unsymmetrical 
alternating  current,  which  we  will  presently  discuss.  Lead  dissolves  as 
PbCl2,  but  is  continually  precipitated  by  the  addition  of  H2S04.  Copper, 
platinum,*  and  palladium  also  dissolve,  and  accumulate  in  the  electro- 
lyte. The  platinum  content  must  not  exceed  50-60  — ,  that  of  the 

litre 

palladium  5  -      — ,  or  else  cathodic  deposition  will  occur.     The  copper 
litre 

can  be  allowed  to  accumulate  considerably. 

Because  of  the  impurities,  but  more  so  because  of  the  decreasing  gold 
content  of  the  bath,  a  fraction  of  the  electrolyte  must  be  regularly 
drawn  off,  and  replaced  by  a  strong  AuCl3  solution.  The  withdrawn 
portion  is  freed  from  gold  by  S02,  the  platinum  precipitated  as  Am2PtCl6, 
any  palladium  extracted  with  ammonia  after  evaporation  to  dry- 
ness,  and  copper  removed  by  scrap  iron.  The  slimes  contain  gold 
(about  10  per  cent,  of  that  cathodically  precipitated),  AgCl,  PbS04, 
and  the  rarer  platinum  metals.  After  removal  of  the  PbS04  by  treat- 
ment with  Na2C03  solution,  followed  by  HN03,  and  the  AgCl  by  fusing 
and  pouring  off,  the  residues  are  generally  recast  into  fresh  anodes. 

The  electrolysis  baths  are  constructed  of  glazed  porcelain  or 
chemical  earthenware,  and,  as  the  electrolyte  is  expensive,  are  small 
in  size.  A  number  of  anodes  and  cathodes  are  arranged  alternately, 
j"  apart,  and  connected  in  parallel,  and  several  baths  are  placed  in 
series.  The  anodes  are  fairly  thin  (e.g.  £"),  to  ensure  their  only  being 
a  short  time  in  the  bath.  They  are  either  provided  with  lugs,  and 
hung  over  the  edges  of  the  bath  as  in  copper  refining,  or  are  better 
suspended  from  platinum  hooks  and  completely  submerged.  The 
cathodes  are  of  very  thin  gold  foil.  All  leads  are  of  silver  or  gold,  as 
copper  would  be  attacked  by  the  traces  of  chlorine  which  are  con- 
tinually evolved.  In  order  to  consume  the  anodes  rapidly  a  high 
current  density  is  necessary.  Further,  the  electrolyte  must  not  contain 
much  of  the  expensive  gold  salt.  To  obtain  an  adherent  deposit 
under  these  conditions  a  high  temperature  is  essential.  In  practice  the 

electrolyte  contains  30-40  ^    -  Au  and  2-3  per  cent,  free  HC1.     A 

litre 

*  Can  however  be  greatly  exceeded. 

'  Platinum  is  almost  passive  in  strong  HC1  (see  p.  151),  but  when  alloyed 
with  gold,  even  up  to  10  per  cent,  and  more,  readily  dissolves. 


xvn.]  GOLD      ,  275 

trace  of  gelatine  may  be  added  to  improve  the  quality  of  the  cathodic 
deposit.  It  is  circulated  through  the  tanks  by  propellers  or  by  gravity, 
the  latter  system  certainly  appearing  preferable.  The  temperature 
varies  in  different  refineries  between  50°-70°.  The  higher  value 
permits  of  more  rapid  work  and  is  therefore  better.  The  tanks 
conveniently  rest  on  a  sandbath,  heated  by  steam  pipes. 

The  current  densities  used  with  anodes  of  normal  silver  content  are 
10-15  amps./dm.2.  Traces  only  of  chlorine  are  evolved,  and  the 
anodes  are  consumed  in  15-20  hours.  This  rapid  rate  of  work  nullifies 
the  last  advantage  of  chemical  methods  of  refining.  The  electro- 
chemical process  formerly  required  some  three  days  for  the  consumption 
of  an  anode.  Higher  anodic  current  densities,  up  to  30  amps./dm.2, 
can  only  be  used  with  anodes  very  poor  in  silver  and  lead.  If  the  silver 
content  is  high  the  current  density  may  drop  to  5-7  amps./dm.2.  The 
voltage  required  is  O'6-l'O  volt,  depending  on  temperature,  composition 
of  electrolyte,  current  density,  etc.  We  have  seen  that  the  yield  at  the 
cathode  is  a  little  higher  than  that  calculated  for  Au'"  —  >  Au  +  3  ©. 
Putting  it  at  103  per  cent.,  and  assuming  a  drop  of  0'8  volt  per  tank, 
we  have  that  one  kilo,  of  gold  requires 

1000       100      96540  X  3        0-8   v  r 

X  103*    -3600-    *  ™L  =  0-32  1LW.H. 


The  power  cost  in  gold  refining  is  negligible. 

Modified  Wohlwill  Process.—  We  have  mentioned  the  modified 
Wohlwill  process1  for  use  with  anodes  of  high  silver  content.  Under 
ordinary  circumstances  an  anode  with,  for  example,  10  per  cent.  Ag  can 
only  carry  a  current  density  of  7'5  amps./dm.2,  and  even  then  must 
be  regularly  scraped  every  f  hour  to  remove  the  AgCl  layer.  Not  only 
is  this  troublesome  in  practice,  but  the  low  rate  of  anode  consumption 
means  heavy  interest  charges.  To  overcome  this  difficulty  Wohlwill 
employs  an  unsymmetrical  alternating  current,  consisting  of  a  direct 
current  on  which  is  superposed  an  alternating  current  of  rather  greater 
(r.m.s.)  value.  The  quantities  of  gold  dissolved  and  deposited  corre- 
spond almost  exactly  to  those  given  by  the  direct  current  alone.  But 
far  higher  current  densities  can  be  employed  than  are  possible  in  the 
absence  of  the  alternating  current  component,  and  hardly  a  trace  of 
chlorine  is  evolved,  the  gold  dissolving  quite  smoothly. 

Thus,  for  example,  the  above-mentioned  alloy  can  be  refined  at  a 
current  density  of  12'5  amps./dm.2,  scraping  being  unnecessary  if  an 
alternating  current  be  employed  I'l  times  as  great  as  the  direct  current 
flowing  ;  whilst  if  this  ratio  be  increased  to  T7,  a  bullion  containing 
20  per  cent.  Ag  can  be  refined  at  a  current  density  of  12  amps./dm.2. 
With  alloys  containing  normal  proportions  of  silver,  far  higher  current 

1  Zeitech.  Ekktrochem.  16,  25  (1010)  ;  D.R.P.  207,665  (1908). 

T  2 


276    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

densities  can  be  employed.  Apart  from  the  great  saving  of  interest  on 
the  gold  stock,  the  amount  of  gold  entering  the  slimes,  and  therefore 
requiring  remelting  and  retreatment,  hardly  exceeds  1  per  cent,  of  the 
amount  deposited,  instead  of  10  per  cent,  as  is  otherwise  usual.  The 
slimes  practically  consist  of  AgCl.  As  we  have  seen,  this  non-pro- 
duction of  gold  slimes  is  a  direct  consequence  of  the  higher  current 
densities  used. 

The  way  in  which  this  alternating  current  component  acts  is  not 
perfectly  clear,  but  is  probably  as  follows.  We  know  that  above  a 
certain  current  density  or  anode  potential  gold  becomes  passive.  The 
effect  of  the  formation  of  the  AgCl  layer  is  to  lessen  the  active  area  of 
the  anode,  and  hence  the  current  it  can  take,  without  chlorine  evolution 
setting  in.1  According  to  Foerster,2  certain  metals,  such  as  iron,  etc., 
are  passive  when  pure,  and  only  become  active  when  charged  with  hydro- 
gen. It  is  possible  that  the  case  is  similar  here,  that  during  the  short 
cathodic  component  of  the  alternating  current  the  metal  is  somewhat 
charged  with  hydrogen,  and  thus  enabled  to  dissolve  during  the  succeed- 
ing anodic  component  far  more  readily  than  under  normal  conditions. 
Only  a  very  small  quantity  of  hydrogen  would  probably  be  thus 
required,  as  we  know  the  solubility  of  the  gas  in  gold  to  be  very  low. 

We  may  notice. in  conclusion  that  the  saving  in  interest,  etc.,  effected 
by  this  device  far  outbalances  the  extra  expense  of  the  alternating 
current. 

6.  Gold  Extraction 

Very  large  quantities  of  gold  are  extracted  from  the  ore  by  the  use 
of  KCN  solutions.  Two  processes  are  used,  the  MacArthur-Forest 
and  the  Siemens  and  Halske,  and  in  the  latter  the  gold  is  precipitated 
electrochemically  from  solution.  The  introduction  of  cyanide  extrac- 
tion into  gold  metallurgy  has  effected  a  revolution  in  the  industry. 

Very  low-grade  ores,  down  to  about  8  ~         gold,  can  be  economically 

ton 

extracted,  and  the  process  has  found  its  largest  application  in  the 
recovery  of  gold  from  the  tailings  and  slimes  of  the  amalgamation 
process.  Further,  higher  grade  ores  containing  the  gold  too  intimately 
embedded  in  the  pyrites  to  allow  of  extraction  by  the  amalgamation 
process,  and  which  previously  were  always  chlorinated,  can  also  be 
successfully  treated. 

KCN   solutions    can    only    dissolve   gold  in    presence    of    air    or 

oxygen.     The  final  result  can  be  expressed  thus :  2Au  +  4KCN  -f  H20 

_j_  JQ2  — >  2KAu(CN)2  +  2KOH,   or  2Au  +  4CN'  +  H20  +  J02 

— »•  2Au(CN)2'  +  20H'.     The  electrochemical  nature  of  the  process  is 

clearly  demonstrated  by  setting  up  an  element  of  which  one  electrode 

1  Of.  pp.  141,  224.  -  See  p.  142. 


XVIL]  GOLD  277 

is  gold  in  air-free  KCN  solution,  the  other  platinum  in  a  KCN  solution 
through  which  oxygen  is  bubbling.1     Such  a  combination  has  an  E.M.F 
of  0-12-0-29  volt,  the  KCN  concentration  varying  from  O'l  per  cent 
to  1  per  cent.    On  short  circuiting,  the  gold  dissolves,  giving  Au  (CN)  { 
ions,  and  at  the  oxygen  electrode  OH'  ions  result.     Bodlander 2  investi- 
gated the  reaction,  and  showed  that  H202  was  sometimes  formed  as  an 
intermediate  product,  in  which  case  the  first  reaction  would  be  2Au  -{- 
02  +  H20  — >  Au20  +  H202,  the   Au20   subsequently   dissolving   in 
the  KCN,  and  the  H202  oxidising  more  gold. 

In  practice,  the  crushed  and  stamped  ore,  after  having  as  much 
gold  as  possible  removed  by  amalgamation,  is  separated  into  three 
fractions  by  levigation,  and  then  treated  with  the  solution  of  KCN,  or 
a  mixture  of  KCN  with  the  cheaper  NaCN.  The  heaviest  fraction 

('  concentrates ')  may  contain  about  25  -       L  gold,  the  second  and 

ton 

third  fractions  ('  tailings  '  and  '  slimes  ')  10-15 .     In  the  Mac- 
ton 

Arthur-Forest  process  the  KCN  content  of  the  solution  used  to  be  0'3-0'4 
per  cent.,  it  is  now  O'l  per  cent,  or  less.  The  velocity  of  solution  of  the 
gold  depending  on  the  amount  of  dissolved  oxygen  in  the  liquors,  air  is 
continually  blown  in  during  the  lixiviation.  When  the  extraction 
is  finished,  the  gold  content  is  precipitated  by  passing  the  solution  over 
zinc  filings  or  a  zinc-lead  couple  containing  5  per  cent.  lead. 

Siemens  and  Halske  Process. — In  this  process  a  0-1-0-2  per  cent, 
solution  is  used  for  the  concentrates,  the  lixiviation  taking  some  weeks. 
The  tailings  and  slimes  only  need  a  0'01-0'05  per  cent,  solution,  and 
the  extraction  is  completed  far  more  quickly — in  a  few  days  or  a  few 
hours,  depending  on  the  fineness  of  division  of  the  gold  present.  The 
lixiviation  is  carried  out  in  huge  wooden  or  iron  tanks  through  which 
air  is  blown.  Kapidity  of  work  is  important.  Otherwise  Cu2S  dis- 
solves, and  iron  pyrites  is  slowly  attacked,  producing  ferric  sulphate,  and 
finally  causing  a  loss  of  cyanide  as  insoluble  Prussian  blue.  The  longer, 
too,  the  solution  is  exposed  to  air  the  greater  will  be  the  loss  of  HCN, 

both  by  hydrolysis  (CN'  +  H20  >  HCN  -f  OH')  with   subsequent 

loss  as  gas,  and  by  action  of  atmospheric  C02  and  oxygen.  Oxidising 
agents  tend  to  hasten  the  solution  of  the  gold,  but  none  has  yet 
found  application  with  the  exception  of  BrCN  and  C1CN,  occasionally 
used  when  the  gold  is  present  as  telluride,  which  is  only  very  slowly 
attacked  by  the  dilute  KCN  solution. 

The  liquors,  containing  from  3-10  -        -  gold,  are  filtered  through 

metre3 

sand  or  carefully  decanted,  and  led  to  the  electrolysis  tanks.     These 

1  Electrochem.  Ind.  7,  156  (1909). 

-  Zeitsch.  Angew.  Chem.  9,  583  (1896). 


278    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

consist  of  long  wooden  vats  (perhaps  30'  X  6'  X  5'),  divided  by  means 
of  double-walled  divisions  into  ten  or  twelve  compartments.  The 
electrolyte  enters  the  end  compartment  at  the  top,  flows  through  and 
out  at  the  bottom,  up  through  the  double-walled  partition,  and  enters 
the  second  division,  again  at  the  top.  In  this  way  it  slowly  traverses 
the  whole  vat,  some  50-80  cubic  metres  passing  through  in  twenty-four 
hours.  Each  compartment  contains  electrodes. 

The  anodes  are  of  sheet  iron,  4-5  mm.  thick.  They  are  slowly 
attacked,  giving  Prussian  blue,  for  which  reason  they  are  enclosed  in 
canvas  bags.  The  addition  of  some  alkali  to  the  electrolyte,  beyond 
that  produced  by  hydrolysis,  considerably  checks  their  corrosion  and 
the  accompanying  loss  of  cyanide.  They  last  indeed  for  some  years. 
Other  materials  have  been  proposed,  notably  Pb02  by  Andreoli,  but  they 
do  not  withstand  the  conditions  so  well.1 

As  cathodes  are  used  spirally  wound  lead  strips,  suspended  from 
crossbars  of  zinced  iron.  The  gold  adheres  satisfactorily  if  the  electro- 
lyte is  kept  free  from  suspended  matter  and  if  the  current  density  is  not 
too  high.  They  are  removed  every  one  or  two  months,  when  they  may 
contain  10  per  cent,  gold,  and  cupelled.  The  resulting  crude  metal 
will  contain  85-95  per  cent,  gold,  the  remainder  being  chiefly  silver 
and  lead.  Other  cathodes  have  been  used  or  suggested,  for  example, 
tin-foil.  A  high  current  density  is  employed,  and  the  spongy  deposit 
is  squeezed  off  every  day  or  two  or  falls  to  the  bottom  of  the  tank. 
These  electrodes  are  thus  permanent.  Neumann  has  suggested  carbon 
cathodes.  A  good  deposit  can  be  produced,  and  they  could  subse- 
quently be  made  anodes  in  a  Wohlwill  refining  bath,  and,  when  the 
metal  had  been  dissolved,  washed  and  returned  to  the  cyanide 
tanks. 

Like  electrodes  in  the  whole  bath  are  connected  in  parallel  and  the 
different  baths  in  series.  The  current  density  at  both  anodes  and 

cathodes  is   about   0*4:  A  higher   one  gives  a  very  loose 

metre2. 

deposit  and  attacks  the  anodes  too  powerfully.  The  voltage  used 
is  about  1*75-3*0  volts  for  the  more  concentrated  KCN  solutions  ; 
for  solutions  with  only  0*01  per  cent.  KCN  it  may  rise  to  4-5  volts. 
The  current  efficiency  is  exceedingly  low,  averaging  less  than  1  per 
cent.  Hydrogen  is  evolved  in  large  quantities.  About  80-90  per 
cent,  of  the  gold  is  deposited.  To  precipitate  the  last  traces  would 
be  uneconomic.  The  liquors  are  either  passed  over  a  zinc-lead  couple, 
or,  after  replenishing  the  cyanide  content,  are  returned  to  the  lixiviating 
tanks. 

Neumann 2  has  made  a  laboratory  study  of  the  influence  of  varia- 

1  Neumann,  Elecirochem.  Ind.  4,297  (/WG). 
-  Loc.  cit. 


XVII.] 


GOLD 


279 


tions  of  current  density  and  concentration  on  the  yield  and  the  voltage, 
and  the  accompanying  Table  XXXVIII  contains  some  of  his 
results. 

TABLE  XXXVIII 


Concentration 
Grams  Au 
per  metre3 

10 
10 
10 
10 
10 

3 

3 


Current 

n 

density 

Per  cent. 

in  amperes 

of  KCN 

per  metre2 

0-05 

0-25 

0-5 

0-5 

0-07 

2-4 

0-07 

4 

0-07 

9 

0-05 

0-25 

0-5 

0-5 

Current 

efficiency 

Voltage 

7*5  per  cent. 

1-1-5  volts 

3-5-4 

1-8 

0-1-0-4 

2-1 

0-08-0-25 

2-5 

0-04-0-16 

3-0 

3 

1-4-1-7 

1-3 

2 

The  current  efficiencies  are  seen  to  be  very  low,  and  to  fall  off  with 
decreasing  gold  content  or  with  increasing  current  density.  Power 
charges  are,  however,  of  small  account  with  such  a  valuable  product, 
and  the  Siemens-Halske  process  can  be  favourably  compared  with  the 
MacArthur-Forest  process  on  four  points  :  it  does  not  use  huge  quan- 
tities of  zinc  (the  zinc  precipitation  always  demands  many  times  the 
theoretical  quantity) ;  it  uses  far  less  KCN — about  one-third ;  the 
precipitated  material  is  more  easily  worked  up  ;  and  a  purer  product 
results.  Nevertheless  the  process  has  recently  become  of  much  less 
importance,  and  there  are  reasons  to  believe  it  may  soon  disappear. 
It  requires  a  larger  and  more  unwieldy  plant,  and  rapid  work  is  im- 
possible. Many  modifications  of  the  Siemens-Halske  process  have  been 
proposed  :  none  have  had  any  success. 

Clancy  Process. — In  conclusion  we  may  briefly  mention  an  electro- 
lytic process  described  by  Clancy 1  for  the  extraction  of  gold  from  very 
refractory  ores,  such  as  tellurides.  The  crushed  ore  is  contained  in  an 
iron  tank  which  acts  as  cathode.  The  anodes,  which  are  of  magnetite,2 
are  cheap,  can  withstand  comparatively  high  current  densities,  and 
have  a  long  life.  The  dilute  electrolyte,  which  is  kept  slightly  alkaline, 
contains  originally  a  mixture  of  KCN,  KCNS,  and  KI,  together  with 
CaCN2  (calcium  cyanamide3).  On  electrolysis,  anodic  reactions  ap- 
parently occur  which  result  in  the  production  of  substances  capable 
of  attacking  the  gold  tellurides,  such  as  ICN,  I2CN2  (Clancy).  The 
liberated  gold  dissolves  in  the  cyanide  present.  To  regenerate  the 
electrolyte,  the  addition  of  fresh  cyanide  is  unnecessary.  The  cheap 
cyanamide  can  be  added  instead,  as  on  electrolysis  it  is  converted 

!  Trans.  Amer.  Electrochem.  Soc.  19,  137  (1911}. 
-  See  p.  363. 
3  See  p.  480. 


280       PKINCIPLES  OF  APPLIED  ELECTKOCHEMISTRY 

anodically  into  Ca(CN)2.  If  this  process  gives  as  good  results  techni- 
cally as  it  seems  to  have  done  on  an  experimental  scale,  it  will 
undoubtedly  find  wide  application. 


Literature 

Ulke.     Die  elektrolytische  Raffi  nation  des  Kupfers. 
v.  Uslar.     Cyanid-Prozesse  zur  Goldgewinnung . 


CHAPTER  XVIII 

ZINO-TIN— NICKEL— IROX— LEAD— VARIOUS 

IN  this  chapter  we  shall  discuss  the  electrometallurgy  of  metals  much 
less  noble  than  those  so  far  dealt  with.  The  difficulties  inherent  in 
their  treatment  are  consequently  much  greater,  and  most  of  the  pro- 
cesses considered  find  small  application  and  are  run  at  a  narrow  margin 
of  profit.  When  real  economic  success  has  been  reached,  it  has  only 
been  after  overcoming  great  obstacles. 

1.  Electrometallurgy  of  Zinc 

Owing  to  the  great  losses  of  metal  (up  to  25  per  cent.)  in  zinc 
metallurgy,  and  the  very  considerable  wear  and  tear  of  the  retorts 
employed,  chemists  have  devoted  much  attention  to  the  extraction  of 
zinc  by  roasting  the  ore,  lixiviating,  and  electrolysing  the  liquors 
produced.  For  several  reasons,  such  processes  have  only  achieved 
slight  success.  Such  reasons  are  (a)  the  high  power  consumption 
necessary  to  deposit  from  aqueous  solution  a  strongly  electropositive 
metal  like  zinc,  using  an  insoluble  anode  ;  (6)  difficulties  encountered 
in  the  cathodic  deposition  ;  (c)  difficulties  in  roasting,  lixiviating  and 
purifying  the  liquors  so  as  to  produce  an  electrolyte  of  constant  and 
definite  composition * ;  (d)  insufficient  demand  for  the  resulting  particu- 
larly pure  product.  At  present  electrolytic  zinc  seems  only  to  be  pro- 
duced at  a  plant  in  Russian  Poland  and  by  Brunner,  Mond,  and  Co., 
Ltd.  (at  Wilmington,  Cheshire). 

Cathodic  Phenomena. — In  the  cathodic  deposition  of  zinc,  two 
important  points  must  be  considered — firstly  the  conditions  under 
which  the  metal  or  hydrogen  respectively  result,  and  secondly  the 
nature  of  the  zinc  deposit.  Zinc  is  far  more  electropositive  than 
hydrogen  (Zn  |  n.  Zn"  =  —  0*76  volt).  One  would  therefore  expect 
the  metal  to  dissolve  freely  in  acid,  and  would  also  expect  to  find  its 
cathodic  deposition  impossible  with  any  appreciable  amount  of  free 
acid  in  the  electrolyte.  As  a  matter  of  fact,  pure  zinc  is  not  attacked 

1  Cf .  pp.  263,  265,  297. 
281 


282    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

by  acids,  and  further  zinc  can  be  electrolytically  deposited  from  acid 
solutions  with  excellent  yields  if  a  sufficiently  high  current  density  be 
used.  The  cause  of  these  contradictions  is  the  very  high  over  volt  age 
necessary  for  the  discharge  of  hydrogen  at  zinc,  which  is  about  0'7  volt, 
even  at  very  low  current  densities,  the  highest  value  for  any  common 
metal  except  mercury.  Consequently,  as  the  hydrogen  must  be 
evolved  at  the  zinc  surface,  the  solution  of  pure  zinc  in  dilute  acid 
does  not  proceed  spontaneously,  and  from  an  electrolyte  containing 
both  zinc  and  hydrogen  ions,  the  former  are  preferentially  discharged, 
unless  only  present  in  small  amount. 

If  however  the  zinc  contains  metallic  impurities  at  which  the  hydro- 
gen overvoltage  is  less  than  at  zinc  itself,  it  will  dissolve  rapidly  in  the 
acid.  Such  metals  are  iron,  silver,  nickel,  copper  or  arsenic,  a  very 
small  quantity  of  which  will  cause  the  zinc  to  dissolve.  Tin,  cadmium, 
and  lead  will  not  act  so  powerfully.  Similarly,  if  the  zinc  cathode  or 
the  electrolyte  is  contaminated  with  one  of  the  above  metals  (which 
are  all  more  electronegative  than  zinc  and  will  therefore  deposit  more 
easily)  then  hydrogen  will  result  instead  of  zinc.  In  any  case,  raising 
the  current  density  always  increases  the  hydrogen  overvoltage  and 
raising  the  temperature  decreases  it.  A  high  current  density  and  a 
low  temperature  therefore  favour  a  good  cathodic  current  efficiency, 
which  is  also  assisted  by  a  high  zinc  and  a  low  acid  concentration  in 
the  electrolyte. 

Nature  of  Cathodic  Deposit. — When  the  electrolysis  is  proceeding 
satisfactorily,  the  zinc  forms  a  compact  greyish  white  finely  crystalline 
mass,  showing  a  slight  tendency  to  give  irregular  growths  at  the  edges 
of  the  electrode.  But  very  often,  without  apparent  cause,  the  deposit 
becomes  spongy  and  dark-coloured,  and  when  this  has  happened,  unless 
by  suddenly  raising  the  current  density,  or  by  other  means,  this  zinc 
sponge  is  covered  over  with  a  layer  of  coherent  metal,  it  is  difficult  to 
prevent  zinc  subsequently  deposited  from  coming  down  in  the  same 
inconvenient  form.  Apart  from  being  very  voluminous,  causing  short 
circuits,  enclosing  electrolyte,  and  being  difficult  to  handle,  it  cannot 
be  melted  up  without  considerable  losses  as  oxide. 

Many  chemists  have  investigated  the  conditions  of  formation  of 
this  zinc  sponge,  particularly  Kiliani,1  Mylius  and  Fromm 2  (from  ZnS04 
solution),  and  Foerster  and  0.  Giinther3  (from  ZnCl2  solution).  Their 
work  has  made  clear  the  conditions  for  the  production  of  satisfactory 
deposits  of  zinc,  but  the  cause  of  the  sponge  formation  is  still  rather 
obscure.  The  view  so  far  generally  taken  is  that  the  sponge  formation 
is  due  to  the  presence  of  zinc  oxide  or  a  basic  zinc  salt  in  the  electrolyte, 
which  somehow  disturbs  the  crystallisation  of  the  metallic  zinc.  Any 

1  Berg-und  Hutten.  Zeit.  1883,  251. 

2  Zeitsch.  Anorg.  Chem.  9,  104  (  AW:,). 

:t  ZtiUch.  Elektrochem.  5,  20  (MM),  6,  301  (M.W). 


xvm.]  ZINC  283 

cause  tending  to  lower  the  H'  concentration  must  then  tend  to  favour 
sponge  formation.  For  example,  Foerster  points  out  that  if  there  are 
irregularities  on  the  surface  of  the  cathode,  the  current  density  may  fall 
locally  or  the  electrolyte  may  fail  to  be  renewed  and  become  depleted 
of  zinc  ions,  which  would  cause  a  further  fall  in  current  density.  Both 
these  causes,  a  deficiency  in  zinc  ions  and  a  lower  current  density,  tend 
to  increase  the  proportionate  number  of  hydrogen  ions  discharged,  and 
to  favour  the  production  of  basic  salts  and  the  formation  of  zinc  sponge. 

Again,  if  any  electronegative  metal  is  present  in  the  electrolyte,  it 
will  be  deposited,  assist  hydrogen  evolution,  and  cause  the  production  of 
zinc  sponge.  When  once  the  sponge  is  deposited  the  discharge  of 
hydrogen  ions  will  increase,  as  the  overvoltage  of  hydrogen  at  zinc 
sponge  is  much  less  than  at  the  massive  metal.  The  behaviour  of 
oxidising  cathodic  depolarisers  rather  confirms  this  general  view. 
Depolarisers  which  oxidise  hydrogen  to  H'  ions — i.e.  which  give  acids  on 
reduction — usually  hinder  the  sponge  formation.  Such  are  the  halogens, 
HC10  and  H2S208.  Oxidising  agents,  on  the  contrary,  which  give 
neutral  or  basic  substances  on  reduction  favour  the  sponge  production. 
Such  are  H202  and  nitrates.  There  is  further  one  fact  which,  at  first 
apparently  a  contradiction,  also  confirms  this  view.  When  zinc  is 
deposited  from  alkaline  zincate  solutions,  sponge  formation  rapidly 
sets  in,  though  the  excess  of  alkali  prevents  any  precipitation  of  oxide 
or  basic  salt.  Hantzsch1  however  has  shown  that  these  zincate 
solutions  are  more  or  less  strongly  hydrolysed,  and  contain  colloidal 
.zinc  hydroxide.  This  will  probably  affect  the  zinc  deposit. 

The  conditions  for  the  production  of  good  zinc  ^deposits  are  as 
follows  : — 

(a)  Moderately  strong  solutions  of  zinc  salt.    40-60  grams/litre  zinc 
is  a  suitable  concentration. 

(b)  Presence  of  acid,  its  amount  depending  on  the  efficiency  of 
circulation  of  the  electrolyte.     The  better  this  is,  the  lower  the  H' 
concentration    can    be.    Depending    on    circumstances,    the    liquors 
should  have  a  free  acid  concentration  of  O'01-O'l  N.      From    the 
point  of  view  of  a  good  cathodic  efficiency,  the  less  present  the  better. 

(c)  Complete  absence  in  the  electrolyte  of  impurities  less  electro- 
positive than  zinc.     Copper  and  arsenic  must  particularly  be  avoided. 
Mylius  and  Fromm  found  that,  in  a  10  per  cent.  ZnS04  solution,  0*004 
per  cent.  As  almost  immediately  caused  the  production  of  zinc  sponge 
and  evolution  of  hydrogen. 

(d)  Current  density  must  not  be  low.      Suitable  limits  are  1-2*5 
amps,  /dm.2 

(e)  Good  circulation. 

(/)  Low  temperature.     Raising  the  temperature  lowers  the  hydro- 
gen overvoltage  and  increases  the  tendency  to  form  basic  salts. 
1  Zeiisch.  Anorg.  Chem.  30,  298  (1902). 


284    PKINCIPLES  OF  APPLIED  ELECTKOCHEMISTRY    [CHAP. 

Zinc  Refining.— The  electrolytic  refining  of  zinc  can  be  readily 
carried  out,  but  generally  will  be  an  uneconomic  operation.  This 
may  be  different  if  the  raw  material  furnishes  slimes  sufficiently  rich 
in  noble  metals.  The  zinc-silver  alloy  obtained  in  the  Parkes  process 
for  desilverising  lead  is  such  a  raw  material,  and  electrolytic  refining 
processes  have  been  devised  by  Hasse  and  by  Roesing.  They  are  not 
now  worked,  the  older  separation  by  distillation  proving  simpler  to 
carry  out.  The  crude  metal  used  was  the  product  of  the  Rossler- 
Edelmann  modification  of  the  Parkes  process,  and  generally  contained 
1 1-12  per  cent.  Ag,  80  per  cent.  Zn,  6-7  per  cent.  Cu,  smaller  quantities 
of  Ni,  Pb,  and  Fe,  and  traces  of  As,  Sb,  and  Bi.  The  anodes  were 
suspended  opposite  to  electrolytic  zinc  cathodes  in  a  slightly  acid  normal 
jiS04  solution,  the  electrolyte  being  circulated  by  gravity.  At  20°, 
nth  a  current  density  of  0'8-0'9  amps. /dm.2,  1'25-1*45  volts  were 
used  per  tank.  The  zinc  produced  contained  O'Ol  per  cent.  Pb,  O'Ol 
per  cent.  Fe,  0'004  per  cent.  Ag.  At  higher  temperatures  it  was  of 
poorer  quality.  The  slimes  consisted  of  copper,  silver,  lead,  zinc 
and  ZnO.  After  removing  the  last  two  constituents  by  dilute  acid, 
a  little  copper  also  dissolving,  they  contained  about  50  per  cent.  Ag, 
10  per  cent.  Pb,  and  40  per  cent.  Cu,  and  were  worked  up  for  their 
separate  constituents. 

The  chief  difficulty  experienced  was  in  keeping  the  electrolyte 
sufficiently  free  from  small  amounts  of  copper  and  silver  dissolved 
anodically.  After  passing  through  a  certain  number  of  tanks,  it  was 
made  to  flow  in  a  thin  stream  over  a  series  of  terraces  on  some  of  which 
were  placed  zinc  shavings,  on  others  ZnO.  The  noble  metals  were  thus 
precipitated,  and,  by  the  combined  action  of  the  ZnO  and  the  air, 
Fe(HO)3  was  also  deposited.  The  liquors  were  finally  settled  or  filtered, 
treated  afresh  with  acid,  and  passed  again  through  the  cells.  Whether 
this  purification  was  too  tedious,  or  whether  it  was  insufficient,  is  not 
known.  An  arrangement  such  as  used  by  Dietzel  for  the  separation  of 
silver  and  copper  would  have  proved  more  effective.  The  economic 
margin  of  the  process  in  any  case  was  small,  and  troubles  of  that  kind 
would  easily  turn  the  scale. 

Zinc  Extraction.  Siemens-Halske  Process. — Electrochemical  pro- 
cesses for  the  extraction  of  zinc  from  its  ores  can  be  divided  into  two 
classes,  using  respectively  as  electrolyte  ZnS04  and  ZnCl2  solutions. 
Of  those  employing  sulphate  solutions,  we  may  mention  the  many 
Siemens  and  Halske  patents.  For  the  lixiviation  of  the  cajcined  ores 
they  have  recommended  at  different  times  acid  Fe2(S04)3  liquors,  an 
alum  solution,  and  dilute  H2S04.  Using  the  first  solution,  ZnS  and 
ZnO  were  dissolved  exactly  as  the  corresponding  copper  compounds 
in  the  Siemens  and  Halske  copper  process/  The  electrolysis  was  also 

1  See  p.  262. 


xviii.]  ZINC  285 

similar — at  the  cathode  deposition  of  zinc,  at  the  anode  oxidation  of 
Fe"  to  Fe'"  ions,  thus  regenerating  the  lixiviating  liquors.  But  in 
addition  to  the  difficulties  experienced  in  the  corresponding  copper 
process,  much  iron  was  precipitated  with  the  zinc,  and  the  deposit 
was  spongy.  Using  alum  liquors,  soluble  basic  aluminium  salts  were 
produced  ;  and,  on  electrolysing,  no  acid  was  set  free,  but  the  neutral 
aluminium  sulphate  regenerated.  The  zinc  deposit  under  these 
conditions  was  probably  unsatisfactory. 

The  best  results  seem  to  have  been  obtained  when  lixiviating  with 
dilute  HoSOi,1  fairly  successful  large-scale  trials  having  been  carried  out 
in  Silesia.  Careful  roasting  is  necessary,  and  the  liquors  must  of  cours/ 
be  freed  from  impurities,  when  good  deposits  of  zinc  (99'98-99'99  per 
cent,  pure)  result.  The  presence  of  alkaline  earths  in  the  ore  is  r 
disadvantage,  causing  a  loss  of  H2S04.  The  question  of  a  suitabL 
anode  material  at  first  gave  much  trouble.  Platinum,  apart  from  its' 
high  first  cost,  dissolves  too  quickly  in  the  technical  liquors,  which  are 
never  free  from  organic  matter.  Lead  fouled  the  electrolyte,  and  Fe304 
proved  useless.  But  Ferchland's  anodes  2  of  electrolytically  deposited 
Pb02  have  proved  to  be  very  stable  chemically.  Such  pure  zinc  was 
obtained  that  it  was  unattacked  by  10  per  cent.  H2S04.  Medhanically, 
however,  they  disintegrate  after  a  certain  time  at  the  high  anodic  current 
densities  recommended.3  The  Mn02  electrodes  prepared  according  to 
Siemens  and  Halske's  patent  do  not  suffer  from  this  drawback,  being 
unaffected  after  a  year's  use.  As  it  was,  using  the  Pb02  anodes,  pure  zinc 
could  be  produced  at  an  average  energy  expenditure  of  3,900  K.W.H. 
per  ton,  this  amount  falling  to  3,300-3,500  K.W.H.  per  ton  when  the 
anodes,  which  were  not  uniform,  were  of  good  quality.  The  unre- 
covered  zinc  did  not  exceed  one-third  of  the  amount  usually  lost  in 
the  distillation  process.  The  chief  difficulties  as  ever  were  encountered 
in  the  roasting  and  lixiviation. 

Laszczynski  Process. — A  similar  process,  due  to  Laszczynski,4  is 
worked  with  success  in  Russian  Poland.  The  blende  is  carefully  roasted 
and  systematically  extracted  with  the  spent  acid  liquors  from  the 
electrolysis.  Dissolved  ferrous  iron  is  oxidised  by  air  or  by  KMn04  and 
precipitated  by  ZnO.  Copper,  arsenic,  and  cadmium  are  removed 
by  H2S.  silver  does  not  dissolve.  The  electrolysis  takes  place  in  lead- 
lined  wooden  tanks,  each  fed  with  1,500  amperes  at  about  4  volts. 
The  current  density  is  about  1  amp./  dm.2.  The  electrolyte  is  agitated 
by  stirrers.  The  process  of  purification  has  not  removed  manganese 
from  the  liquors.  This  is  easily  oxidised  anodically  to  permanganate, 
and  is  then  capable  of  attacking  the  cathodic  zinc  deposit.  This 

1  Engelhardt  and  Huth,  Metall.  7,  1  (1910).  2  See  p.  154. 

3  30-75  amps./ dm.'2.    The  idea  is  to  produce  some  SgOs"  ions,  which  act  against 
the  zinc  sponge  formation.     At  the  cathode,  l'5-3  amps./ dm.*  were  used. 

4  Zeitsch.  Elektrochem.  15,  456  (1909).     Compare  also  p.  261. 


286    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

oxidation  is  practically  entirely  prevented_byj3losely  surrounding  the 
anodes  with  a  diaphragm  of  some  suitable  material.1  The  fresh  liquors 
contain  about  10  per  cent,  zinc  ;  the  electrolysis  is  stopped  when  this 
has  fallen  to  4  per  cent.  The  electrolyte  then  contains  about  9  per  cent, 
free  acid.  A  94  per  cent,  current  efficiency  is  said  to  result,  and  the 
zinc  produced  is  99*97  per  cent,  pure  and  unattacked  by  12  per  cent. 
H2S04. 

Hoepfner  Process. — There  has  also  been  no  lack  of  proposed  processes 
for  zinc  extraction  involving  the  electrolysis  of  aqueous  ZnCl2  solutions. 
The  production  of  these  liquors  from  the  ore  is  less  simple  :  on  the  other 
hand  their  decomposition  voltage  is  lower  than  that  of  ZnS04  solutions. 
Of  these  processes,  only  that  due  to  Hoepfner  need  be  considered. 
A  plant  was  for  some  time  in  operation  at  Fiirfurt,  and  for  years  it 
has  been  worked  in  a  modified  form  by  Brunner,  Mond,  and  Co.  at 
Winnington.  Unfortunately  very  few  details  of  the  working  of  this 
latter  plant  are  available. 

The  calcined  zinc  ores  were  formerly  stated  to  be  extracted  in 
presence  of  C02  gas  with  the  CaCl2  waste  liquors  obtained  in  the 
ammonia-soda  process.  ZnCl2  goes  into  solution  as  follows  : — 

ZnO  +  CaCl2  +  C02  — >  CaC03  +  ZnCL. 

After  removal  of  iron,  manganese,  and  traces  of  electronegative  metals, 
the  solution  is  electrolysed  between  graphite  anodes  and  cathodes 
consisting  of  large  iron  discs,  revolving  on  horizontal  axes,  and 
only  partially  immersed  in  the  bath.  This  renews  the  electrolyte 
in  the  cathode  layer,  and  makes  the  zinc  deposit  denser  and 
more  even.  The  graphite  anodes  are  suitably  hooded  for  leading 
away  the  chlorine.  Judging  from  the  great  difficulty  intro- 
duced in  the  present  case  by  diaphragms  (see  below),  it  is 
possible  that  they  are  here  dispensed  with,  and  that  the  liquors 
are  merely  suitably  circulated.  The  cathodic  current  efficiency  is 
80  per  cent.,  the  losses  being  chiefly  due  to  chlorine  dissolved  in  the 
bath.  This  may  either  chemically  attack  the  zinc,  or  depolarise  the 
H"  discharge,  preventing  however  any  escape  of  gaseous  hydrogen 
(HC1  being  regenerated)  and  thus  favouring  a  good  zinc  deposit.  The 
anodic  current  efficiency  is  85  per  cent.,  and  the  gas  60-70  per  cent, 
pure.  There  are  considerable  losses  due  to  leakage  in  the  many 
collecting  hoods  used.  The  chlorine  is  utilised  for  bleach. 

The  following  description  applies  to  the  Furfurt  plant.2  The  raw 
material  contained  10-12  per  cent,  zinc,  chiefly  as  sulphide,  and  was 
roasted  with  20  per  cent.  NaCl  for  20-22  hours  at  600°-650°.  The 
charge  was  lixiviated  hot,  and  yielded  a  solution  containing  10-11  per 
cent,  zinc,  with  Na8S04,  NaCl,  and  various  impurities.  The  Na2S04 
was  removed  by  crystallisation  at  —  5°  ;  iron,  manganese,  and  nickel 

1  Cf.  p.  262.  '  Zcitsch.  Ekktrochem.  10,  G88  (mi}. 


xvm.]  TIN 


287 


hydroxides  were  precipitated  by  bleach  solution  and  powdered  lime- 
stone. From  the  filtered  liquors,  traces  of  lead,  copper,  thallium, 
arsenic,  etc.,  were  removed  by  zinc  dust,  and,  after  a  second  filtration, 
a  solution  was  obtained  with  9*5  per  cent,  zinc,  22  per  cent.  NaCl,  a 
little  gypsum,  and  traces  of  lead  and  thallium. 

This  was  acidified  with  HC1  and  electrolysed  in  wooden  tanks, 
V-shaped  in  end  vertical  cross-section.  Each  vat  contained  several 
large  revolving  disc-shaped  cathodes,  only  about  one-third  immersed  in 
the  electrolyte.  Between  each  pair  of  cathodes  was  an  anode  chamber 
containing  a  number  of  hard  carbon  anodes  (graphite  was  not  then 
available),  separated  from  the  cathode  compartments  by  diaphragms, 
and  provided  with  suitable  chlorine  exits.  The  diaphragms  caused 
much  difficulty.  Nitrated  muslin,  also  muslin  with  the  fibres  carefully 
coated  with  hydrated  silica,  were  used,  but  a  really  resistant  material 
was  never  obtained.  The  anodes  behaved  satisfactorily.  The  electro- 
lyte circulated  through  the  tanks  by  gravity.  The  solution  leaving  the 
last  tank  still  contained  about  2  per  cent,  zinc,  below  which  concen- 
tration it  was  uneconomical  to  reduce  it.  During  the  electrolysis  a 
continuous  addition  of  free  HC1  was  necessary,  the  free-acid  content 
being  kept  at  O'08-Ol  per  cent,  (about  0'03  n.). 

With  a  current  density  of  1  amp. /dm.2  a  tank  took  1,000  amperes 
at  3'3-3'8  volts.  The  current  efficiency  was  about  95  per  cent. 
Assuming  3'6  volts,  one  ton  of  zinc  would  require  about  3,100  K.W.H. 
The  cathode  metal  was  99'97  per  cent,  pure,  containing  0'01-0'02 
per  cent.  Pb  and  a  trace  of  iron.  The  process  was  given  up  because  of 
the  cost  and  trouble  of  calcining  and  lixiviating  ores  poor  in  zinc. 
Such  difficulties  are  almost  invariably  the  most  serious  ones  encountered 
when  working  out  electrolytic  processes  for  the  extraction  of  metals 
from  their  ores.  Of  next  importance  is  generally  some  question  of 
suitable  material  for  diaphragm  or  anode — in  this  case  diaphragm. 


2.  Electrometallurgy  of  Tin 

Tin  can  be  extracted  from  its  ores  very  simply  and  in  a  sufficiently 
pure  condition  by  ordinary  metallurgical  methods.  There  is  no  scope 
for  electrolytic  processes  in  that  field.  The  same  statement  holds  good 
of  tin  refining.  Crude  tin  can  be  readily  and  economically  refined 
electrolytically  in  a  Na2S  solution,  but  there  is  no  particular  demand 
for  the  pure  product  resulting,  and  the  impurities  are  not  sufficiently 
valuable  for  their  recovery  alone  to  warrant  the  process  being  worked. 
Thus  in  one  case  the  anode  slimes  had  Sn  12-15  per  cent.  ;  Pb  20- 
30  per  cent.  ;  Sb  20-30  per  cent.  ;  Fe  1-2  per  cent.  ;  Cu  1  per  cent.  ; 
Bi  0'5  per  cent.  ;  Ag  0'5-1'5  per  cent.  ;  Au  O'OOl  per  cent.  On  the 
other  hand,  the  recovery  of  tin  from  tin-plate  scrap  and  refuse  can  be 


PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

readily  and  economically  carried  out  by  electrolytic  methods.  Such 
methods  have  been  largely  used  during  recent  years,  though  probably 
detinning  by  chlorine  gas  will  become  more  and  more  prominent  in 
the  future. 

The  raw  material  is  either  tin-plate  scrap,  produced  during  the  manu- 
facture of  tin  cans  and  tinned  iron  ware,  or  discarded  tin  cans  from  the 
refuse-heaps  of  our  towns.  This  second  source  of  material  is  by  no 
means  fully  exploited  at  present.  Formerly  tin  scrap  contained  about 
5  per  cent,  of  its  weight  of  tin — now  the  deposit  is  much  thinner, 
and  is  only  1*5-3  per  cent,  of  the  total  weight.  The  electrochemical 
processes  proposed  fall  into  three  groups.  In  those  of  the  first  type 
the  tin  is  dissolved  chemically,  and  deposited  electrochemically  in 
another  tank.  In  the  second  type  an  acid,  in  the  third  an  alkaline, 
electrolyte  is  used,  and  in  both  cases  the  process  is  carried  out  in  one 
operation. 

Two  examples  of  the  first  kind  of  process  are  the  Bergsoe  and 
Browne-Neil  processes,  worked  in  Copenhagen  and  North  America 
respectively.  In  the  Bergsoe  process,  the  tin-scrap  is  suspended  in  iron 
baskets  in  a  cold  SnCl4  solution  which  flows  through  a  series  of  stripping 
tanks.  The  liquors  leaving  the  last  tank  charged  with  SnCl2  flow 
into  the  electrolysis  vats,  where  they  are  electrolysed  between  tin 
cathodes  and  graphite  anodes.  Very  pure  tin  is  deposited,  and  at  the 
anodes  SnCl4  is  regenerated.  The  chief  advantage  of  the  Bergsoe 
process  is  the  pure  product ;  its  disadvantages  are  the  slow  rate  of 
working,  and  the  gradual  fouling  of  the  electrolyte  with  FeCJ3.  In 
the  Browne-Neil  process,  the  tin-plate  is  stripped  by  means  of  a  boiling 
FeCl3  solution,  into  which  the  scrap,  packed  in  cages,  is  lowered.  When 
the  tin  has  dissolved,  the  cage  is  rapidly  withdrawn,  as  the  uncovered 
iron  is  next  attacked.  Fresh  tin  scrap  is  lowered  in  until  all  the  FeCls 
has  been  reduced  as  foUows  :  2FeCl3  -f  Sn  — >  2FeCl2  -f  SnCl2.  The 
resulting  liquors  circulate  through  concrete  tanks  containing  tin  cathodes, 
the  graphite  anodes  being  enclosed  in  separate  chambers  of  porous 
clay  which  are  fed  with  the  spent  FeCl2  solution  leaving  the  cathode 
compartment.  At  the  cathodes  tin  is  deposited,  whilst  at  the  anodes 
FeCla  is  regenerated.  The  chief  drawback  is  the  unavoidable  solution  of 
much  iron  by  the  FeCl3  during  the  dipping.  Neither  of  these  processes 
has  yet  been  extensively  applied. 

In  processes  of  the  second  class  mentioned,  the  proposal  is  to 
make  the  tin-plate  anode  in  an  acid  solution,  whereby  it  dissolves, 
and,  when  sufficient  has  accumulated  in  the  electrolyte,  plates  out 
cathodically.  No  process  of  this  kind  has  ever  proved  practicable, 
owing  to  the  very  rapid  deterioration  of  the  electrolyte  caused  by 
the  iron  dissolving. 

Goldschmidt  Process. — By  far  the  most  important  process  used 
is  that  employing  NaOH  as  stripping  agent  and  electrolyte,  and  first 


xvni.]  TIN  289 

worked  out  by  H.  Goldschmidt  at  Essen.  It  depends  on  the  facts 
that  tin  dissolves  anodically  in  an  NaOH  solution,  whilst  iron  is  passive 
under  similar  conditions.  From  the  resulting  solution  the  tin  is  readily 
cathodically  deposited  when  its  concentration  in  the  electrolyte  has 
reached  quite  a  moderate  value. 

Anodic  Behaviour  of  Tin.  —  In  practice  tin  is  found  to  dissolve 
anodically  in  the  tetravalent  form,  giving  stannate  as  follows  : 

Sn+40  —  -^Sn"" 
Sn""  +  60H'  —  >  Sn03"  +  3H20. 

This,  together  with  the  known  fact  that  alkaline  stannite  solutions 
decompose  spontaneously,  giving  metallic  tin  and  stannate,  led  to  the 
belief  that  the  relations  between  the  Sn""  and  Sn"  ions  and  metallic 
tin  were  similar  to  those  between  Cu"  and  Cu*  ions  and  metallic  copper  — 
i.e.  that  the  equilibrium  corresponding  to  the  equation  2Sn"  ^± 
Sn""  -f-  Sn  lies  very  much  over  in  favour  of  the  right-hand  side. 
Against  this  was  the  fact  that  tin  dissolves  anodically  in  acid  solution 
as  Sn"  ions.  Further,  H.  Goldschmidt  and  Eckardt1  found  that  pure 
tin,  though  readily  becoming  passive,  really  dissolves  in  alkaline  solu- 
tions also  in  the  stannous  condition. 

The  subject  was  fully  investigated  by  Foerster  and  Dolch.2  They 
showed  that  tin  is  really  not  analogous  to  copper  in  this  connection,  but 
that  Sn-Sn",  not  Sn-Sn"",  is  the  stable  system.  The  reason  that 
anodic  tin  becomes  so  readily  passive  in  alkaline  solution  is  that,  at  a 
certain  concentration,  a  colloidal  tin  compound  is  precipitated  on  the 
electrode.  This  prevents  the  diffusion  away  of  the  Sn"  (or  Sn02")  ions, 
the  anode  potential  being  consequently  raised  to  the  value  necessary 
for  oxygen  evolution.  The  oxygen  of  course  rapidly  oxidises  the 
stannite  to  stannate,  the  results  observed  under  technical  conditions 
thus  being  explained.  Any  insoluble  impurities  present  in  the  tin 
assist  the  setting  up  of  passivity.  The  higher  the  temperature  and 
the  lower  the  current  density,  the  more  tin  can  be  dissolved  before 
the  formation  of  stannate  commences.  If  the  electrode  be  scraped, 
it  again  becomes  active.  The  chemical  precipitation  of  tin  from 
alkaline  stannite  solution  was  shown  to  be  preceded  by  a  great  diminu- 
tion of  the  Sn""  (or  Sn03")  concentration,  due  to  a  gradual  formation 
of  non-ionised  colloidal  meta-stannic  acid.  It  is  only  when  the  Sn"" 
concentration  in  the  solution  has  in  this  way  become  very  low  that 
the  Sn"  concentration  exceeds  that  corresponding  to  the  equation 

—  K,  and  the  reaction  2Sn"  —  >  Sn""  -f  Sri  sets  in. 


[Sn     ] 

When  the  layer  of  tin  has  dissolved,  a  surface  of  iron  is  exposed  to 

1  Zfit.sch.  Phys.  Chem.  56,  385  (1906). 

-  Zfitsch.    Elektrochem.    18,  599    (1910).     See    also    Foerster  and  Yamasaki, 
Zeitsch.  Elektrochem.  17,  361  (1911). 


290    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

the  alkali.  Iron  is  more  electropositive  than  tin,  and  would  therefore 
dissolve,  were  it  not  passive.  OH'  ions  are  instead  discharged,  and 
oxygen  liberated.  A  certain  amount  of  iron  does,  however,  enter  and 
foul  the  electrolyte.  This  is  probably  due  to  the  presence  of  small 
quantities  of  Cl'  ions  in  the  liquors,  with  their  well-known  effect  of 
removing  passivity. 

At  the  cathode  we  have  the  discharge  of  Sn""  and  H'  ions  to 
consider.  Insufficient  data  prevent  a  discussion  as  to  which  takes 
place  preferentially.  The  discharge  of  Sn""  ions,  whether  through  the 
intermediate  formation  of  Sn"  ions  or  directly,  is  probably  rendered 
difficult  by  reaction  resistances,  whilst  in  the  case  of  H'  ions  there  is 
the  overvoltage  to  consider,  amounting  to  a  minimum  of  about  O5  volt 
at  tin  and  O08  volt  at  iron.  (The  tin  is  deposited  on  an  iron  cathode.) 
In  practice,  current  efficiencies  of  70-90  per  cent.,  reckoned  on  tetra- 
valent  tin,  result.  The  remaining  10-30  per  cent,  of  the  current  is 
carried  by  H'  ions,  discharged  of  course  at  the  iron,  not  at  a  tin  surface. 

In  the  actual  process,  the  anode  material  is  loosely  packed  in  iron 
cages.  These  are  suspended  within  large  iron  tanks,  serving  as  cathodes. 
If  composed  of  old  tin  cans  from  the  refuse-heap,  a  preliminary  treat- 
ment is  necessary  because  of  the  impurities  present  and  the  large  and 
inconvenient  bulk  of  the  material.  The  cans  are  compressed  and 
then  lacerated  by  rollers  with  sharp  cutting  points.  Fatty  matter, 
enamel,  etc.,  are  removed  by  caustic  soda,  and,  after  washing,  the  material 
is  brought  into  the  electrolysis  tanks.  (Solder  may,  if  necessary,  be 
removed  by  heating  in  a  furnace.)  If  this  treatment  is  omitted,  it; 
causes  very  rapid  fouling  of  the  bath.  Owing  to  the  necessity  of  allow- 
ing the  electrolyte  access  to  all  parts  of  the  tin  scrap,  the  anode  cages 
cannot  be  large  or  tightly  packed.  The  weight  of  tin  scrap  per  cage  is 
therefore  small — ten  to  twenty  kilos.  A  tank  of  three  cubic  metres 
capacity  may  contain  six  such  baskets,  which  will  occupy  one-half  to 
three-fourths  of  the  bath  volume. 

The  electrolyte  originally  contains  about  10  per  cent.  NaOH. 
This  diminishes  during  the  process  (C02  absorption,  etc.),  and  falls 
perhaps  to  7-8  per  cent.,  at  which  figure  it  is  kept  by  constant  regenera- 
tion. To  lower  the  voltage  and  improve  the  nature  of  the  deposited  i 
tin,  the  temperature  is  kept  at  70-80°  by  a  suitable  arrangement  of 
circulation  and  steam  coils.  The  cathodic  current  density  is  about 
80-100  amps./metre2,  and  1*5-2  volts  are  used  per  tank.  A  cage  is! 
three  to  four  hours  in  the  bath.  The  current  efficiency  is  70-90  per 
cent.,  the  deficiency  being  due  to  hydrogen  evolution.  Assuming  T7 
volts  and  80  per  cent,  current  efficiency,  the  recovery  of  one  ton  of  tin 
would  require 

96540  X  4  X  1000  X  1000  X  100  X  1'7 
119    X  SOX  3600X1000 


XVIIL]  TIN  291 

If  the  tin  recovered  be  2  per  cent,  of  the  scrap  treated,  then  the 
treatment  of  10,000  tons  of  scrap  per  annum  would  require,  assuming 
300  twelve-hour  working  days  to  the  year,  an  installation  capable  of 

1920  X  10000 
furnishing  5Q  x  12  x  300  =  110  K'W'  (approximately). 

The  tin  is  mostly  deposited  in  spongy  form,  though  a  few  crystals 
are  obtained.  It  contains  about  2  per  cent,  each  of  lead  and  iron,  a 
little  copper,  and  some  stannic  hydroxide.  The  lead  is  deposited 
electrochemically,  being  dissolved  from  the  solder  in  the  anode  material. 
The  iron  is  chiefly  mechanically  admixed,  having  dropped  from  the 
anode  baskets.  Tin  not  immediately  under  those  is  materially  purer. 
The  product  is  collected,  washed,  pressed  into  briquettes,  and  melted 
up  with  coke  and  a  suitable  flux.  A  metal  results  containing  99  per 
cent.  Sn  with  1  per  cent.  Pb  and  small  quantities  of  copper  and  iron. 
This  is  not  further  purified.  The  detinned  iron  still  contains  at  least 
0' 1-0*2  per  cent.  Sn.  This  is  partly  due  to  the  presence,  at  the  actual 
surface  of  contact  of  tin  and  iron,  of  an  alloy  which  is  attacked  with 
difficulty  by  the  NaOH.  To  dissolve  the  last  traces  of  tin  would  require 
too  much  time,  would  be  wasteful  of  power,  and  would  result  in  iron 
dissolving  or  becoming  loose  and  falling  to  the  bottom  of  the  bath. 
There  are  further  bound  to  be  pieces  of  tin-plate  more  heavily  tinned 
than  the  average,  or  which  do  not  get  their  fair  share  of  current.  Some- 
times as  much  as  0'3-0'5  per  cent.  Sn  remains  undissolved  on  the  iron. 
The  detinned  material  is  a  valuable  product,  and  is  utilised  in  the 
open-hearth  furnace  for  steel-making. 

An  important  point  is  the  regeneration  of  the  electrolyte.  If  the  free 
alkali  content  becomes  too  low,  stannic  hydroxide  will  settle  out ;  and 
the  NaOH  is  being  continually  used  up  during  the  process  by  absorption 
of  C02  or  by  iron  oxide  dissolving.  A  fraction  of  the  liquors  is  regularly 
withdrawn  and  replaced  by  fresh  NaOH  solution.  The  foul  electrolyte 
is  saturated  with  C02,  whereby  the  tin  and  iron  are  precipitated.  The 
resulting  solution  is  then  causticised  by  lime  in  the  ordinary  manner 
and  subsequently  returned  to  the  tanks.  The  precipitate  of  stannic 
hydroxide  is  freed  from  iron  by  HC1,  and  can  then  be  worked  up  in  a 
variety  of  ways. 

It  remains  to  consider  the  disadvantages  of  the  process.  The  first 
is  incomplete  recovery  of  the  tin,  the  reasons  for  which  we  have  dis- 
cussed. There  is  also  a  considerable  loss  due  to  the  electrolyte  which 
adheres  to  the  detinned  iron  when  it  is  removed  from  the  bath.  Its 
content  of  tin  cannot  be  recovered  economically  by  washing.  Even 
more  important  is  the  large  wages  item  due  to  the  small  units  employed, 
and  to  the  necessity  of  sorting  out  the  raw  material  before  treatment. 
These  facts  make  it  very  probable  that  in  the  future  the  electrolytic 
detinning  process  will  be  replaced  by  the  chlorine  detinning  process  of 
K.  Goldschmidt,  in  which  the  treatment  is  more  rapid  and  the  losses 

u  2 


292    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY  [CHAP. 

considerably  less.  The  pure  dry  chlorine  required  can  be  furnished  in 
practically  unlimited  quantities  by  the  various  electrolytic  alkali- 
chlorine  processes  at  work. 


3.  Electrometallurgy  of  Nickel 

Nickel,  in  its  sulphide  and  arsenide  ores,  is  generally  accompanied 
by  copper  and  iron.  The  iron  can  be  largely  removed  as  oxide  and 
slag  by  a  smelting  process  which  produces  a  rich  copper-nickel  matte. 
This  matte  can  be  separated  into  two  parts,  one  of  which,  containing 
little  copper  and  nearly  all  the  nickel,  being  worked  up  to  a  metal  con- 
taining 98-99  per  cent,  nickel.  For  the  manufacture  of  nickel  steel 
this  material  may  suffice.  A  higher-grade  product  is,  however,  often 
in  demand,  particularly  for  anodes  in  nickel-plating.  This  purer 
product  can  conveniently  be  prepared  by  electrolytic  methods,  which 
can  be  applied  in  two  ways.  The  crude  pyrometallurgical  nickel  can  bo 
electrolytically  refined.  Or  the  matte  obtained  at  an  earlier  stage  can 
be  suitably  lixiviated,  the  impurities  removed  chemically  or  electro- 
chemically,  and  the  purified  liquors  electrolysed,  using  an  insoluble 
anode.  Processes  of  both  kinds  have  been  or  are  used. 

Anodic  Behaviour  of  Nickel.—  We  will  first  consider  the  behaviour 
of  nickel  electrodes  in  electrolytes  containing  nickel  salts.  A  nickel 
anode  shows  very  markedly  the  phenomena  of  passivity.  The  equili- 
brium potential  value  l  of  Ni  |  n.NiS04  at  room  temperature  is  about 
—  0'25  volt.  But  to  bring  about  the  process  Ni  -f  2  ©  —  >  Ni"  at 
such  an  anode,  a  large  excess  anodic  polarisation  is  necessary.  Thus,  for 
example,  Schoch  found  that  in  n.NiS04  at  26°,  a  nickel  anode  only 
dissolves  at  current  densities  below  0'036  amp.  /dm.2.  When  this 
value  is  exceeded,  it  becomes  passive  and  oxygen  is  evolved,  its  potential 
being  about  +  O28  volt,  and  the  excess  polarisation  0*53  volt.  With 
other  electrolytes,  or  at  other  concentrations,  the  results  are  similar. 
The  passivity  of  nickel,  like  that  of  other  metals,  decreases  with  rise 
of  temperature  or  in  the  presence  of  Cl'  or  H'  ions.  Hence  at  a  higher 
temperature,  or  in  a  solution  of  NiS04  containing  acid  or  NaCl,  or  in  a 
NiCl2  solution,  a  nickel  anode  can  be  subjected  to  much  greater  current 
densities  than  the  above  without  passivity  setting  in.  For  details 
the  reader  is  referred  to  the  papers  cited. 

A  further  point  should  be  here  mentioned,  viz.  that  when  subjected 
to  the  same  anodic  current  density  in  the  same  electrolyte,  samples  of 
nickel  prepared  in  different  ways  dissolve  with  very  different  potentials. 
The  reasons  for  this  we  have  discussed.2  Electrolytic  nickel,  unless 
freshly  prepared  and  thus  rhjipjrd  with  hydrogen,  requires  the  highest 


1  Schoch,    Amer.    Chem.   Jour.    41,   208,    232   (1M9)  ;     Schweitzer,    Ze.itttch. 
Ehktrochem.  15,  602  (  / 

2  P.  i:r>. 


XVIIL]  NICKEL  293 

anodic  polarisation.  Rolled  nickel  sheet  dissolves  more  easily,  cast 
nickel  more  easily  still.  Best  of  all  is  cast  nickel  the  surface  of  which 
has  been  roughened  in  some  way,  perhaps  by  acid.1  If  the  current 
density  is  sufficiently  high  for  passivity  complications  to  enter,  these 
potential  differences  may  even  rise  in  some  cases  to  one  volt.2  Wrought 
or  cast  nickel  anodes  of  uneven  structure  often  therefore  yield  a  large 
proportion  of  undissolved  slimes,3  which  is  very  inconvenient  in 
electroplating. 

Cathodic  Behaviour  of  Nickel. — In  connection  with  the  cathodic 
deposition  of  nickel,  there  are  several  questions  to  be  discussed.  The 
equilibrium  potential  for  Ni  |  n.  NiS04  is  about  —  0'26  volt,  and  hence, 
unlike  copper,  nickel  cannot  be  quantitatively  deposited  out  of  solutions 
containing  moderate  quantities  of  free  acid.  The  hydrogen  overvoltage 
at  nickel  amounts  to  a  minimum  of  0'2  volt  at  room  temperature,  but 
this  is  nullified  by  the  fact  that,  just  as  the  small  velocity  of  the  process 
Ni  -f-  2  ©  — >  Ni"  causes  passivity  at  a  nickel  anode,  so  does  the 
low  velocity  of  the  converse  process  Ni" — >  Xi  -f  2  ©  necessitate  a 
considerably  higher  cathodic  polarisation  than  corresponds  to  the 
equilibrium  value  for  nickel  deposition.4  Tliis  has  been  made  clear  by 
the  work  of  Schoch  5  and  Schweitzer.5  At  moderate  current  densities 
(0'3-1*0  amp./ dm.2)  and  at  room  temperature  the  excess  cathodic 
polarisation  necessary  is  0'3-0'4  volt.  Thus,  Schweitzer  found  the 
following  cathodic  potentials  necessary  for  nickel  deposition  from 
n.  NiCl2  solution  at  16°  in  a  hydrogen  atmosphere  : 

TABLE    XXXIX 

Current  density  Cathode  potential 

O'O  (equilibrium  potential)  —  0'31  volt  (about) 

0-01  amp./ dm.-  —  0-462 

0-03  —  0-486 

0-06  —  0-51 

0-11  -0-535 

0-9  -  0-645 

This  excess  polarisation  increases  with  the  current  density,  but  decreases 
with  rise  of  temperature.  Above  90°  it  is  only  about  Ol  volt.  The 
hydrogen  overvoltage,  of  course,  behaves  similarly. 

Whatever  the  temperature,  then,  the  H'  concentration  must  be 
kept  down  or  else  the  current  efficiency  of  nickel  deposition  will  be 
low.  At  the  same  time  there  is  a  lower  limit  set  to  the  acidity  of  the 
electrolyte,  as  otherwise  basic  salts  would  separate  out,  foul  the  bath, 
and  affect  the  quality  of  the  nickel  deposit. 

1  In  that  case  it  is  charged  with  hydrogen,  which  helps  its  solution.     Cf.  p.  142. 

-  Brown,  Trans.  Amer.  Electrochem.  Sec.  4,  83  (1903). 

3  See  p.  295.  4  See  p.   121.  '  Loc.  cit. 


294    PKINCIPLES  OF  APPLIED  ELECTROCHEMISTRY  [CHAP. 

Another  point  to  be  noticed  is  the  difficulty  of  obtaining  at  room 
temperature  thick  cathodic  deposits  of  the  metal.  Unless  exceedingly 
low  current  densities  are  employed,  the  deposit  peels  off  in  tliin  flakes 
and  shavings,  and  a  coherent  thick  plate  cannot  be  made.  In  the 
case  of  iron,1  a  similar  phenomenon  has  been  traced  to  the  simultaneous 
deposition  of  hydrogen,  which  distributes  itself  unequally  in  the 
metal,  thus  producing  strains  and  causing  the  flaking.  It  would 
seem  natural  to  attribute  the  flaking  in  the  present  case  to  the 
same  cause.  According  to  Engemann,2  however,  this  is  not  so,  the 
amounts  of  hydrogen  dissolved  in  the  deposited  metal  being  too  low. 
The  phenomenon  is  rather  due  to  the  presence  of  traces  of  iron  in 
the  electrolyte.  This  metal  is  discharged  from  solution  more  easily 
than  nickel,  and  the  first  layers  of  the  deposit  therefore  contain  a 
higher  proportion  of  it  than  do  subsequent  layers.  Strains  are  in 
this  way  set  up,  and  result  in  the  flaking.  If  electrolyte  and 
anode  are  perfectly  free  from  iron  there  is  no  difficulty  in  getting  a 
satisfactory  deposit.  Low  temperature,  low  H'  concentration  in 
the  electrolyte,  and  high  current  density,  all  of  which  favour  an 
irregular  deposition  of  iron,  also  favour  the  flaking  of  the 
cathodic  deposit.  At  higher  temperatures  (50°-90°)  there  is  no 
difficulty  in  getting  good  deposits,  even  at  current  densities  of 
2'5  amps./ dm.2.3 

Laboratory  work  on  the  electro-deposition  of  nickel  has  been  carried 
out  by  Foerster  4  and  by  Kern  and  Fabian.5  As  we  have  seen,  Foerster 
showed  the  necessity  with  ordinary  electrolytes  of  working  above  room 
temperature  if  thick  deposits  are  to  result.  Generally  the  higher  the 
temperature  and  the  nickel  content  of  the  electrolyte,  the  better  the 
deposit.  Any  iron  and  cobalt  present  in  the  nickel  anode  are  deposited 
cathodically.  Very  little  separation  can  be  effected,  owing  to  the 
chemical  and  electrochemical  similarity  of  the  metals  concerned  and 
their  tendency  to  form  solid  solutions  with  one  another.  Kern  and 
Fabian  worked  with  different  electrolytes,  and  varied  the  concentration 
of  free  acid,  temperature,  and  current  density.  They  observed  the 
cathodic  current  efficiency  and  the  bath  voltage.  In  all  their  experi- 
ments the  anodes  were  of  crude  cast  nickel,  containing  92  per  cent. 
Ni,  5  per  cent.  Fe,  some  copper  and  carbon,  and  were  placed  1"  from 
the  cathodes.  The  following  table  contains  a  selection  of  their  results. 
The  heavy  deposits  given  by  neutral  NiSOd  solutions  were  due  to  the 
formation  of  basic  salts.  This  tendency  was  much  less  marked  with 
NiCl2  solutions. 

1  See  p.  300.  , 

2  Zeitsch.  Elektrochem.  17,  910  (Mil). 

r.trr,  KrUftrh.  Klcktrochcm.  4,  160  (AW). 
4  Loc.  <•</. 
6  Electrochem.  2nd.  6,  365  (1008). 


CVIII.] 
Electrolyte 

e 

NICE 
TABLE 

Current 
density 

:EL                                295 

XL 

Nida 

NiS04 

Voltage 

Current 
efficiency 

v  .             Current 
Volta8e    efficiency 

8  per  cent.  Ni  +  0-5  / 
mol.  equiv.  of  free 

acid 

8  per  cent.  Ni  +  O'l  ( 
mol.  equiv.  of  free 
acid 

20°  C. 
40° 
60° 

}  10  } 
,  amp. 

)  foot2  1 

Volt 
0-49 
0-36 
0-21 

Per  cent. 
3-20 
1-25 
1-01 

Volt 
0-89 
0-65 
0-42 

Per  cent. 
0-86 
0-62 
0-39 

20°      )             f 
40°            10    - 
60°       J 

0-73          62-6 
0-52          79-0 
0-35          71-0 

0-91 

0-78 
0-51 

1-60 

1-60 
1-50 

8  per  cent.  Ni  +  0-05-  f 
0'04  mol.  equiv.  of  -j 
free  acid 

8  per  cent.  Ni  ;  neutral  -' 

20° 
40° 
60° 

H 

0-86 
0-53 
0-44 

67-2 
75-7 
80-7 

0-97 
0-79 
0-61 

o-o 

0-7 
2-8 

20° 
40° 
60° 

I  -  i 

0-78 
0-59 
0-44 

96-6            1-50 
99-4            1-35 
99-2            0-88 

102-2 
106-9 
101-1 

8  per  cent.  Ni;  neutral- 

20° 
40° 
60° 

H 

1-06 
0-73 
0-64 

91-2            3-45 
94-7            2-30 
88-3            1-40 

— 

In  the  electrolytic  refining  of  nickel  the  impurities  to  be  considered 
are  copper,  cobalt,  iron,  carbon,  and  sulphur.  Of  these,  only  the  last 
two  can  be  eliminated  if  the  process  be  carried  out  in  one  operation, 
using  crude  soluble  anodes  ;  for  copper  is  precipitated  more  readily 
than  nickel,  and,  as  we  have  seen,  Foerster  showed  that  no  separation 
of  iron  and  cobalt  is  possible  in  this  way.  The  only  nickel  refining 
process  of  this  type  of  which  the  author  knows  was  that  worked  by  the 
Balbach  Smelting  and  Refining  Co.  (U.S.A.)  some  years  back.1  The 
anodes  contained  94-97  per  cent.  Ni ;  0'75  per  cent.  Fe  ;  O2-O6  per 
cent.  Cu ;  0'25  per  cent.  Si ;  2-3  per  cent.  C.  The  electrolyte 
was  a  hot  (50°-60°)  NiS04  solution,  the  current  density  being 
1*5  amps./dm.2  and  a  bath  taking  1*7— 1*8  volts.  A  H.P.  year  is 
said  to  have  produced  2-4  tons  nickel,  which  corresponds  to  an 
expenditure  of  1,600-3,300  K.W.H.  per  ton.  The  metal  was  of 
excellent  quality,  containing  99'5-99'7  per  cent.  Ni-f  Co,  0'1-0*2  per 
cent.  Cu,  0*25  per  cent.  Fe,  with  traces  of  carbon,  sulphur,  arsenic, 
and  silicon.  This  process  has  now  been  discontinued  for  some  years. 
The  cause  is  stated  to  have  been  the  large  quantities  of  anode 
residues  produced  in  the  electrolysis.  The  crude  metal  contained 
a  large  percentage  of  carbon,  gave  brittle  unhomogeneous  castings, 

1  Electrochem.  2nd.  1,  208  (1903)  ;  Zeitsch   Elektrochem.  10,  821  (1904). 


296    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY  [CHAP. 

and,  when  once  the  outer  layer  had  been  dissolved,  crumbled  away 
very  readily.  Its  high  melting-point  rendered  this  a  particularly 
heavy  drawback.  Patents,  in  fact,  were  taken  out  for  the  use  of 
frames  packed  with  powdered  nickel  as  anodes. 

Most  other  electrometallurgical  nickel  processes  have  employed  in- 
soluble anodes  for  the  final  electrolysis,  previously  removing  impurities 
from  the  electrolyte  in  various  ways.  Thus  electrolytic  means  have 
been  applied  to  the  removal  of  copper  from  the  liquors,  and  this  elec- 
trolysis of  solutions  containing  copper  and  nickel  has  been  studied 
by  Neumann  x  and  by  E.  Giinther.2  Neumann  used  anodes  containing 
48*9  per  cent.  Ni,  5O1  per  cent.  Cu,  O63  per  cent.  Fe,  with  some 
carbon  and  silicon,  in  an  electrolyte  (circulated)  of  initial  composition 


—  CuS04,  5H20,  180  ;  H2S04,  85  c.c.  per  litre.    He  found  that 

litre 

the  anode  dissolved  readily  and  copper  was  cathodically  deposited,  the 
electrolyte  growing  continually  poorer  in  copper,  and  richer  in  nickel 
and  iron.  Finally,  the  quality  of  the  deposit  began  to  deteriorate, 
hydrogen  being  evolved.  This  happened  at  a  current  density  of 


1*5   amps.  /dm.2,    when    the  electrolyte  still  contained   20  —  -    of 

litre 

copper.  .  No  nickel  was  deposited. 

Giinther  used  anodes  containing  Cu  26*4  per  cent.,  Ni  50*2  per  cent., 
Fe  21*2  per  cent.,  with  some  sulphur  and  carbon.  The  electrolyte  was 

acid  CuS04  with  384  giramS  copper  and  62'7  giramS  H2S04.      With 
litre  litre 

electrodes  5  cm.  apart,  and  a  current  density  of  2  amps.  /dm.2,  the 
cathodic  current  efficiency  was  95-97  per  cent.,  and  the  bath  voltage 
0'56,  rising  to  T16  volts.  The  copper  produced  was  99'97  per  cent. 
pure.  At  the  start  the  electrolyte  was  stirred  by  blowing  in  air.  When 
the  copper  content  had  fallen  to  1  per  cent,  circulation  was  commenced. 
By  this  means  the  copper  concentration  was  lowered  to  0'5  per  cent. 
before  the  deposit  became  spongy.  It  is,  however,  better,  working 
with  2  amps./dm.2,  not  to  allow  the  copper  content  to  fall  below  1 
per  cent.  The  electrolysis  should  then  be  discontinued,  and  the 
remainder  of  the  copper  removed  by  H2S. 

Hoepfner  Process.  —  Of  technical  processes,  that  of  Hoepfner  should 
first  be  noticed.  It  received  very  extended  trials,  and  underwent 
many  modifications,  but  never  became  really  successful.  In  one  form 
which  it  assumed,  the  nickel  ore,  containing  copper  and  iron,  was 
partially  roasted  to  render  most  of  the  iron  insoluble,  and  then  ex- 
tracted with  a  CaCl2  solution  containing  CuCl2,  the  anode  liquors 

1  Zeitsch.  Elektrochem.  4,  316  (1898). 

2  Ibid.  9,  2  40  (  / 


MCKEL  297 

of  the  subsequent  electrolysis.     Copper  and  nickel  sulphides  dissolved 

as  follows  : 

Cu2S  +  2CuCl2  —  >  2Cu2Cl2  +  S 

CuS  +  CuCl2  -  >  Cu2Cl2  +  S 

NiS+  2CuCl2  -  >  Cu2Cl2  +  NiCl2  +  S 

After  removing  silver  and  iron  chemically,  the  purified  electrolyte 
charged  with  NiCl  and  Cu2Cl2  was  electrolysed  as  described  on  p.  264. 
The  greater  part  of  the  copper  in  the  cathodic  compartment  was 
thus  removed  electrolytically,  and  the  last  portions  chemically.  The 
purified  solution  containing  CaCl2  and  NiCl2  was  then  electrolysed, 
using  a  nickel  sheet  cathode  and  a  graphite  anode.  Nickel  of  excellent 
quality  resulted,  whilst,  at  the  anode,  either  chlorine  was  evolved  and 
utilised  in  lixiviating  the  roasted  ore,  or  else,  as  in  the  copper  electro- 
lysis, the  anode  compartment  was  fed  with  the  solution  of  Cu2CL 
and  NiCl2  resulting  from  the  lixiviation  of  the  ore,  and  Cud,  was 
regenerated. 

Savelsberg-Wannschaff  Process.  —  The  circumstances  which  caused 
the  abandonment  of  the  Hoepfner  copper  process  —  difficulties  con- 
nected with  diaphragms  and  lixiviation  of  the  calcined  ore  —  affected  the 
nickel  process  adversely  also.  It  was  worked  for  some  years  in  Papen- 
burg,  but  was  later  replaced  there  by  a  process  devised  by  Savelsberg 
and  Wannschaff.1  A  matte  with  65-70  per  cent.  Ni,  containing  iron, 
but  very  free  from  copper,  is  finely  ground  with  water  or  CaCl2  solution 
and  treated  with  chlorine.  Nickel  and  iron  dissolve,  sulphur  is  liberated 
and  partially  oxidised  to  H2S04,  whilst  the  excess  of  HC1  produced  is 
neutralised  by  adding  Fe203.  The  solution,  after  filtration  from  CaS04, 
sulphur,  silica,  Fe203.  etc.,  contains  chlorides  of  nickel  and  iron.  It  is 
heated  to  60°-70°,  treated  with  fresh  powdered  ore,  and  air  is  blown  in. 
The  iron  in  solution  is  thus  replaced  by  nickel,  and  Fe(HO)3  is  preci- 
pitated. After  decantation  and  filtration,  the  liquors,  containing  NiCl2, 
are  electrolysed  between  sheet-metal  cathodes  and  graphite  anodes,  the 
latter  being  hooded  to  collect  the  chlorine.  The  electrolyte  contains 


initially  100  nickel,  and  leaves  the  bath  with  30  .     The 

litre  htre 

cathodic  current  density  is  1-1*2  amps.  /dm.2.  The  working  tem- 
perature is  not  stated  ;  4-4'5  volts  per  bath  are  used,  and  the  average 
current  efficiency  is  93  per  cent.,  though  99  per  cent,  is  sometimes 
reached.  The  product  is  perfectly  compact,  though  of  a  somewhat 
warty  appearance,  due  to  bubbles  of  hydrogen  adhering  to  the  electrode 
and  becoming  gradually  covered  with  metal.  It  contains  99'9  per  cent. 
Ni  +  Co  ;  0'06  per  cent.  Fe  ;  0'02  per  cent.  Cu  ;  0*02  per  cent.  Si02  ; 

i  Zeitech.   EleldrocTiem.    10,  821   (1904).     Also    Billiter,  Die   Elektrochemischen 
Verfahren,  etc.,  voL  i.,  p.  282  (1909). 


298    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY  [CHAP. 

sulphur,    carbon,    arsenic,    antimony,    etc.,    etc.,    traces    or    absent. 
Assuming  93  per  cent,  current  efficiency  and  4*3  volts,  we  have 
1  ton  nickel  requires 

100  X  2  X  96540  X  1000  X  1000  X  4'3 
93  X  58-7  X  3600  X  1000 

Browne  Process.  —  The  next  process  which  will  be  considered  is  that 
of  Browne,1  carried  out  successfully  for  years  by  the  Canadian  Copper 
Co.  at  Brooklyn,  and  now  again  in  operation  elsewhere  by  the  same 
company.  A  copper-nickel-iron  matte,  containing  about  equal  quan- 
tities of  copper  and  nickel,  is  desulphurised,  half  of  the  product  cast 
into  anodes,  the  other  half  granulated.  This  granulated  material  is 
treated  with  chlorine  in  presence  of  a  brine  solution,  and  yields  the 
electrolyte  for  use  with  the  anodes  just  mentioned,  containing  NiCL, 
Cu2Cl2,  FeCl2,  and  NaCl.  The  anodes  (in  Brooklyn)  had  54'3  per 
cent.  Cu  and  43'  1  per  cent.  Ni,  the  remainder  being  iron  and  sulphur. 
The  electrolysis  took  place  in  cement  tanks,  using  thin  electrolytic 
copper  cathodes.  A  coherent,  but  rather  crystalline,  copper  deposit 
resulted. 

The  current  efficiency  was  about  90  per  cent,  (copper  deposited  from 
the  cuprous  state),  the  deficiency  being  due  to  the  presence  of  Cu" 
ions  ;  0'3-0'35  volts  were  used.  On  leaving  this  series  of  tanks,  the 
r^tio  Ni  :  Cu  in  the  liquors  was  about  80  :  1.  The  rest  of  the  copper  was 
precipitated  by  Na2S,  the  iron  oxidised  and  removed  as  hydroxide, 
and,  after  concentration,  practically  all  the  NaCl  crystallised  out.  In 
the  final  electrolysis  the  cathodes  were  of  nickel  strip,  the  graphite 
anodes  being  provided  with  porous  clay  diaphragms  and  hoods. 
A  cathodic  current  efficiency  of  93'5  per  cent,  resulted,  3'5-3'6 
volts  being  required  per  tank.  The  nickel  deposit  was  of  excellent 
quality,  analysing  Ni  99'85  per  cent.,  Cu  0'014  per  cent,,  Fe  0'085 
per  cent. 

Before  finally  leaving  this  subject,  it  should  be  mentioned  that  the 
statement  has  been  made  that  electrolytic  nickel  is  at  present  also 
produced  by  the  Orford  Copper  Co.,  New  Jersey,  using  anodes  of  nickel 
matte  in  a  NiCl2  solution  as  electrolyte,  and  getting  cathodic  deposits 
of  excellent  quality.  Sulphur  will,  of  course,  remain  undissolved  at 
the  anode,  and  in  view  of  the  difficulties  experienced  in  working  the 
Marchese  copper  process  technically  one  is  inclined  to  doubt  the  relia- 
bility of  the  information.2  E.  Giinther,3  working  on  a  small  scale, 
studied  the  behaviour  of  nickel  matte  anodes  in  NiS04  solution.  They 
dissolved  with  surprising  ease,  and  the  cathodic  nickel  deposits 


Elektrochem.  9,302 

('/..  lu.urvn,   p.  2«i|. 

MetalLl,n(H«>l). 


XVIIL]  IRON  299 

were  of  good  quality.  He  experienced,  however,  some  trouble  with 
the  anodes  (owing  to  their  brittleness,  irregular  structure,  etc.), 
and  these  difficulties  would  undoubtedly  be  far  greater  on  a  technical 
scale. 

4.  Electrolytic  Refining  of  Iron 

In  recent  years  this  subject  has  been  much  studied,  chiefly  from 
the  point  of  view  of  preparing  pure  iron  for  experimental  alloy  work. 
Lately,  however,  the  well-known  firm  of  Langbein  and  Pfanhauser,  of 
Leipzig,  using  the  process  of  Fischer,  has  commenced  to  electro- 
lytically  refine  iron  for  use  in  transformer  cores,  etc.  The  pure  product 
has  very  favourable  magnetic  properties. 

We  already  know  that  an  iron  anode  markedly  exhibits  the  pheno- 
mena of  passivity.  The  conditions  for  its  ready  solution  are  high 
temperature,  low  current  density,  and  presence  of  Cl7  and  H*  ions 
in  the  electrolyte.  It  is  particularly  from  a  study  of  the  behaviour  of 
this  metal  that  Foerster  has  based  his  views  on  passivity.1 

Cathodic  Deposition. — Most  attention  has,  however,  been  devoted 
to  the  cathodic  deposition  of  iron.  We  may  particularly  mention  the 
work  of  Burgess  and  Hambuechen,2  Foerster,  Mustad  and  Lee,3  and 
PfafL4  There  are  several  points  to  be  considered.  Firstly,  iron  is 
much  more  electropositive  than  hydrogen.  The  equilibrium  potential 
Fe  j  n.  FeS04  is  about  —  0*46  volt,  whilst  even  in  neutral  solution 
the  potential  of  the  hydrogen  electrode  is  —  Oil  volt.  The- reversible 
deposition  of  hydrogen  from  such  a  solution  should  therefore  take 
place  rather  more  easily  than  that  of  iron,  and  the  small  hydrogen 
overvoltage  at  that  metal  barely  equalises  matters.  But  we  have 
seen  5  that  iron,  like  nickel,  is  one  of  those  metals  whose  cathodic 
deposition  encounters  strong  irreversible  reaction  resistances.  This 
fact,  of  course,  favours  the  preferential  discharge  of  H*  ions,  and  if 

satisfactory    current    efficiencies    are    to   be  obtained  the  ratio  — — 

H 

must  be  as  high  as  possible.  It  is  limited  by  the  solubility  of  the 
iron  salt  employed,  and  by  the  fact  that  at  low  H*  concentrations 
basic  salts  are  liable  to  precipitate,  affecting  the  quality  of  the 
cathodic  deposit.  A  rise  in  temperature,  however,  decreases  the 
reaction  resistance,  and  should  therefore  increase  the  current  efficiency. 
The  same  should  result  from  an  increase  in  current  density,  which 
is  known  to  largely  increase  the  overvoltage  effect. 

i  See  p.  142. 

•2  Electrochem.  Ind.  2,  184  (1904).     Trans.  Amer.  Electrochem.  Soc.  19,  181  (1911). 

a  Abhand.  Bumen  Ges.  2,  34  (1909). 

•»  Zeitsch.  Elektrochem.  16,  217  (1910). 

»  P.  121. 


300    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY   [(HAP. 
The  figures  of  Tables XLI  andXLII  show  these  deductions  to  be  true. 


TABLE    XLI 

Kkvtn.Iytc  2-3  n  .  FeS04 


0 

Current  efficiency. 

Cathodic  current  density 
1  '6  amps,  /dm.2 

3-2  amps./  dm.  - 

Acidity 
0-05  ». 

0-1  n. 

0-01  H. 

0-05   ;/. 

0-1  n. 

21-4 
19-6 
26 

48-7 
78-8 

18° 
37° 
55° 
75° 
95° 

Per  cent. 
30-33 
40-45 
58-64 
J80-85 

4-8 
6-7 
13-2 
34-8 
58-8 

86-8 
89-0 
90-8 
94-8 

61-6 
52-0 
()0-0 
72-0 

TABLE  XLII 
Electrolyte  FeCL  +  0-05  n  .  HC1.     Current  density  1-6  amps,  /dm.2 


Current  efficiency 

6 

4-6  n  .  FeCl, 

2-3  n  .  FeCL 

0-8  n  .  FeCL 

Per  cent. 

18° 

4<Hi 

21-0 

9-0 

37° 

46*3 

30-4 

12-7 

55° 

62-7 

46-0 

21-1 

75° 

81-7 

64-6 

34-0 

The  best  results  for  a  good  current  efficiency  are  therefore  :  (a)  liigh 
Fe"  concentration  ;  (b)  H'  concentration  only  high  enough  to  prevent 
precipitation  of  basic  salts  ;  (c)  high  temperature  ;  (d)  high  current 
density. 

Nature  of  Deposit. — There  are  two  further  points  connected  with 
the  character  of  the  cathodic  deposit.  By  varying  the  conditions  the 
iron  can  be  produced  either  exceedingly  hard  and  brittle  or  else  ductile. 
Also  the  tendency  for  any  but  very  thin  deposits  to  flake  off  is  very 
marked,  as  in  the  case  of  nickel.  Both  these  facts  have  been  shown  to 
result  from  the  presence  or  otherwise  of  hydrogen  in  the  metal.  If  it  be 
present,  the  deposit  is  hard  and  brittle,  and  moreover  readily  rusts ;  if 
absent,  a  soft  ductile  iron  results.1  If  it  is  unequally  and  irregularly 
dissolved  it  sets  up  strains,  and  causes  the  metal  to  flake  off,  as  Lee 
has  clearly  shown.  He  found  the  percentage  of  dissolved  hydrogen  to 

1  The  nature  of  the  deposit  also  appears  to  depend  on  the  iron  salt  used  :  some- 
t lines  it  is  crystalline;  sometimes,  as  when  produced  at  high  temperatures  and 
high  current  densities  from  a  FeCL,  solution,  it  is  smooth  and  dark  grey  in  colour. 
Engemann  (Zeitech.  Eleklrochem.  17,  910,  1911)  has  shown  electrolytic  nickel  deposi- 
tion to  be  somewhat  similarly  alfected. 


XVIIL]  LEAD  301 

increase  somewhat  with  decrease  of  acid  in  the  electrolyte,  with  increase 
of  iron  in  the  electrolyte,  and  with  increase  of  current  density.  Far 
greater,  however,  was  the  effect  of  increased  temperature,  which  caused 
the  hydrogen  content  to  rapidly  diminish.  Thus,  under  certain  con- 
ditions, iron  deposited  at  18°  contained  O085  per  cent,  hydrogen,  at 
37°  0-039  per  cent.,  at  55°  0'024  per  cent.,  at  75°  0-0096  per  cent.  At 
high  temperatures,  then,  iron  can  be  electrolytically  deposited  using 
large  current  densities,  and  yet  contain  little  hydrogen  and  be  ductile. 
Moreover,  the  rapid  diffusion  possible  under  those  conditions  equalises 
the  concentration  of  the  dissolved  gas,  and  thus  removes  the  tendency 
to  flake  off. 

Burgess  and  Hambuechen,  using  anodes  of  wrought  iron,  employ  a 

nrpQ  TYI  g 

bath  of  ferrous  ammonium  sulphate  with  40  ^ iron.     It  is  not 

litre 

circulated.  They  work  at  30°  with  a  cathodic  current  density  of 
O'6-l  amp./dm.2,  the  bath  taking  about  1  volt.  (This  value,  high 
compared  with  the  voltage  of  the  copper  refining  tank,  is  of  course 
due  to  the  irreversibility  at  both  anode  and  cathode.)  The  current 
efficiency  is  practically  100  per  cent.,  the  very  uniform  product 
99-97  per  cent,  pure  (free  from  C,  Si,  Mn),  hard  and  brittle.  One  kilo 
of  refined  iron  is  produced  per  K.W.H. 

Pfaff  recommends  a  FeS04  bath  not  less  than  twice  normal,  and 
acidified  with  H2S04  (O'Ol  normal).  He  works  at  70°,  using  a  cathodic 
current  density  of  2  amps. /dm.2,  and  obtains  a  perfectly  homogeneous, 
dense,  finely  crystalline  product.  The  current  efficiency  is  about  87  per 
cent.,  and  the  bath  voltage  0'7  volt.  In  order  to  remove  hydrogen 
bubbles  adhering  to  the  cathode  surface,  he  blows  in  air. 

The  two  processes  more  or  less  worked  technically  employ  FeCl2 
solutions.  Merck1  works  at  70°,  with  a  solution  containing  100  grams 
FeCl2  to  100  grams  H20.  (NH4C1  is  also  said  to  be  added.)  The 
electrolyte  is  circulated,  and  the  current  density  is  3-4  amps./dm.2. 
Experiments  of  Pfaff  indicate  that  FeS04  solutions  behave  morevsatis- 
factorily.  Fischer  uses  a  bath  containing  450  grams  FeCl2  +  500  grams 
anhydrous  CaCla  -f-  750  grams  water.  The  working  temperature  is  at 
least  90°,  and  the  current  density  10  amps./dm.2  (according  to  one 
account  20  amps./dm.2).  Ductile  iron  deposits  of  any  thickness,  and 
at  least  99- 95  per  cent,  pure,  result. 


5.  Electrometallurgy  of  Lead 

The  refining  of  crude  lead  has  so  far  been  mainly  carried  out  for  the 
recovery  of  the  silver.  Of  the  two  processes  used,  the  Pattinson  and 
the  Parkes  processes,  the  latter  has  in  recent  years  become  the  more 

1  D.R.P.  126,839. 


302    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY  [CHAP. 

important.  Both  involve  a  considerable  fuel  consumption,  and  can- 
not be  economically  worked  on  a  small  scale.  In  the  Parkes  process 
there  is  a  further  continual  loss  of  zinc.  In  neither  case  is  the  bismuth 
eliminated,  and  the  resulting  product  is  not  as  suitable  for  the  manu- 
facture of  lead  colours — white  lead,  chrome  yellow — as  a  bismuth-free 
lead  would  be.  A  simple  and  efficient  electrolytic  refining  process, 
taking  into  account  the  high  electrochemical  equivalent  of  lead,  would 
seem  to  be  offered  an  excellent  field. 

There  are  two  difficulties  to  be  overcome.  Firstly,  electrolytes 
containing  the  anions  commonly  used  in  electrolytic  refining — Cl', 
N03',  SO/'— cannot  be  employed.  PbCl2  and  PbS04  are  too  insoluble. 
NOs'  ions  would  bring  any  anode  silver  into  solution  unless  the  current 
density  were  exceedingly  low,  and  this  would  be  subsequently  catho- 
dically  deposited ;  further,  metallic  lead  chemically  reduces  NOg'toNO/, 
the  solution  becoming  yellow,  and  under  certain  conditions  complex 
nitrites  settling  out.  Secondly,  lead  is  not  readily  deposited  cathodically 
in  compact  form,  but  gives  crystalline  growths  which — apart  from  the 
inconvenience  of  handling — spread  towards  the  anode  and  cause  short 
circuits.1  Only  by  using  exceedingly  concentrated  Pb(N03)2  or  PbAc2 
solutions  could  Glaser2  get  a  coherent  lead  deposit.  The  only  early 
process  which  ever  promised  anything  was  that  of  Keith,  whose  elec- 
trolyte was  a  solution  of  PbS04  in  NaAc.  The  cathodic  lead  was 
practically  pure  except  for  bismuth,  but  was  crystalline,  and  tended 
to  continually  form  short  circuits.  The  process  proved  unable  to 
compete  with  the  Parkes  process. 

Betts  Process.3— In  1902  Betts  proposed  the  use  of  a  PbSiF6  solution 
as  electrolyte,  with  the  addition  of  a  trace  of  gelatine.  The  resulting 
lead  is  compact,  there  is  little  or  no  trouble  from  crystalline  growths, 
and  all  impurities  present  in  appreciable  quantity  enter  the  slimes. 
The  power  cost  is  low,  and  certain  difficulties  encountered  in  working 
up  the  slimes  have  now  been  partly  overcome.  This  process  has 
been  working  on  a  large  scale  for  years  in  America  and  England, 
and  must  be  regarded  as  a  serious  competitor  of  the  older  methods 
of  refining. 

Senn  *  has  made  a  laboratory  study  of  the  process.  He  first  showed 
that  the  nature  of  the  electrolyte  is  essentially  important.5  Using  a 
solution  roiitjiiniii.il  S-.'J  per  cent,  lead  and  Jlv  per  cent.  H2SiF6,  and 


1  Cl.  p.  Ui.T. 

2  Zeitech.  KlektrocJiew .  7,  :{iir, 

',,/,,„•/„,„.  /„,/.  1,  KIT  (/w;;);   M,-I«U.  6,  2M  (1!.W). 

4  Zt  it. *,-!,.    /-'My,-,,,-/// ///.    11,  oo«)   (IMi:,). 

5  Mathers  (Trans.  Am.fr.  Electrochem.  Soc.  17,  261  (W10),  has  slx.un  an  arid 
solution  of  Pb(ClO4).j  to  be  also  a  very  .suitable  electrolyte-.     It  has,  moreover,  a 
higher  conductivity  than  has  the  a«  -i<l  I 'USi  !•',..     Then-  is  ;l  project  that  it  will  als,, 
shortly  come  into  technical  use. 


xvm.]  LEAD  303 

a  current  density  of  T07  amps. /dm.2,  a  good  coherent  lead  deposit, 
tending  merely  to  crystal  formation  at  the  edges,  resulted.  With 
1-72  amps,  /dm.2  the  product  was  highly  crystalline.  On  adding  gelatine 

(Ol  ^      -   gave  the  maximum  effect),  the  higher  current  density  fur- 
litre 

nished  a  coherent  though  crystalline  deposit  with  a  few  needles  at  the 
edges  only  ;  and  with  T07  amps,  /dm.2,  a  fine  silky  deposit  with  no  trace 
of  crystalline  growths  resulted.  Using  Pb(N03)2  or  PbAc2,  coherent, 
crystalline  and  very  brittle  deposits  were  given  after  adding  gelatine, 
but  the  latter  salt  required  far  more.  Both  electrolyte  and  gelatine, 
therefore,  assist  in  the  favourable  results.  Temperature  seemed  of 
little  influence.  And,  provided  that  the  total  SiF6  content  of  the  elec- 
trolyte remained  constant,  the  lead  concentration  could  be  considerably 
lowered  without  affecting  the  results.  The  cathodic  current  efficiency 
was  about  98  per  cent.,  and  the  anode  dissolved  quantitatively  as 
Pb"  ions.1  The  anodic  loss,  indeed,  exceeded  100  per  cent.,  on  account 
of  chemical  solution  of  the  lead  in  the  free  acid. 

Senn  finally  showed  that  a  lead  anode,  saturated  with  copper  or 
containing  20  per  cent.  Bi,  could  be  dissolved  at  0'5-1'5  amps,  /dm.2, 
these  metals  remaining  in  the  slimes.  With  10  per  cent.  Sb  present, 
1  amp. /dm.2  could  be  safely  used.  At  1/5  amp. /dm.2  the  antimony 
dissolves  and  is  cathodically  deposited.  With  a  normal  anodic  con- 
tent of  any  of  these  metals  no  appreciable  quantity  should  be  found 
in  the  cathodic  lead. 

Technically  the  electrolysis  takes  place  in  large  tanks  of  tarred 
\vood  or  cement.  Anodes  and  cathodes  are  suspended  alternately  and 
connected  as  in  the  multiple  system  of  copper  refining.  The  anodes  are 
wedge-shaped,  thicker  at  the  top  than  at  the  bottom.  Their  average 
composition  (mean  of  ten  analyses)  is  Pb  98' 1  ;  Ag  0'61  ;  Sb  0'7  ; 
Cu  0-23  ;  As  0*19  ;  Bi  O'l ;  Sn  0'03  ;  Fe  O'Ol  ;  Au  0'007.  They  are 
provided  with  lugs  which  rest  on  the  positive  busbars.  The  cathodes 
(formerly  prepared  by  electrolysis  in  a  separate  tank)  are  now  made  by 
pouring  out  a  thin  sheet  of  refined  molten  lead  on  to  an  iron  plate. 
They  are  wrapped  round  copper  rods  which  hang  on  the  negative  bus- 
bars. The  composition  of  the  electrolyte  can  vary  considerably. 
An  average  liquor  initially  contains  70-80  grains  lead  (as  PbSiF6)  and 
100-110  grams  free  H2SiF6  per  litre.  It  is  prepared  by  acting  on 
fluor-spar  and  silica  with  H2S04,  and  adding  white  lead  to  the  H2SiF6 
solution  produced.  The  free  acid  keeps  back  the  hydrolysis  of  the  SiF6" 
ions  and  increases  the  conductivity ;  0*1-0'2  per  cent,  gelatine  or  glue  is 
present.  The  specific  resistance  is  3'6  ohms /cm.3.  The  liquor  is 
circulated  by  gravity.  The  fall  in  level  per  tank  is  slight,  and  contact 


1  See  also  Elbs  and  Niibling,  Zeitsch.  Elektrochem.  9,  781  (1903). 


304    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY  [CHAP. 

with  air  during  circulation  is  avoided  as  far  as  possible,  as  dissolved 
oxygen  attacks  the  antimony  in  the  slimes,  and  when  that  metal  is  once 
in  solution  it  is  cathodically  deposited. 

During  the  electrolysis  the  electrolyte  deteriorates.  The  two  most 
important  changes  are  the  disappearance  of  free  acid  and  of  glue.  The 
former  change  is  caused  by  (a)  the  discharge  of  H'  ions  at  the  cathode, 
and  (6)  the  lead  dissolving  chemically,  whereby  its  concentration  in  the 
electrolyte  rises.  Regular  addition  of  fresh  H2SiF6  (6-12  Ibs.  per  ton 
of  lead  produced)  is  necessary.  Glue  is  consumed  at  the  rate  of  J— 1  Ib. 
per  ton  of  cathodic  lead,  and  must  also  be  continually  renewed.  Most  of 
it  is  cathodically  precipitated  with  the  metal  or  adsorbed  in  the  slimes. 
Other  changes  are  the  production  of  HF  by  the  hydrolysis  of  SiF6" 
ions  with  formation  of  hydrated  silica,  and  the  slow  accumulation  of 
iron,  zinc,  antimony,  etc. 

The  current  density  is  17-20  amps. /dm.2  The  working  voltage 
(including  that  lost  in  leads)  is  about  0*28  volt  with  fresh  anodes,  but 
gradually  rises  to  0*40  volt,  owing  to  the  increasing  resistance  of  the 
slimes.  The  actual  polarisation  voltage  is  only  about  0'02  volt  per 
tank.  The  working  temperature,  kept  up  by  the  current,  is  30°-35°. 
Anodes  and  cathodes  are  in  the  bath  for  about  eight  days.  At  the  end 
of  that  time  the  anodes  still  contain  about  25  per  cent,  of  unattacked 
material  (including  lugs).  This  proportion  is  greater  than  in  the  case 
of  copper  refining  (10-15  per  cent.).  Their  remelting  is,  however,  easy, 
and  further  electrolysis  means  constantly  rising  voltage,  due  to  the 
increasing  thickness  of  slimes.  During  the  electrolysis,  any  anodic 
zinc,  nickel,  or  cobalt,  together  with  some  of  the  iron  and  tin,  dissolve. 
None  of  these  accumulates  sufficiently  to  deposit  appreciably  on  the 
cathode.  We  have  also  seen  that  dissolved  oxygen  can  cause  anti- 
mony from  the  slimes  to  enter  the  electrolyte  and  appear  in  the  cathodic 
deposit. 

The  refined  lead  is  very  pure.  An  average  analysis  of  eight  samples 
gives  Pb  99-996  ;  Fe  0-0013  ;  Sb  0-0006  ;  Cu  0-0005  ;  Ag  O'OOOl  ;  Sn 
0*0001  ;  As  trace  ;  Zn,  Bi,  Ni,  etc.,  absent.  The  cathodic  current 
efficiency  averages  about  90  per  cent,  at  the  current  densities  quoted. 
By  altering  the  latter,  it  can  easily  rise  to  95  per  cent,  or  fall  to  85  per 
cent.  The  loss  is  due  to  slow  H'  discharge  and  to  chemical  solution  of 
the  lead.  Taking  the  average  cell  voltage  as  0'34,  we  find  that  one 
ton  of  lead  requires 

96540  X  2  X  1000  X  1000  X  100  X  0'34  _ 
207-1  X  90  x  3606~xTOOO~ 

Treatment  of  Slimes. — The  greatest  difficulty  in  the  Betts  process 
has  been  the  treatment  of  the  slimes.  An  efficient  treatment  is  essential 
in  the  refining  of  a  material  like  lead,  where  the  demand  for  the  particu- 
larly pure  cathode  product  is  limited.  An  averago  sample  of  the 


XVIIL]  LEAD  305 

slimes  (which  do  not  fall  off,  but  retain  the  form  of  the  anode)  contains 
perhaps  25  per  cent.  Sb,  20  per  cent.  As,  10  per  cent.  Pb,  10  per  cent. 
Cu,  25  per  cent.  Ag,  5  per  cent.  Sn,  3  per  cent.  Bi,  1  per  cent.  Fe,  0*1 
per  cent.  Au.  But  besides  metallic  constituents,  hydrated  silica,  PbF2, 
and  large  quantities  of  admixed  fluo-silicate  solution  are  present.  The 
silica  and  PbF2  result  from  the  hydrolysis  of  the  SiF6"  ions  in  the 
electrolyte,  and  this  in  its  turn  is  caused  by  the  low  H*  concentration 
in  the  immediate  regions  of  the  anode  (due  to  the  migration  away  of 
positive  ions)  and  by  chemical  action  of  the  lead  on  the  dissolved 
acid.  The  deficiency  of  H'  ions  and  the  resulting  precipitation  of  non- 
conducting substances  both  contribute  towards  the  high  resistance  of 
the  slimes  which  we  have  already  noticed. 

The  slimes  are  first  carefully  washed  on  the  counter-current  prin- 
ciple, and  the  resulting  liquors  returned  to  the  electrolysis  tanks.  The 
subsequent  treatment  does  not  yet  appear  quite  settled.  Of  the  many 
methods  suggested,  we  will  briefly  note  the  two  following.  The  slimes, 
now  free  from  H2SiF6,  are  washed  with  alkali  and  boiled  with  a  6  per 
cent.  Na2S  solution  containing  free  sulphur.  The  antimony  dissolves, 
but  only  a  little  of  the  arsenic,  and  is  precipitated  electrochemically.1 
The  filtered  slimes  are  roasted.  Arsenic  is  driven  off  and  the  lead  enters 
the  slags.  The  product  is  then  extracted  with  hot  H2S04  in  presence 
of  air.  Silver  and  copper  dissolve,  the  silver  is  precipitated  on  copper 
strips,  and  copper  sulphate  subsequently  crystallised  out.  Further 
treatment  of  the  residue  with  H2S04  will  finally  leave  any  gold  un- 
dissolved.  Another  method  is  to  cupel  the  slimes,  after  washing  and 
drying,  in  a  basic  hearth  with  the  addition  of  soda.  As203  and  some 
Sb203  are  driven  off,  and  PbO  and  Sb203  are  found  in  the  slag  and 
worked  up  to  antimonial  lead.  Ag,  Bi,  Au,  Cu  and  Pb  remain  in  the 
metallic  mass,  which  is  apparently  treated  with  H2S04  as  in  the  first 
process  given. 

The  chief  advantages  of  the  Betts  process  do  not  consist  in  its  rela- 
tive cheapness — there  is  little  difference  here  between  it  and  the  Parkes 
process — but  in  the  fact  that  it  gives  a  lead  free  from  bismuth  and 
antimony,  allows  of  the  recovery  of  these  substances,  and  involves  a 
smaller  loss  in  lead  and  noble  metals.  On  a  small  scale,  as  has  been 
mentioned,  the  Betts  process  can  be  quite  easily  worked,  while  the  older 
processes  fail. 

Electrolytic  Extraction  of  Lead.— Whilst,  however,  electrolytic 
methods  can  be  successfully  applied  to  lead  refining,  it  is  very  doubtful 
whether  they  can  be  economically  used  for  the  extraction  of  the  metal 
from  its  ores.  The  pyrometallurgy  of  lead  is  simple  and  inexpensive, 
and  recent  improvements  have  reduced  the  silver  losses  to  a  very  low 
figure. 

i  P.  307. 


306    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY  [CHAP. 

The  Salom  Process1  for  the  electrolytic  reduction  of  galena  was 
nevertheless  worked  |or  some  years  on  a  large  scale  at  Niagara.  In 
the  final  form  it  took,  the  finely-powdered  galena,  which  must  be  of  high 
quality,  was  fed  continuously  into  a  ring-shaped  cell  of  antimonial 
lead  containing  a  10  per  cent.  H2S04  solution  as  electrolyte.  The 
base  and  sides  of  the  cell  slowly  revolved  and  constituted  the  cathode, 
whilst  the  anode  was  stationary  and  insulated  by  rubber  rings.  The 
cathodic  reaction  is  PbS  +  2H'  -  — >  Pb  +  H2S  +  20,  and  not 
chemical  reduction  by  liberated  nascent  hydrogen.2  The  lead  sponge 
formed  was  scraped  out  regularly  at  a  later  stage  in  the  revolution  of  the 
cathode,  and  H2S  passed  off  through  suitable  exits.  2'5-2'9  volts  were 
required  per  cell,  and  with  well-ground  material  a  current  efficiency  of 
70  per  cent,  was  obtainable.  The  power  expenditure  per  ton  of  lead 
sponge  was  thus  only  about  1,000  K.W.H.  The  lead  sponge  was  chiefly 
calcined  to  litharge,  and  it  also  was  intended  to  use  it  for  accumulators, 
though  for  this  purpose  it  would  doubtless  be  too  impure.  The  disad- 
vantages of  the  process  which  led  to  its  abandonment  were :  (a)  the 
necessity  of  using  a  very  pure  galena  ;  (b)  the  fine  preliminary  grinding 
required  ;  (c)  the  incomplete  reduction  obtained,  usually  only  95  per 
cent.  ;  (d)  difficulties  connected  with  leakage  of  the  H2S  ;  (e)  the  fineness 
of  division  of  the  product  rendering  it  impossible  to  melt  it  up  without 
losses.  The  first  is  particularly  vital. 

6.  Bismuth  and  Antimony 

In  conclusion  certain  processes  proposed  for  the  electrolytic  refining 
or  production  of  bismuth  and  antimony  will  be  briefly  noted.  Mohn 3 
has  described  experiments  on  the  refining  of  crude  bismuth  anodes  of 
the  following  composition — 94  per  cent.  Bi,  2'2  per  cent.  Pb,  3'1  per 
cent.  Ag,  0*5  per  cent.  Cu,  O'l  per  cent.  Sb,  O'l  per  cent.  Au.  The 
electrolyte  contained  7  per  cent.  Bi  as  BiCl3  and  9-10  per  cent,  free 
HC1.  Anodic  and  cathodic  current  density  were  about  2  amps. /dm.2 
and  6  amps./dm.2  respectively.  The  bath  took  T2  volts.  The  results 
were  not  very -satisfactory.  Lead,  antimony  and  copper,  and,  unless 
care  was  taken,  some  silver  also,  dissolved.  Of  these,  silver  at  once 
enters  the  cathodic  deposit,  .and  copper  and  antimony  can  only  be 
allowed  to  accumulate  in  the  electrolyte  to  a  small  extent.  After 
melting  up  with  nitre  and  soda  to  remove  any  arsenic  or  antimony,  a 
99'8  per  .cent,  pure  product,  containing  otherwise  only  silver,  resulted. 
According  to  Foerster  and  Schwabe,4  an  electrolyte  of  bismuth  fluo- 
silicate  promises  much  better  results.  The  single  potentials  shown 

1  Electrochem.  2nd.  1,  18  (lf)02);  Zeitsch.  Elektrochem.  9,380  (1903). 
3  Bernfeld,  Zeitsch.  Phys.  Chem.  25,  46  (1898). 

3  Electrochem.  2nd.  5,314  (l'.nfi). 

4  Zeitsch.  Elektrochem.  16,  279  (1910). 


XVIIL]  ANTIMONY  307 

by  silver,  lead  and  bismuth  against  solutions  of  their  fluosilicates  are 
far  apart,  and  a  good  separation  of  these  metals  should  thus  be  possible. 
Using,  in  fact,  a  crude  bismuth  anode  containing  silver  and  lead,  it  was 
found  that  the  silver  remained  undissolved  in  the  slimes,  the  lead 
accumulated  in  the  electrolyte,  and  a  very  pure  cathodic  bismuth 
resulted.  The  character  of  the  deposit  was  also  compact  and  dense. 
Antimony  has  been  prepared  by  Siemens  and  Halske  by  electrolytic 
deposition  from  alkaline  sulphantimonite  solutions.  The  product  is 
very  pure.  Such  a  liquor  with  3*5  per  cent.  Sb  also  results  from  one 
method  of  working  up  the  slimes  of  the  Betts  lead-refining  process.  It 
is  electrolysed  until  its  content  has  sunk  to  1  per  cent.,  a  current 
efficiency  of  45  per  cent,  being  obtained.  Betts  l  has  also  experimented 
with  acid  solutions  of  SbCl3  +  PeCl2  and  of  SbF3  +  FeS04.  At  the 
anode  ferric  salts  were  regenerated,  which  could  be  subsequently  used, 
for  example,  to  dissolve  up  more  antimony  or  antimony  compounds. 
At  the  cathode  a  pure  deposit  of  antimony  of  good  quality  resulted, 
provided  that  copper  and  arsenic  were  absent. 


Literature 

E.  Giinther.     Die  Darstellung  des  Zinks  auf  elektrolytischem  Wege. 
Betts.     Lead  Refining  by  Electrolysis. 

1  Trans.  Amer.  Electrochem.  Soc.  8,  187  (1'JOZ). 


CHAPTER  XIX 

ELECTROPLATING    AND    ELECTROTYPING 

Electroplating  is  the  art  of  covering  a  metallic  surface  by  electro- 
deposition  with  an  adherent  coating  of  some  other  metal,  the  form  of 
the  original  surface  being  fully  retained.  The  second  metal  may  be 
deposited  for  decorative  purposes,  or  because  of  its  superior  resistance 
to  chemical  and  atmospheric  influences.  Electrotyping  is  the  art  of 
reproducing  the  form  of  an  object  by  electro-deposition  on  a  cast  or 
negative. 

Electroplating — General  Considerations.— The  conditions  govern- 
ing the  cathodic  deposition  of  metals,  the  nature  of  the  deposit,  and 
related  phenomena,  have  been  already  considered,1  and  to  these  dis- 
cussions the  reader  is  referred.  Special  attention  should  be  paid  to  the 
paragraphs  dealing  with  irreversible  effects,  hydrogen  overvoltage, 
concentration  polarisation,  depolarisation  by  deposition  as  alloy,  effect 
of  H'  and  metallion  concentration,  presence  of  organic  addition  agents, 
current  density,  deposition  from  complex  salts,  etc.,  etc. 

Certain  points  should  be  particularly  emphasised  here.  The  first 
essential  in  an  electrogalvanic  deposit  is  that  it  should  be  continuous, 
regular,  and  should  adhere  perfectly  to  the  object  plated.  The  latter 
is  therefore  always  first  very  thoroughly  cleaned  by  methods  presently 
described.  But  another  condition  must  be  satisfied.  If  the  metal  to 
be  plated  and  the  deposited  metal  do  not  cohere  well,  or  if  their  co- 
efficients of  expansion  are  somewhat  different,  then  the  deposit  will  not 
adhere  to  the  plated  object,  but  will  easily  flake  off. 

This,  however,  is  not  so  if  the  metals  have  any  tendency  to  alloy 
together.  Under  such  circumstances  the  electro-deposition  of  the 
plating  metal  is  depolarised  by  the  plated  metal,  and  the  first  thin 
layer  of  the  former  is  deposited,  not  as  pure  metal,  but  as  an  alloy  with 
the  material  of  the  object  treated.  As  electro-deposition  continues  the 
alloy  becomes  gradually  richer  in  the  deposited  metal,  until  it  finally 
contains  no  admixture  of  the  other  constituent.  This  layer  of  alloy  is 

1  Pp.  116,  118-120.  Varioua  points  have  also  been  considered  in  the  last  two 
chapters. 

308 


ELECTROPLATING  309 

extremely  thin,  but  nevertheless  the  increase  in  cohesion  is  sufficiently 
great  and  the  change  of  coefficient  of  expansion  sufficiently  gradual  to 
result  in  a  strong  and  adhesive  deposit.1  Haber 2  has  suggested  that 
this  alloying  is  essential  if  the  deposit  is  to  adhere.  Nickel  does  not 
deposit  satisfactorily  on  either  tin,  zinc,  or  lead  unless  a  preliminary 
coating  of  copper  or  brass  is  plated  on,  and  the  very  frequent  use 
of  intermediate  coatings  of  these  substances  in  electroplating  is 
accounted  for  by  their  strong  alloying  tendencies. 

Another  point  is  that  non-adhesive  deposits  often  result  if,  when 
the  object  is  placed  in  the  bath,  the  process  is  not  immediately  started, 
or  if  the  electrolysis  is  interrupted  before  its  completion.  The  cause 
is  the  formation  of  a  thin  oxide  layer  on  the  clean  surface.  This 
phenomenon  is,  of  course,  only  noticeable  with  metals  more  electro- 
positive than  hydrogen.  In  the  case  of  aluminium,  it  renders  electro- 
plating exceedingly  difficult. 

As  a  galvanic  deposit  must  be  continuous,  and  usually  capable  of 
a  high  polish,  its  crystalline  structure  should  be  as  fine  as  possible. 
This  explains  the  frequent  use  of  baths  containing  the  plating  metal 
as  complex  anion.  We  have  seen 3  how  such  solutions  generally  furnish 
compact  finely-grained  deposits. 

Technical.4 — The  first  process  in  electroplating  is  the  cleaning  of 
the  surface  to  be  treated.  If  coated  with  rust  or  oxide,  and  if  of  a 
convenient  size,  a  preliminary  cleaning  is  given  by  a  sandblast,  wire 
brushes,  or  other  suitable  arrangement.  Grease  is  carefully  removed  by 
dipping  into  a  hot  alkaline  solution.  ^Then  the  last  traces  of  oxide  are 
dissolved  and  a  bright  metallic  surface  produced  by  means  of  a  '  pickling ' 
solution,  naturally  varying  with  the  metal  treated.  Acid  solutions— 
HOI,  HN03,  H2S04 — are  usually  employed.  The  details  we  need  not 
consider.  Occasionally  the  article  (particularly  if  of  iron  or  steel)  is 
cleaned  by  making  it  cathode  in  a  bath  containing  H2S04  or  HC1.  In 
both  the  chemical  and  electrochemical  treatment  with  acid,  the  gas 
evolution  removes  much  of  the  scale  or  oxide  mechanically.  The 
object  is  finally  thoroughly  washed  with  water,  and  introduced  into 
the  plating  bath,  the  current  being  usually  immediately  switched  in 
(see  above). 

The  plating  tanks  are  generally  wooden  lead-lined  vessels  of  con- 
venient size,  though  the  smaller  ones  are  often  constructed  of  glazed 
earthenware.  As  the  objects  are  continually  removed  and  replaced, 
an  arrangement  of  the  different  tanks  in  series  would  prove  very 
inconvenient.  They  are  therefore  connected  in  parallel,  and  fed  by 

1  This  can  be  otherwise  when  two  metals  are  deposited  together  irregularly. 
See  p.  294. 

-  Grundriss  der  technischen  Elektrochemie,  p.  280  (1898). 

:<  P.  125. 

4  Metatt.  Chem.  Engin.  8,  274  (1910);  Pfanhauser,  Metall  1,  313  (1904). 


310    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY  [CHAP. 

a  low-voltage  dynamo  capable  of  giving  a  large  current.  Each  tank 
should  be  independently  provided  with  ammeter,  voltmeter,  and 
regulating  resistance.  The  objects  under  treatment  (cathodes)  are 
usually  suspended  by  hooks  from  copper  rods  laid  across  the  top  of  the 
tank,  these  being  in  turn  connected  with  the  negative  pole  of  the  source 
of  current.  The  anodes  are  of  purest  sheet  metal,  if  necessary  enclosed 
in  linen  or  parchment  bags  to  retain  the  slimes.  Occasionally,  where 
the  employment  of  an  anode  of  the  plating  metal  (e.g.  of  gold) 
would  prove  too  expensive,  an  insoluble  anode  is  used. 

Anodes  and  cathodes  are  as  far  as  possible  arranged  symmetrically, 
and  so  as  to  occupy  the  whole  cross-section  of  the  bath.  To  ensure  a 
uniform  deposit  on  an  article  of  irregular  shape,  auxiliary  anodes 
placed  in  special  positions  are  sometimes  employed.  Small  objects, 
such  as  nails,  etc.,  are  filled  into  suitable  baskets  or  drums,  which  are 
immersed  in  the  bath.  In  other  cases — e.g.  the  galvanising  of  wire — 
special  arrangements  are  adopted. 

The  current  density  is  as  high  as  is  consistent  with  the  furnishing 
of  a  satisfactory  deposit.  It  varies  (with  the  plating  bath  and  the 
temperature)  between  0*1-2*0  amps. /dm.2.  The  thickness  of  deposit 
given  is  usually  very  small,  thousandths  of  a  mm.  for  cheap  articles, 
and  rarely  exceeds  a  few  tenths  of  a  mm.  At  the  end  of  the  operation 
the  articles  are  removed  from  the  bath,  dried,  and  in  most  cases  finally 
polished.  The  utmost  cleanliness  is  necessary  throughout  all  the 
operations. 

Zinc-plating  (Wet  Galvanising). — As  is  known,  many  iron  objects 
are  coated  with  zinc  to  protect  them  against  rust — e.g.  roofing,  fencing, 
wire,  etc.  This  is  at  present  effected  either  by  dipping  the  cleaned 
article  in  a  molten  zinc  bath,  or  by  electro-deposition.  The  former 
method  (hot  galvanising/)  gives  a  more  brilliant  coat,  but  possesses 
several  disadvantages.  It  is  wasteful,  forming  deposits  of  high-melting 
iron-zinc  alloy  in  the  bath  ;  it  gives  a  coat  th"  thickness  of  which  is  not 
easily  controllable  and  which  is  often  irregular,  and  its  mechanical  and 
resistive  properties  fall  short  of  those  of  the  electro-deposit.1  As 
a  consequence,  the  method  of  electro-,  cold,  or  wet  galvanising  wins  more 
and  more  favour. 

The  conditions  favouring  the  formation  of  a  good  zinc  deposit  at 
high  current  efficiency  are  set  out  on  p.  283.  In  accordance  with 
these  conditions,  we  find  that  Langbein  recommends  a  bath  containing 
per  hundred  litn-s  : 

20  kilos.  ZnS04,  7H20, 
4  kilos.  Na-sSO,.  1<>ILO, 
1  kilo.  ZnCl2, 

0-r>  kilo,  boric  ,-icid  crystals. 

.     A'/'/  //'or //i/;/.     /W.   3,    17    (1!><>;>). 


xix.]  ELECTROPLATING  311 

This  can  be  taken  as  typical  of  other  solutions.  The  sodium  sulphate 
increases  the  conductivity  ;  the  boric  acid  gives  the  required  acidity, 
which  is  kept  constant  by  the  continuous  regulated  addition  of 
dilute  H2S04.  The  current  density  used  is  generally  about  50-100 

—  —  -,  as  the  electrolyte  is  not  very  efficiently  circulated.     Sometimes 
metre2 

it  rises  to  200  -   —  •     The  bath  is  worked  at  room  temperature  or 
metre2 

at  40°-50°  (particularly  with  objects  of  a  highly-developed  surface). 
3-6  volts  are  used,  depending  on  the  conductivity  of  the  electrolyte 
(always  rather  low),  temperature,  and  current  density. 

Nickel-plating.  —  The  mechanical  strength  and  chemical  resistivity 
of  this  metal,  together  with  its  capability  of  taking  a  high  polish,  have 
resulted  in  an  enormous  application  in  the  electroplating  industry.  It 
can  be  used  for  coating  practically  all  the  cheaper  metals.  Except 
with  iron  and  steel,  to  which  nickel  adheres  if  the  object  is  thoroughly 
clean,  a  preliminary  copper  coating  is  first  deposited. 

The  cathodic  and  anodic  behaviour  of  nickel  has  been  discussed  on 
pp.  292-294.  A  thin  deposit  of  nickel  which  does  not  flake  off  is 
obtainable  at  room  temperature  if  the  nickel  content  of  the  solution 
is  high,  the  acid  content  low,  and  the  current  density  not  too  great. 
To  get  satisfactory  thicker  deposits  under  ordinary  conditions,  the  tem- 
perature must  be  raised  to  about  70°,  when  higher  current  densities  can 
be  employed.  Nickel  anodes  readily  become  passive  in  most  electrolytes 
(not  when  chlorides  are  present)  unless  the  current  density  is  kept  very 
low.  But  their  behaviour  also  depends  largely  on  their  method  of  pre- 
paration. Both  rolled  and  electrolytic  nickel,  particularly  the  latter, 
dissolve  much  less  readily  —  i.e.  become  passive  sooner  —  than  cast  nickel. 

As  most  nickel  electro-deposits  are  thin,  the  nickel-plating  bath  is 

generally  run  at  room  temperature.      The  nickel,  about  10       ---  , 

litre 


is  present  as  sulphate.  A  large  quantity  of  (NHi^SC^  is  usually  added 
to  increase  the  conductivity,  and  the  bath  is  acidified  with  citric  acid 
and  kept  acid  with  H2S04  just  as  is  the  case  with  zinc.  The  cathodic 
current  density  is  generally  low,  0'3-0'6  amp./dm.2.  In  some  cases 
1  amp./dm.2  can  be  used.  At  the  anode  the  figure  is  still  lower. 
The  voltage  varies  between  l'8-3'5  volt.  For  thicker  deposits, 
although  special  electrolytes  have  been  suggested  for  use  at  ordinary 
temperatures,  it  is  better  to  work  at  higher  temperatures.  The  bath 
must  be  much  richer  in  nickel,  but  large  current  densities  can  be  em- 
ployed. The  current  efficiency  1  of  cathodic  nickel  deposition  seldom 
exceeds  90  per  cent.,  owing  to  hydrogen  evolution.  For  the  same  reason, 
both  the  acid  and  nickel  concentrations  of  the  electrolyte  alter. 

1  Brown,  Tram.  Amer.  Electrochem.  Soc.  4,  83  (1903). 


312    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY  [CHAP. 

Langbein  has  overcome  this  difficulty  by  utilising  the  above- 
mentioned  differences  in  ease  of  solution  of  different  varieties  of 
nickel  anodes.  If  all  anodes  are  of  cast  nickel,  which  does  not 
readily  become  passive,  then  the  nickel  concentration  of  the  bath 
will  steadily  increase,  and  the  acid  concentration  decrease.  If  all 
consist  of  rolled  nickel,  only  a  small  fraction  of  metal  will  dissolve 
anodically,  large  quantities  of  acid  being  formed  instead  with  libera- 
tion of  oxygen.  By  using  suitable  numbers  of  cast  and  rolled  anodes 
the  composition  of  the  bath  can  be  regulated,  and  the  continual 
addition  of  acid  or  nickel  salts  avoided. 

Copper-plating.— This  process  is  usually  only  an  intermediate  stage 
prior  to  the  electro-deposition  of  other  metals.  Occasionally  it  is 
carried  out  for  its  own  sake,  in  which  case  the  article  is  subsequently 
dipped  into  an  alkaline  polysulphide  bath,  in  order  to  produce  the  film 
of  Cu2S  known  as  '  oxidised  copper/  A  bath  of  potassium  cupro- 
cyanide  is  almost  invariably  used.  It  is  prepared  from  KCN  and 
some  cupric  salt,  Na2S03  being  added  to  reduce  the  copper  to  the 
cuprous  condition,  and  to  avoid  the  production  of  poisonous  cyanogen 
gas  which  otherwise  occurs. 

Spitzer1  has  investigated  the  electro-deposition  of  copper  from 
cyanide  solutions  at  room  temperature.  He  showed  that  an  irreversible 
effect  (probably  connected  with  a  slow  rate  of  the  reaction  Cu(CN)2/ 
— *•  2CN'  -f-  Cu') 2  is  present,  the  polarisation  needed  for  copper 
deposition  considerably  exceeding  the  equilibrium  value.  Thus,  with 
a  solution  of  O'l  n.  CuCN  -f-  0*2  n.  KCN  (which  corresponds  closely 
with  the  technical  electrolyte),  whilst  the  equilibrium  value  for  a  copper 
electrode  was  —  O61  volt,  the  potential  was  —  O77  volt  at  a  current 
density  of  O001  amp. /cm.2  and  —  1*12  volts  at  O003  amp. /cm.2. 
The  hydrogen  equilibrium  potential  in  such  a  solution  is  —  0*7  volt,  and 
the  hydrogen  overvoltage  at  a  copper  surface  at  these  low  current 
densities  about  0' 2-0*3  volt.  We  see  therefore  that  even  at  low  current 
densities  large  quantities  of  hydrogen  must  be  evolved  simultaneously 
with  the  copper  deposition.  In  fact,  Spitzer  found  that  as  the  current 
density  rose  from  O'OOl  to  O02  amp. /cm.2,  the  current  efficiency  fell 
from  58  per  cent,  to  10*6  per  cent.  With  excess  of  KCN  still  worse 
results  were  obtained,  in  respect  both  to  voltage  and  current  efficiency. 
The  experience  of  the  electroplater  corresponds  fully  with  these  results. 
Although  the  current  density  is  low  (0*3  amp.  /dm.2),  yet  the  deposition 
of  copper  is  accompanied  by  a  violent  gas  evolution,  and  the  voltage 
necessary  is  3  volts. 

Brass-plating.— This  operation  also  is  chiefly  carried  out  to  provide 
a  surface  to  which  other  electro-deposited  metals  can  firmly  adhere. 
A  number  of  motor-car  fittings  are,  however,  electrolytically  brassed. 

11.  :M.r,  ( !!>(>.',}.  -  S<-<-  j».    ]•_><>. 


XIX.]  ELECTROPLATING  313 

The  electrolyte  contains  a  mixture  of  K2Zn(CN)4  and  KCu(CN)2. 
As  the  deposit  contains  about  20  per  cent.  Zn  :  80  per  cent.  Cu,  the 
brass  anode  used  should  have  the  same  composition.  In  sulphate 
solutions  the  potentials  of  zinc  and  copper  are  far  apart,  a  fact  on  which 
the  E.M.F.  of  the  Daniell  cell  depends.  But  in  cyanide  solutions 
they  lie  far  closer  together,1  and,  if  a  considerable  excess  of  cyanide 
be  present,  the  zinc  becomes  nobler  than  the  copper.  In  practice  this 
last  factor  does  not  come  into  play,  as  a  large  excess  of  cyanide  is 
avoided. 

But  two  other  circumstances  bring  the  zinc  and  copper  discharge 
potentials  near  to  one  another,  and  thus  render  possible  the  cathodic 
deposition  of  brass.  The  first  is  that  the  irreversible  polarisation2 
necessary  for  the  discharge  of  these  metals  from  cyanide  solution 
increases  with  current  density  far  more  rapidly  with  copper  than  with 
zinc,  and  even  at  quite  low  current  densities  (0'003  amp.  /cm.2)  the 
potentials  are  already  very  close  together.  Secondly,  Zn"  discharge 
is  depolarised  by  the  copper  content  of  the  cathode  in  virtue  of  the 
mutual  tendency  to  alloy  formation,  and  zinc  is  therefore  deposited  at 
a  polarisation  below  that  necessary  in  the  absence  of  copper.  With  a 
normal  current  density  of  0'003  amp.  /cm.2,  with  an  equal  number  of 
gram-atoms  of  copper  and  zinc  in  solution,  and  with  no  great  excess 
of  KCN,  the  brass  produced  contains  20  per  cent.  Zn.  Excess  of 
KCN  causes  a  greater  hydrogen  evolution  and  a  decreased  current 
efficiency.  Too  little  KCN  increases  the  distance  between  the  zinc 
and  copper  potentials,  and  diminishes  the  proportion  of  the  former 
metal  deposited. 

The  electro-deposition  of  brass  has  also  been  studied  by  Field,3 
who  paid  particular  attention  to  current  efficiency  and  the  composition 
of  the  cathodic  deposit,  investigating  the  influence  of  such  factors  as 
excess  of  KCN  and  temperature. 

Silver-plating.  —  We  have  already  4  seen  in  what  form  silver  usually 
deposits  from  its  simple  salts.  As^a  silver  electro-deposit  must  essentially 
be  capable  of  polish,  a  cyanide  bath  is  always  used  in  electroplating. 
This  gives  a  fine  deposit,  sometimes  already  brilliant,  sometimes  milk- 
white,  which  can  easily  be  polished.  The  electrolyte  contains  8-20 

silver,  and  is  usually  prepared  from  AgCl  and   KCN  in  the 

* 


ratio  l[AgCl]  :  3-4[KCN].  The  fine  silver  anodes  readily  dissolve.  The 
current  density  is  0'1-0'25  amp./dm.2,  and  the  voltage  O'5-l'O  volt,  de- 
pending on  the  resistance  of  the  bath  and  the  current  density.  Very 
little  hydrogen  is  evolved,  the  current  efficiency  being  about  99  per 
cent.  Like  all  cyanide  baths  exposed  to  air,5  the  electrolyte  gradually 

1  See  p.  120.  '  See  p.  120. 

:f  Trans.  Farad.  Soc.  5,  172  (1909).  4  P.  266.  »  Cf.  p.  277. 


314    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY  [CHAP. 

deteriorates,  forming  carbonate,  ammonia,  etc.,  and  must  be  regularly 
renewed.  The  deposition  of  silver  from  cyanide  solutions  has  been 
studied  by  Foerster  and  Brunner.1  The  discharge  potentials  of  silver 
and  hydrogen  ions  lie  so  far  apart  that,  even  with  a  solution  low  in 
silver,  the  formation  of  hydrogen  is  only  to  be  feared  at  comparatively 
high  current  densities.  The  usual  small  deficit  in  the  cathodic  silver 
deposit  is  essentially  due  to  the  depolarisation  of  H'  discharge  by 
dissolved  air.  If  the  electrolyte  be  agitated,  the  loss  increases.  On 
the  other  hand,  excess  of  KCN,  by  increasing  the  alkalinity,  makes 
H*  discharge  so  difficult  that  in  a  solution  containing  l[AgCNJ  : 
40[KCN],  in  presence  of  air  and  with  stirring,  a  current  density  of 
2  amps./dm.2  can  be  used  without  appreciable  cathodic  loss  due  to 
hydrogen. 

Gold-plating.— A  cyanide  bath  is  also  used  hero,  prepared  by  the 
addition  of  KCN  to  some  suitable  gold  salt.  If  used  at  low  temperatures 

(for  the  gilding  of  large  objects),  it  may  contain  up  to  ten      c 

gold  as  KAu(CN)2 ;  at  temperatures  of  about  70°  (for  the  gilding  of 
smaller  objects),  when  diffusion  is  much  greater,  a  concentration  of  one 

-   gold  suffices.     The   KCN  added   is   about   five   to   six  times 
litre 

the  quantity  necessary  to  convert  the  gold  into  AuCN.  The  anodes 
are  generally  of  fine  gold,  but  graphite  can  also  be  used.  At  a  current 
density  of  O'OOl  amp. /cm.2,  or  a  little  higher,  a  fine,  uniform,  well- 
coloured  deposit  results.  The  voltage  varies  considerably,  depending 
on  the  arrangement  of  the  bath  and  the  exact  composition  of  the 
electrolyte.  Average  values,  using  a  gold  anode,  are  one  volt  for  a 
hot  and  four  volts  for  a  cold  bath.  With  a  graphite  anode  they  are 
naturally  higher.  The  gold  anode  is  sometimes  observed  to  become 
passive. 

Coehn  and  Jacobsen2  have  studied  the  anodic  and  cathodic  processes 
at  gold  electrodes  in  a  gold  cyanide  bath.  With  a  saturated  (14  per 
cent.)  KAu(CN)2  solution,  gold  deposition  took  place  at  a  potential 
0'2  volt  positive  to  that  of  hydrogen  in  the  same  solution.  As  technical 
current  densities  are  very  small,  and  as  gold  cyanide  solutions  show  no 
abnormal  cathodic  behaviour  like  cuprous  cyanide  solutions,  this 
explains  the  fact  that  little  or  no  hydrogen  evolution  takes  place  in  the 
plating  bath.  At  the  anode,  passivity  phenomena  were  observed, 
depending  in  a  curious  manner  on  current  density  and  KCN  concen- 
tration, and  fully  explaining  the  variations  noticed  in  practice.  With 
a  solution  containing  1-2  per  cent,  free  KCN  and  a  <  urrent  density  of 
0-0004-0-002  amp./cm.2,  the  gold  dissolved  quantitatively  as  AuCN. 

1  Zeitsch.  Elektrochem.  13,  r,t;i  (/.'/;). 
Anofg.  ('hem.  55,  821  (/W7). 


xix.]  ELECTROTYPING  315 

If  the  amount  of  KCN  were  decreased,  passivity  set  in,  accompanied 
by  gas  evolution.  If  it  were  increased  above  the  1-2  per  cent.,  brown 
or  yellow  colorations  due  to  polymerised  cyanogen  compounds  were 
produced  at  the  anode,  which  again  became  passive,  necessitating  an 
increased  anodic  polarisation  of  O'T-0'8  volt.  A  still  further  concen- 
tration increase  resulted  again  in  solution  of  the  gold. 

Platinum-plating.  —  Up  to  now  platinum-plating  has  been  little 
practised,  owing  to  the  difficulty  of  producing  satisfactory  deposits, 
but  the  possibility  of  making  platinum-coated  anodes,  crucibles, 
etc.,  etc.,  justifies  a  brief  mention  here.  Langbein  recommends  a 

(NH4)2PtCl6  bath,  containing  about  7  -  platinum,  together  with 


a  considerable  quantity  of  sodium  citrate.  A  certain  excess  of 
can  be  added  to  diminish  the  resistance.  Other  baths  are  prepared 
by  adding  ammonium  or  sodium  phosphates  to  an  H2PtCle  solution. 
Instead  of  citrates  or  phosphates,  Jordis  recommends  the  use  of  lactates. 
The  electrolysis  is  best  carried  out  at  80°-90°,  the  current  density 
being  O01  amp./dm.2.  As  the  anode  (whether  platinum  or  not)  is 
insoluble,  the  voltage  is  high  —  three  to  six  volts.  McCaughey  and 
Patten1  have  studied  platinum  deposition  from  solutions  containing 
citrates.  Their  work  makes  it  clear  that  the  problem  is  a  somewhat 
complex  one.  The  use  of  alkaline  platinite  solutions  might  perhaps 
give  satisfactory  results. 

Electrotyping.  —  Practically  all  electrotypes  consist  of  copper, 
though  sometimes,  when  the  surface  is  liable  to  much  wear,  they  are 
coated  with  a  thin  layer  of  iron.  An  acid  CuS04  bath  is  used,  similar 
to  that  employed  in  refining.  The  first  step  is  the  preparation  of  a 
negative  cast  of  the  original.  This  usually  consists  of  plaster  of  Paris 
or  of  wax  of  suitable  mechanical  properties,  and  great  care  must  natu- 
rally be  taken  that  it  is  in  all  respects  an  absolutely  true  one.  Its  surface 
is  then  well  brushed  over  with  graphite  to  render  it  conducting,  it  is 
attached  at  its  edge  to  a  copper  lead  connected  with  the  negative 
current  pole,  and  placed  in  the  bath.  This  contains  200  grams  CuS04, 
5H20  and  30  grams  H2S04  per  litre,  and  is  kept  at  room  temperature. 
The  anodes,  sheets  of  electrolytic  copper,  are  enclosed  in  linen  bags  to 
retain  traces  of  slimes.  The  current  density  is  about  2  amps.  /dm.2, 
and  the  bath  voltage  about  one  volt.  For  more  rapid  working,  the 
temperature  is  raised  and  the  electrolyte  stirred  by  vigorously  blowing 
in  air.  In  that  way  current  densities  of  5-8  amps.  /dm.2  can  be  em- 
ployed, at  the  cost  of  a  considerably  increased  bath  voltage.  When 
the  copper  positive  is  sufficiently  thick  and  rigid,  the  whole  is  taken 
from  the  bath,  the  negative  cast  detached,  and  the  electrotype  backed 
with  some  low-melting  lead  alloy  to  provide  the  necessary  strength. 

1  Trans.  Amer.  Electrochem.  8oc.  15,523  (1909);  17,  275  (1910). 


316    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY  [CHAP. 


An  electrolytic  iron  coating,  if  applied,  is  deposited  on  account  of 
its  hardness,  due,  as  we  have  seen,  to  dissolved  hydrogen.  Suitable 
conditions  are  those  under  which  Burgess  and  Hambuechen  worked.1 
Elmore  Process.  —  We  must,  in  conclusion,  describe  certain  electro- 
typing  processes  applied  to  a  particular  purpose  —  viz.  the  production 
of  seamless  copper  tubes.  The  best  known  is  that  of  Elmore,2  worked 
in  Leeds,  France,  and  Germany.  The  electrolyte  consists  of  acid 
CuS04  solution,  the  anodes  are  of  cast  copper,  at  least  98  per  cent. 
pure.  The  cathode  is  a  drum,  cylinder,  or  rod  of  the  required  diameter, 
usually  of  brass  or  of  coppered  cast  iron,  slowly  revolving  horizontally 
partly  within  and  partly  without  the  electrolyte.  To  facilitate  the 
removal  of  the  tube  subsequently  formed,  it  is  covered  with  a  thin 
layer  of  oil  and  graphite.  Contact  with  the  negative  current  pole  is 
made  by  brushes.  The  bar  anodes  are  arranged  symmetrically  in  a 
curved  wooden  frame  under  the  lower  surface  of  the  revolving  cathode 
(Fig.  67).  The  baths  are  run  at  room  temperature.  No  figures  have 
been  published  on  the  current  density.  It  and  the  bath  voltage  will 
probably  be  similar  to  those  used  in  the  ordinary  copper  electrotyping 
process  (2  amps.  /dm.2  at  the  cathode,  and  1-1*5  volts). 

The  particular  feature  of  the  process  is  the  means  used  to  produce  a 
dense  and  even  cathodic  deposit.  This  is  effected  by  causing  pieces  of 

agate,  pressed  on  the  cathode  sur- 
face by  springs,  to  travel  continu- 
ously from  one  end  of  the  cathode 
to  the  other  and  back  again.  This 
movement,  combined  with  the 
rotation  of  the  cathode,  leads  to 
all  parts  of  the  surface  of  the 
latter  being  regularly  and  fre- 
quently subjected  to  pressure,  and 
effectually  prevents  any  formation 
of  irregular  or  uneven  growths  on 
the  same.  To  subsequently  de- 
tach the  tubes  from  their  cores, 
they  are  gently  heated,  and 

worked  loose  by  pressure.  Their  great  tensile  strength  allows  them 
to  be  easily  drawn;  consequently  a  few  standard  sizes  only  snv 
directly  made  in  the  bath.  Tubes  up  to  five  feet  in  diameter  ;m<l 
sixteen  feet  in  length  have  been  produced. 

A  modification  of  this  process  due  to  Jullien  and  Dessolle  3  is  success- 
f  M  lly  at  work  in  France.  Besides  the  longitudinal  motion,  the  burnisher 
is  provided  with  a  superposed  alternating  movement  in  another  plane, 
and  better  effects  are  said  to  result. 


f      \ 


rvodes 

Fin.  07.— Elmore  Process. 


1  s'-«-  I'- 


BbeAwftem.  n 
•'''French  Patent  .'{«]!),  7-Mi 


.  3,  ir.o 


xix.]  ELECTROTYPING  317 

Cowper-Coles l  proposes  the  use  of  very  rapidly  revolving  cathodes 
and  higher  temperatures.  Working  in  this  way,  far  higher  current 
densities  can  be  employed,  and  the  friction  between  electrolyte 
and  cathode  nevertheless  ensures  an  excellent  deposit.  Although 
50-60  amps./dm.2  can  be  used,  this  renders  the  voltage  too  high. 
20  amps./dm.2  is  a  convenient  figure,  and  with  a  strongly  acid,  well- 
conducting  electrolyte,  the  bath  voltage  is  then  only  0'8  volt. 
Cowper-Coles  has  also  worked  out  a  very  ingenious  method  of 
producing  copper  wire  from  the  cylinders  and  tubes  which  result. 


Literature. 

Schlotter.     Galvanostegie,  vol.  i. 

1  Electr.  61,  680  (1908). 


CHAPTER  XX 

HYPOCHLORITES  AND  CHLORATES 
1.  General  Theory 

WHEN  aqueous  Nad  is  electrolysed  between  platinum  electrodes,  the 
primary  electrode  reactions  are  perfectly  simple.  E.P.  Na'  — >  Na 
is  —  2' 72  volts.  If  therefore  Na'  ions  are  to  be  discharged  cathodically 
and  reversibly  at  room  temperature  from  a  given  solution,  log[Na']  must 

2'72 

be  —      (about  47)  times  as  great  as  log[H'].     Putting  [H'J  in  a  neutral 

\)  v/OO 

aqueous  solution  at  O8  X  10~',  we  have  [Na']  =  1040.  It  is  therefore 
obvious  that  H'  ions  are  preferentially  discharged  even  from  a  very 
strongly  alkaline  solution,  and,  although  requiring  a  certain  over- 
voltage,  the  cathodic  hydrogen  production  is  primary,  and  not 
secondary,  as  is  often  assumed.1 

At  the  anode  the  possible  processes  are  the  discharge  of  OH7  and 
Or  ions.  For  the  oxygen  electrode  we  have  02  |  n.OH'  =  =  +  0'41 
volt.  Hence  reversible  oxygen  discharge  from  a  neutral  aqueous  solu- 
tion will  commence  at  an  anodic  potential  of  O41  —  0*058  log  0'8  X 
10~7  =  +  0*82  volt.  Anodic  oxygen  evolution,  however,  is  subject 
to  a  very  high  overvoltage,  which,  even  at  platinised  platinum,  rises 
to  nearly  one  volt  at  moderate  current  densities,  corresponding 
t<»  a  final  anodic  potential  of  -f  1/8  volts.  The  equilibrium  value 
<1.  :  W.C1'  is  +  1-S6  volts  at  250.  For  C12  |  n.NaCl  it  will  be  about 
-f  1*37  volts.  At  platinised  platinum,  Cl'  discharge  is  almost  rever- 
sible, and  consequently,  at  such  an  electrode,  chlorine  and  not  oxygen 
will  be  moie  easily  liberated.  At  polished  platinum  the  relations 
remain  unaltered,  increased  overvoltage  causing  in  each  case  an 
increased  polarisation  of  about  0*6  volt. 

The  electrolysis  will  thus  yield  hydrogen  and,  as  H'  ions  disappear, 
-in  alkaline  solution  at  the  cathode,  and  chlorine  at  the  anode-.  If  it 
be  carried  out  so  that  the  different  products  are  kept  separate  as  far 

1  If  the  Na'  diacharge  is  depolarised,  the  results  ean  of  eourse  be  different. 
See  pp.  121,  342. 

318 


HYPOCHLORITES  AND  CHLORATES       319 

as  possible,  we  have  an  electrolytic  alkali-chlorine  cell.  Such  cells  will 
be  discussed  in  the  next  chapter.  If,  on  the  other  hand,  the  products 
be  allowed  to  mix  and  react,  there  result,  according  to  the  conditions, 
NaCIO  solutions — bleaching  liquors — or  NaC103  solutions  (KC10  or 
KC103  if  KC1  be  the  starting  material).  The  study  of  these  conditions 
is  the  subject  of  the  present  chapter.  It  may  be  remarked  at  the 
outset  that  it  is  chiefly  due  to  the  exhaustive  work  of  Foerster  and  his 
pupils  that  our  knowledge  is  in  its  present  satisfactory  state. 

As  a  necessary  preliminary  we  must  consider  the  rather  complex 
chemical  reactions  involved.  When  a  halogen  reacts  with  a  solution 
containing  OH'  ions,  with  the  exception  of  fluorine,  which  directly 
decomposes  the  water,  liberating  oxygen,  the  following  equilibrium  is 
set  up  : 

X2  +  OH'^±X'4-HXO  (i) 

Halogen  Halogen 

oxy-acid 

Thus  chlorine  gives  Cl'  ions  and  HC10.  The  reaction  goes  more  com- 
pletely from  left  to  right  the  more  reactive  the  halogen — i.e.  the  greater 
its  ionising  tendency.  For  example,  a  saturated  aqueous  chlorine 
solution  is  about  30  per  cent,  hydrolysed  at  0°,  and  more  so  at  higher 
temperatures,  the  reaction  being  endothermic.  At  25°  the  equili- 
brium constant  K  =  -!  is  1-5  X  1Q-11.1  But  bromine 
LH01OJ  .  L^l  J 

water  only  contains  traces  of  HBrO,  and  in  an  iodine  solution  there  is 
no  perceptible  hydrolysis.  If  an  equivalent  of  alkali  be  present  per 
molecule  of  chlorine,  the  reaction  is  pretty  complete  in  dilute  solutions. 
In  more  concentrated  solutions  free  chlorine  and  alkali  can  be  detected. 
With  bromine  or  iodine  the  hydrolysis  is  far  less,  and  considerable 
quantities  of  either  halogen  can  exist  in  presence  of  free  alkali. 

Now,  as  one  of  the  resultants  of  the  above  equation  is  an  add,  the 
reaction  will  proceed  further  if  sufficient  OH'  ions  are  present.  We 
shall  have 

HXO  +  OH'  — >  XO'  +  H20,  (ii) 

taking  place  the  more  completely  the  stronger  the  acid.  If  the  latter 
is  a  so-called  '  strong  '  acid,  the  neutralisation  will  be  almost  complete, 
and,  as  reaction  (i)  mil  progress  in  consequence  of  the  removal  of  the 
HXO,  the  final  result  of  the  interaction  of  a  molecule  of  halogen  and 
two  of  alkali  can  be  written 

X2  +  2ROH  — >  RXO  +  RX  +  H2O  (iii) 

corresponding  to  the  well-known  equation  C12  +  2NaOH  — ->  NaCIO 
H-  NaCl  -f  H20.  This  last  equation,  in  fact,  nearly  represents  the  truth. 
The  amount  of  free  HC10  resulting  from  the  action  of  two  equivalents 
of  alkali  on  one  molecule  of  chlorine  is  quite  small.  \Vith  bromine 

1  Jakowkin,  Zeitsch.  Phys.  Chem.  29,  613 


320    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTEY  [CHAP. 

and  iodine  the  case  is  very  different.  Both  free  acid  and  free  halogen 
are  present,  in  greater  quantity  with  iodine  than  with  bromine.1  HIO 
is  so  weak  that  it  is  believed  it  can  also  ionise  to  I'  -f-  OH'  2  —  i.e.  that 
it  is  a  so-called  amphoteric  electrolyte. 

Further  reactions  are,  however,  possible.  A  hypohalogenite-halide 
mixture  does  not  represent  the  stablest  system  which  the  action  of 
halogen  on  alkali  can  furnish.  The  stablest,  as  a  matter  of  fact,  is 
oxygen  +  halide.  We  accordingly  find  that  hypochlorite  solutions 
slowly  decompose  thus  : 

2C10'  —  >2C1'  +02  (iv) 

•This  reaction  under  ordinary  conditions  is  exceedingly  slow,  but  is 
greatly  accelerated  by  small  quantities  of  cobalt  or  nickel  salts.  A 
well-known  method  of  preparing  oxygen  depends  on  this  fact.  More 
interesting  and  important  is  the  mechanism  of  production  of  HO/  ions 
in  hypohalogenite  solutions.  This  takes  place  as  follows  3  : 

2HXO  +  XO'  —  ->  X03'  +  2X'  +  2IT  (  v) 

Solutions  containing  CIO'  or  BrO'  ions  alone  are  stable,  as  also  are 
solutions  containing  free  HC10  or  HBrO  only.  When  mixed  they  react 
as  above.  The  H'  ions  liberated  produce  more  undissociated  HXO 
from  the  XO'  ions  present.  The  HXO  concentration  thus  remains 
fairly  constant  throughout,  whilst  that  of  the  XO'  decreases.  The 

velocity  constant  of  this  reaction  (k  in  the  equation  --  =  kfHXOJ2 

dt 

[XO'J)  is  0-0023  at  25°  for  X  =  Cl.  For  bromine  it  is  far  greater,  about 
0'25,  and  for  iodine  greater  still. 

If  we  also  notice  that  the  reaction  velocity  depends  on  the  square 
of  the  free  acid  concentration,  but  only  varies  directly  as  the  XO' 
concentration,  we  see  that  a  solution  prepared  from  definite  molar  con- 
centrations of  bromine  and  alkali  will  give  brornate  far  more  rapidly 
than  a  solution  containing  the  same  original  molar  concentrations  of 
chlorine  and  alkali  will  give  chlorate.  Not  only  is  the  velocity  constant 
greater  in  the  first  case,  but  HBrO,  being  weaker,  is  neutralised  to  a 
lesser  extent  than  HC10  by  the  alkali.  With  iodine  and  alkali,  the 
tendency  to  give  halogenate  is  still  stronger.  HIO  solutions  are  only 
stable  when  exceedingly  dilute.  Otherwise  the  reaction  2HIO  -f  10' 
—  >  I03'  -f  21'  +  2H'  commences  at  once,  the  very  small  10'  concen- 


1  Foerster  and  Gyr,  Zeitech.  Elektrochem.  9,  1 

-  Le  Blanc  has  shown  that  iodine  ran  <li>-  nlve  anodically  in  acid  solution  to  give 
HIO.-J,  a  far  stronger  acid  than  HIO.  The  primary  formation  of  1  .....  ions  is  to 
be  assumed  here. 

3  Foerster  and  Jorre,  Jour.  Prakt.  Chem.  59,  53  (  18MJ).  Foerster,  loc.  cit. 
63,  141  (HUH).  Kretzschmar,  Zeitsch.  Elektrochem.  10,  789  (1'JOl).  Foerster  and 
Gyr,  loc.  cit.  9,  6  (1W3). 


xx.]      HYPOCHLORITES  AND  CHLORATES        321 

tration  produced  by  the  OH'  ions  of  the  water  (HIO  -f  OH7  - 
10'  +  H20)  sufficing.  At  the  same  time  the  I'  ions  liberated  react 
with  the  HIO  (F  +  HIO  —  ->  I2  +  OH'),  the  equation  being  the 
reverse  of  equation  (i).  Thus,  except  when  very  dilute  or  in  presence 
of  a  large  excess  of  alkali,  an  HIO  solution  rapidly  decomposes,  giving 
iodine  and  HI03. 

Returning  to  the  case  of  chlorine,  it  has  been  seen  that  if  one  mole- 
cule only  of  halogen  reacts  with  two  equivalents  of  alkali  the  rate  of 
formation  of  chlorate  is  very  slow,  partly  owing  to  the  prevailing  low 
HC10  concentration.  The  equation  C12  +  2ROH  — >  RC1  +  RC10  + 
H20  represents  pretty  closely  the  final  stage.  If  now  a  small  extra 
amount  only  of  HC10  be  added,  the  rate  of  production  of  chlorate  will 
be  very  greatly  increased,  for  this  small  addition  may  mean  a  very 
large  relative  increase  in  the  HC10  concentration,  on  which  the  reaction 
velocity  essentially  depends.  In  practice  this  addition  can  be  effected 
by  using  rather  more  than  one  molecule  of  chlorine  to  two  equivalents 
of  alkali.  Fresh  HC10  is  formed  according  to  equation  (i),  there  is  no 
alkali  to  neutralise  it,  and  reaction  (v)  will  therefore  commence.  This 
explains  the  fact,  long  known  (Gay  Lussac),  that  an  excess  of  chlorine 
over  and  above  the  amount  demanded  by  the  usual  equation 

6KOH  +  3C12  — >  KC103  +  5KC1  +  3H20 

is  necessary  for  the  formation  of  chlorate  from  chlorine  and  alkali. 
For  the  thorough  comprehension  of  this  series  of  reactions,  and  of  the 
differences  shown  by  the  different  halogens,  this  equation  indeed  is 
quite  inadequate. 

There  is  one  other  way  in  which  hypochlorite  solutions  may  decom- 
pose, giving  rise  to  chlorate.  The  reaction  3C10'  — >  C103'  -f-  2C1' 
takes  place  with  very  low  velocity  at  ordinary  temperatures,  rather 
more  rapidly  with  increased  alkali  content  of  the  solution.  As  a  factor 
in  the  reaction  producing  chlorate  under  technical  conditions,  it  hardly 
needs  mention.  Even  at  higher  temperatures  it  is  quite  dwarfed  by 
other  reactions. 

2.  Hypochlorites — Theory 

It  has  been  shown  that  the  primary  products  of  the  electrolysis  of  a 
pure  neutral  NaCl  solution  are  hydrogen  and  NaOH  at  the  cathode, 
chlorine  at  the  anode.  For  every  molecule  of  halogen  evolved,  we 
obtain  two  equivalents  of  alkali.  If  the  anodic  and  cathodic  liquors  be 
continually  mixed,  the  chlorine  will  react  on  the  OH'  ions  according  to 
equations  (i)  and  (ii) x  thus  :— 

C12+OH'^±C1'     +  HC10 
HC10  +  OH'  ^±  CIO'  +  H20. 

1  P.  319. 


322    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY  [CHAP. 

When  conditions  have  become  constant,  the  actual  concentrations  of  the 
substances  involved  in  these  equilibria  will,  of  course,  differ  at  the 
two  electrodes.  At  the  anode,  owing  to  the  excess  of  chlorine,  there 
will  be  a  high  HC10  concentration ;  at  the  cathode  this  will  be  very 
low.  Hence  at  the  anode,  or  near  it, -the  direct  chemical  formation  of 
NaC103  according  to  the  equation  2HC10  +  CIO'  — >  C103'  +  2C1'  + 
2H*  is  possible.  The  better  the  circulation  of  the  liquid,  the  less  will 
this  reaction  take  place,  and  in  any  case  it  amounts  to  little  if  the 
working  temperature  be  low.  Under  favourable  conditions,  very  little 
chlorine  should  escape,  and  the  electrolysis  should  produce  NaCIO 
almost  quantitatively  according  to  the  total  equation — 

NaCl  +  H20  — *  NaCIO  +  H2, 

this  representing  the  result  of  the  passage  of  two  faradays  through 
the  cell. 

As  the  chemical  decomposition  of  neutral  hypochlorite  solutions  is 
very  slow,  this  process  could  proceed  to  the  formation  of  a  saturated 
solution  but  for  the  cathodic  and  anodic  behaviour  of  the  CIO'.  At 
the  cathode,  whatever  the  electrode  material,  it  is  very  readily  re- 
duced by  the  nascent  hydrogen  (2C10'  +  H2  — >  201'  +  H20).  Failing 
steps  to  prevent  this  reduction,  a  hypochlorite  concentration  will 
finally  be  reached  at  which  as  much  is  reduced  as  is  simultaneously 
re-formed  from  chlorine  and  alkali.  Its  yield  will  thus  fall  to  zero. 
This  reduction  can  be  avoided  by  adding  a  little  K2Cr04  to  the  electro- 
lyte. This  important  point  was  first  discovered  by  Imhoff,  and  the 
mechanism  of  its  reaction  recognised  by  Miiller.1  It  is  to  be  ascribed 
to  the  reduction  of  Cr04"  ions  to  Cr'"  ions  and  the  formation  of  a  thin 
diaphragm  of  insoluble  chromium  chromate  around  the  cathode.  This 
prevents  contact  of  the  bulk  of  the  solution  with  the  electrode,  and 


-0-08  -0-18  -0-28  -0-38  -0-48  -0-58  -0-68  -0-78  -0-88  Volt 
Cathode  Potential. 

FIG.  68. 

thus  with  the  nascent  hydrogen.  If  the  electrolyte  be  made  strongly 
acid  or  alkaline,  when  in  either  case  such  a  film  would  be  dissolved,  the 
addition  no  longer  works,  or  its  action  is  weakened. 

1  Zeitsch.  Elektrochem.  5,1469  (1899) ;  7,  398  (Ml) ;  8f  909  (1902). 


xx.]  HYPOCHLORITES  AND  CHLORATES  323 

The  current  potential  diagram  (Fig.  68)  clearly  shows  its  influence  on 
the  cathode  potential.  Curves  I  and  T  are  taken  without  the  addition 
of  chromate.  Hydrogen  evolution  is  depolarised  and  considerable 
currents  pass  at  a  lower  cathodic  polarisation  than  the  normal  one  for 
hydrogen  discharge  (the  dotted  line).  If,  when  the  point  a  is  reached 
on  curve  I,  some  chromate  be  added,  the  reduction  ceases,  the  current 
immediately  drops,  and  curve  II  is  followed.  Another  rise  in  the  curve 
only  commences  when  the  hydrogen  discharge  potential  is  exceeded. 
If  now  the  polarisation  be  gradually  lessened,  curve  III  is  followed 
continuously,  showing  that  depolarisation  effects  no  longer  enter.  As 
typical  of  the  effect  produced  by  the  chromate  addition  on  the  yield  we 
may  take  a  result  of  Miiller's.  Working  with  neutral  30  per  cent.  NaCl 
at  45°-50°  and  using  smooth  platinum  electrodes,  the  yield  of  active 
oxygen  was  only  32*8  per  cent.,  the  reduction  being  54*5  per  cent. 
With  the  addition  of  0*18  per  cent.  K2Cr04,  these  figures  became  69' 6 
per  cent,  and  3' 6  per  cent,  respectively.  Other  addition  agents  have 
been  recommended.  Vanadium  salts  and  sodium  resinate  may  be 
mentioned.  Important  is  the  addition  of  high-molecular  non-aromatic 
sulphur  organic  compounds  in  presence  of  calcium  salts,  patented  by 
Thiele.1  The  mode  of  action  appears  to  be  similar. 

At  the  anode,  the  disturbing  reaction  which  limits  the  possible 
concentration  of  electrolytic  hypochlorite  solutions  is  CIO'  discharge. 
Instead  of  this,  according  to  most  analogies,  proceeding  thus,  2C10'  + 
H20  — >  2HC10  +  J02  +  20,  C103'  ions  are  produced.  Cl'  ions 
are  simultaneously  formed,  and  the  whole  has  been  shown 2  to  take 
place  probably  as  follows  : — 

6010'  +  3H20 >  6H'  +  2C10/  +  4C1'  +  1 J02  +  60        (vi) 

The  Cl'  ions  are  subsequently  discharged  and  the  H'  ions  neutralised  by 
the  six  OH'  ions  simultaneously  formed  cathodically.  (HC10  behaves 
similarly,  forming  HC103,  and  evolving  oxygen  and  chlorine.)  When 
a  stationary  state  has  been  reached,  with  a  constant  NaCIO  concen- 
tration, the  CIO7  ions  formed  in  a  given  time,  minus  the  number  catho- 
dically reduced,  will  equal  the  number  discharged  according  to  the 
above  equation. 

Now  the  production   of  six   CIO'  ions  [(a)  2C1' >  C12  +  20  ; 

(6)  C12  +  OH'  — >  CIO'  +  Cl'  +  H']  necessitates  the  passage  of  twelve 
coulombs  through  the  cell.  Hence,  at  the  stationary  state,  two-thirds 
of  the  current  will  be  spent  in  producing  chlorine  which  reacts  with  the 
alkali,  giving  CIO'  ions,  and  one-third  in  the  discharge  of  these  CIO'  ions. 
As  the  former  reaction  produces  available  oxygen,  ultimately  as  NaC103, 
whereas  the  second  reaction  furnishes  free  oxygen,  we  shall  finally  have 
a  system  in  which  the  hypochlorite  concentration  is  constant,  in  which 

i  D.R.P.  141372  (1902),  205087  (1906).     See  also  p.  333. 
-  Foerster  and  Muller,  Zeitsch.  Elektrochem.  8,  665  (1902). 

Y  2 


324    PRINCIPLES  "OF  APPLIED  ELECTROCHEMISTRY  [CHAP. 

NaC103  is  produced  with  a  66' 6  per  cent,  current  efficiency  and  in 
continually  increasing  concentration,  and  in  which  the  remaining 
33-3  per  cent,  of  the  current  gives  oxygen.  Hydrogen,  of  course,  is 
continually  discharged  at  the  cathode. 

This  has  been  experimentally  proved  over  a  considerable  range  of 
temperature,  current  density,  and  NaCl  concentration.  We  are  here, 
however,  chiefly  concerned  with  the  constant  NaCIO  concentration 
which  can  finally  be  reached.  This  will  depend  on  several  factors, 
anode  material,  NaCl  concentration,  current  density  and  temperature 
being  the  chief.  Fig.  69  gives  the  current — anode  potential  curves 
for  the  electrolysis  of  NaCl  and  NaCIO  solutions.  Curve  1  was 


0-9      1-0      M      1-2      1-3      1-4      l-5»     1-6      1-7     1-8     1-9     2-0      2-1 
Anode  Potential  in  Volts. 

Fia.  69. 

taken  with  a  platinised  platinum  anode,  2  and  3  with  smooth 
platinum,  the  difference  between  1  and  3  being  due  to  the  increased 
overvoltage  at  the  latter.  We  see  that  CIO'  discharge  takes  place  far 
more  easily  than  Cl'  discharge,  and  for  a  given  initial  strength  of  NaCl 
and  a  given  anode  potential  CIO'  ions  will  soon  accumulate  in  the 
electrolyte  sufficiently  to  take  part  in  the  electrolysis.  With  a  platinised 
platinum  anode  (the  hypochlorite  curve  is  not  given)  the  distance 
between  the  two  curves  is  not  so  great.  Therefore  a  larger  accumu- 
lation of  hypochlorite  is  necessary  before  the  CIO'  ions  are  discharged, 
and  the  final  equilibrium  concentration  is  also  higher. 

The  whole  course  of  the  electrolysis  of  a  neutral  brine  solution  at 
smooth  and  platinised  platinum  anodes  is  shown  in  Fig.  70.  The 
electrolyte  initially  consisted  of  220  c.c.  of  5'1  w..NaCl,  containing  0'2 
per  cent.  KjCrO^  It  was  electrolysed  at  12-13°  with  2  amperes  at  an 
anodic  current  density  of  0*067  amps. /cm.2.  The  full  curves  are  for  a 
platinised  platinum  anode,  the  dotted  curves  for  smooth  platinum. 
The  difference  between  the  final  hypochlorite  concentrations  is  clearly 
shown,  and  we  see  how  chlorate  formation  and  oxygen  evolution  com- 
mence sooner  at  the  polished  than  at  the  platinised  electrode. 

The  higher  the  NaCl  concentration,  the  more  easily  Cl'  ions  are 


XX.] 


HYPOCHLORITES  AND  CHLORATES 


325 


discharged,  and  the  greater  must  be  the  CIO'  concentration  for  their 
discharge  to  commence.  A  high  Nad  concentration  favours  a  high 
NaCIO  concentration.  A  high  current  density  causes  efficient  mixing, 
owing  to  the  rapid  gas  evolution.  This  prevents  local  concentration  of 
CIO'  ions  at  the  anode,  and  makes  a  higher  concentration  in  the  whole 


4567 
Time  In  Hours. 

FIG.  70. 


10 


electrolyte  a  necessary  condition  for  their  discharge.  It  is  found 
in  practice  that  a  high  current  density  favours  a  high  hypochlorite 
content.  The  temperature  finally  should  be  kept  low,  not  so  much  to 
lower  the  rate  of  chemical  decomposition  of  the  hypochlorite  as  for  the 
following  reason.  The  CIO'  concentration  necessary  for  anodic  chlorate 
formation,  and  therefore  the  corresponding  final  stationary  state,  is 
only  slightly  dependent  on  the  temperature.  On  the  other  hand,  the 
equilibrium 

ci2 + 20H'  ^±  H2o  +  cr + cio' 

moves  strongly  in  favour  of  the  left-hand  side  with  fall  of  temperature. 
If,  therefore,  a  solution  which  has  reached  the  stationary  state  at  a  high 
temperature  be  cooled,  a  considerable  re-formation  of  chlorine  and  alkali 
will  take  place,  and  the  cooled  liquors  will  contain  less  hypochlorite 
than  if  they  had  been  prepared  at  room  temperature.  The  accompanying 
Table  XLIII  x  clearly  shows  the  effect  of  these  different  influences  on 
the  final  equilibrium  NaCIO  concentration. 

1  Foerster  and  Miiller,  Zeitsch.  Elektrochem.  9,  196  (1903). 


326    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY  [CHAP. 

TABLE  XLIII 


.. 

Concentration 
of  NaCl 

Tern- 
perature 

Anodic  current 
density 
in  amps.  /  cm.2 

Grams  of  hypochlorite  oxygen 
per  100  c.c. 

Platinised 
platinum  anode 

0-61 

Polished 
platinum  anode 

4-8n. 

13° 

0-017 

0-34 

0-17 

0-89 

0-68 

50° 

0-017 

0-31 

0-17 

0-17 

0-64 

0-42 

l-7w. 

13° 

0-017 

0-48 

0-28 

0-17 

0-65 

0-47 

50° 

0-017 

0-23 

0-15 

0-17 

0-40 

0-35 

The  highest  concentration  of  electrolytic  hypochlorite  so  far  prepared  in 

the  laboratory  corresponds  to  an  active  oxygen  content  of  1O56    . 

litre 

(46*8  ^p  ---  active  chlorine).    This  figure  has  been  exceeded  techni- 
litre 

cally.     The  next  table1   gives  the  results  of  electrolysis  on  a  labo- 
ratory scale  (using  platinised  platinum  electrodes)  of  an  electrolyte 


containing  280 


litr. 


NaCl  and  2    ^-a-DQ?    K2Cr04.      (In   the    last 


litre 


experiment  only  100  NaCl  were  used.) 

Jit  re 


TABLE  XLIV 


e 

Anodic 
current 
density  in 

Voltage 

Grams 
hypochlorite 
oxygen 

Grams 
bleaching 
chlorine 

i    Current 
efficiency 

One  gram 
hypochlorite 
oxygen  re- 

amps./cm.2 

per  litre 

per  litre 

watt-hours 

per  cent. 

13° 

0-017 

2-4 

4-20 

18-6 

96 

8-4 

13° 

0-017 

2-4 

5-24 

23-2 

90 

8-95 

10° 

0-07 

3-1 

6-8 

30-1 

96 

10-84 

13° 

0-17 

3-6 

5-28 

23-4 

99 

12-2 

13° 

0-17 

3-6 

8-7 

88-5 

87 

13-5 

14° 

0-17 

4-7 

5-20 

23-0 

95 

16-6 

From  the  preceding  discussion  it  will  be  seen  that  the  most  favour- 
able conditions  for  the  electrochemical  production  of  hypochlorite 
solutions  are  : 

1  Foerster  and  Miiller,  Zeitech.  Elektrochem.  8,  8  (1902). 


xx.]       HYPOCHLORITES  AND  CHLORATES       327 

(1)  Strong  Nad  solution,  both  to  lower  the  resistance  and  to  permit 
the  production  of  stronger  hypochlorite  solutions. 

(2)  Low  temperature. 

(3)  High  anodic  current  density. 

(4)  Presence  of  K2Cr04. 

(5)  Use  of  platinised  platinum  electrodes. 

In  practice  these  conditions  are  modified  for  other  reasons.  The 
use  of  concentrated  brine  means  a  high  salt  consumption  and  also 
larger  shunt  current  losses.1  A  low  temperature  involves  artificial 
cooling.  Platinised  [platinum  electrodes  are  too  fragile — the  deposit 
easily  drops  away.  Even  platinum  electrodes,  unless  of  particular 
construction,  mean  a^large  initial  outlay.  Then  there  are  other  points. 
The  circulation  must  be  adequate,  or  free  chlorine  will  escape.  If  the 
crude  brine  contains  a  large  proportion  of  calcium  or  magnesium 
salts,  particularly  if  the  bleaching  liquor  is  to  be  stored  before  use, 
a  preliminary  purification  is  necessary,  otherwise  difficultly  soluble 
calcium  and  magnesium  hydroxides  are  precipitated  at  the  cathode. 
The  solution  thereby  becomes  acid,  and  can  decompose  as  follows : 

2HC10  +  CIO'  — >  CIO/  +  2C1'  +  2H*  (v) 

Often  these  salts  are  not  completely  removed,  and  the  cathode  becomes 
gradually  covered  with  a  crust  which  raises  the  cell  voltage.  This  can 
be  readily  removed  by  reversing  the  current  for  a  little  while.  Again, 
the  energy  efficiency  diminishes  rapidly  as  the  hypochlorite  concentra- 
tion rises.  It  is  often  cheaper  to  make  dilute  solutions  than  concen- 
trated solutions  which  are  diluted  before  use.  A  point  to  be  considered 
in  designing  the  apparatus  is  its  freedom  from  exposed  metal  (copper 
or  nickel)  parts  which  are  liable  to  attack  by  the  spray  given  off  during 
the  electrolysis.  If  this  happens,  the  liquors  will  become  contaminated 
with  copper  or  nickel  salts,  and  the  decomposition 

2C10'  — >  2C1'  +  Oa  (iv) 

will  be  accelerated. 

3.  Hypochlorites— Technical 

A  large  number  of  different  electrolysers  have  been  designed,  and 
the  most  successful  will  here  be  described.  They  mostly  use  bi-polar 
electrodes.  These  are  advantageous  because  they  allow  of  very 
compact  apparatus,  and  also  because  exposed  metallic  connections  and 
bus-bars  are  dispensed  with.  A  considerable  drawback  is  the  liability 
to  shunt  current  losses  and  consequent  decreased  current  efficiencies. 
We  have  already  encountered  such  losses  in  the  series  system  of  copper 
refining.2  But  here  the  danger  is  considerably  greater  ;  for  whereas  in 
the  Hayden  process  the  voltage  drop  between  adjacent  electrodes  is 

1  See  pp.  258,  392.  2  See  p.  258. 


328    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY  [CHAP. 

only  about  0'13  volt,  in  a  hypochlorite  electrolyser  it  rarely  falls  below 
four  and  can  exceed  six  volts.  There  is  thus  a  correspondingly  greater 
inducement  for  the  current  to  pass  directly  from  one  end  electrode  to 
the  other  through  the  electrolyte. 


1       2       3      4       5      6       7       8      9      10     11     12     13     14 
Grams  Active  Chlorine  per  Litre. 

FIG.  71. 

Kellner  Cell  :  Vertical  Type. — The  Kellner  apparatus  in  one  form 
consists  of  a  tall  rectangular  earthenware   trough,    divided  into   a 


2      3      4      5      6      7      8      9      10     11     12     13    14 
Grams  Active  Chlorine  per  Litre. 

FIG.  72. 


number  of  vertical  chambers  by  a  series  of  glass  plates.  These  plates 
are  wound  round  with  Pt-Ir  wire  or  partly  covered  on  both  sides  with  a 
continuous  network  of  the  same,  and  constitute  the  bi-polar  electrodes. 


XX.] 


HYPOCHLORITES  AND  CHLORATES 


329 


The  winding  or  network  does  not  extend  over  the  whole  length  of  the 
plates — only  about  half  is  covered,  and  both  ends  are  left  free.  This  is 
done  to  diminish  the  shunt  current  losses  by  increasing  the  electrolyte 
resistance  which  has  to  be  overcome.  The  end  electrodes  are  of  stouter 
network,  and  are  provided  with  platinum-coated  leads.  The  brine 
enters  a  chamber  in  the  bottom  of  the  cell,  and,  ascending,  distributes 
itself  amongst  the  different  compartments,  where  it  is  electrolysed.  It 
leaves  the  cell  by  spouts  with  which  each  compartment  is  separately 
provided,  and  falls  back  into  the  main  supply  tank  placed  underneath. 
There  it  is  cooled  by  means  of  a  coil,  and  pumped  back  into  the  electro- 
lyser  until  the  required  strength  has  been  reached.  An  electrolyser 
with  twenty  compartments  (nineteen  bi-polar  and  two  end  electrodes) 
takes  110  volts,  therefore  5*5  volts  to  each  compartment.  This  high 
figure  is  due  to  the  very  heavy  current  density  (under  normal  work- 
ing conditions  0' 5-0*75  amps./ cm.2)  and  to  the  high  overvoltages 
involved. 

Figs.  71 1  and  72  show  the  relations  existing  between  current 
efficiency,  energy  expenditure,  and  available  chlorine  concentration, 
using  brine  solutions  of  different  strengths.  Table  XLV  indicates 
the  results  with  different  quantities  of  added  Na2Cr04,  and  also  gives 
the  expenditure  of  salt  per  kilo,  of  active  chlorine.  A  10  per  cent. 
NaCl  solution  was  used. 

TABLE  XLV 


Grams  of 
active 
chlorine 
per  litre 

With  0-08  per  cent.  NaaCrO., 

With  0-12  per  cent.  Na«CrO4 

Current    K.W.H.  per  Kilos.  NaCl 
efficiency      kilo.  CL     per  kilo.  C12 

Current 
efficiency 

K.W.H.  per  Kilos.  NaCl 
kilo.  CL     per  kilo.  CL 

3-8 

per  cent. 
79-0 

5-1 

26-3 

per  cent. 

4-0 

88-3 

4-5 

25-0 

6-8 

70-9 

5-6 

14-7 

6-9 

76-3 

5-2 

14-5 

9-0 

66-2 

6-0 

11-1 

9-4 

65-0 

6-2 

10-6 

It  is  customary  in  practice  to  use  10  per  cent,  brine  with  0'1-0'5 
per  cent,  added  Na2Cr04,  and  to  work  at  12°-15°.    An  apparatus  such 

as  is  described,  carrying  100  amperes  and  yielding  liquors  with  10  ^ — 

litre 

available  chlorine  at  a  63  per  cent,  current  efficiency,  will  produce  in 
twenty-four  hours  nearly  forty  kilos,  of  active  chlorine. 


1  Engelhardt,  Hypockloriie  und  eZe^rwcAe^etcAe(Technisch-konstruktiverTeil), 
p.  1GO. 


330    PRINCIPLES  OF  APPLIED  ELECTKOCHEMISTRY  [CHAP 

Haas  and  Oettel  Cell. — This  apparatus  employs  bi-polar  carbon  elec- 
trodes. Its  latest  construction  is  shown  diagrammatically  in  Figs.  73 
and  74.  An  earthenware  trough,  open  above,  is  divided  into  narrow 
vertical  compartments  by  plates  of  Acheson  graphite  a  a  a,  securely 
cemented  into  position.  From  both  sides  of  the  top  of  the  trough 
projects  out  a  series  of  open  earthenware  channels  d  d  d,  separated 
from  one  another  by  earthenware  ribs,  and  serving  as  outlets  for  the 


FIG.  73.— Haas  and  Oettel  Cell. 
Plan. 


FIG.  74.— Haas  and  Oettel  Cell. 
End  Elevation. 


different  electrolysis  chambers.  The  base  of  the  trough  is  provided 
with  an  opening  (C)  common  to  all  the  different  compartments.  Current 
is  led  in  and  out  at  the  two  end  electrodes.  The  electrolyser  stands  in 
a  tank  containing  brine,  the  normal  level  of  which  is  rather  below 
the  level  of  the  above-mentioned  earthenware  channels. 

When  the  current  is  switched  in,  the  strong  gas  evolution  raises  the 
level  of  the  cell  liquid  to  the  point  where  it  can  enter  these  channels, 
through  which  it  flows,  and  out  into  the  supply  tank,  whilst  fresh  brine 
is  continually  sucked  in  below.  Thus  the  circulation  is  automatically 
effected  by  the  electrolysis  and,  moreover,  controlled  by  it.  For  the 
higher  the  current  density  the  greater  is  the  heat  developed,  and  the 
more  rapidly  are  the  liquors  carried  through  to  the  main  tank  which 
contains  cooling  coils.  The  discharge  channels  serve  to  increase 
the  liquid  ohmic  resistance  between  adjacent  compartments,  and  thus 
diminish  shunt  current  losses.  In  earlier  types  of  the  electrolyser 
(still  in  use)  this  was  more  effectually  done  by  a  system  of  longer  and 
narrower  closed  channels,  but  in  practice  these  were  liable  to  choke  up 
and  cause  disturbances. 

Oettel1  gives  the  following  details  for  an  apparatus  of  the  older 
type,  using  17  per  cent.  NaCl.  It  contained  28  compartments  and 
took  116'5  volts  and  61*5  amperes,  corresponding  to  4'16  volts  per 
compartment  and  a  current  density  of  about  10  amps./ dm.2.  The 
temperature  was  22'5°. 


1  Zeitsch.  Elektrochem.  7,  319  (W(H). 


XX.] 


HYPOCHLORITES  AND  CHLORATES 


TABLE   XLVI 


331 


Grams  active                 ^^ 

chlorine                      „  . 
,.,                        efficiency 
per  litre 

K.W.H.  per 

kilo,  of 
active  chlorine 

Kilos,  salt 
per  kilo,  of 
active  chlorine 

Per  cent. 

2-55 

95-0 

3-31 

66-6 

4-59 

82-4 

3-82 

37-0 

5-90 

72-1 

4-36 

28-8 

7-41 

68-2 

4-61 

22-9 

8-82 

64-8 

4-85 

19-3 

10-50 

61-9 

5-08 

16-2 

11-22 

59-1 

5-32 

15'1 

12-30 

56-7 

5'54 

13-8 

13-35 

54-8 

5-74 

12-7 

14-31 

52-8 

5-96 

11-9 

With  the  newer  electrolysers  the  results  are  rather  less  favourable. 
One  kilo,  of  active  chlorine  requires  about  6'4  K.W.H.  with  a  salt 

consumption  of  14  kilos.,  the  concentration  being  about   12   - 

litre 

active  chlorine.  The  reason  is  the  increased  shunt  current  losses,  the 
current  efficiency  dropping  to  46  per  cent. 

The  Haas-Oettel  apparatus  uses  brine  without  the  addition  of 
chromate  or  other  substances.  It  is  enabled  to  do  this  probably 
because,  owing  to  the  porosity  of  the  carbon,1  a  considerable  fraction  of 
the  H'  discharge  actually  takes  place  within  the  electrode  material, 
and  the  nascent  gas  does  not  thus  come  into  contact  with  the  CIO'  ions 
in  the  bulk  of  the  electrolyte.  The  carbon  electrodes,  however,  also 
exert  another  influence.  The  formation  of  a  small  quantity  of  C02, 
resulting  from  oxygen  discharge  at  the  porous  anodes,  is  inevitable. 
This  produces  free  HC10,  accelerates  the  chemical  production  of  NaC103, 
and  lowers  the  hypochlorite  current  efficiency.  Finally,  although  the 
substitution  of  carbon  for  platinum  means  a  considerable  initial  saving, 
repairs  and  renewals  are  more  expensive.  The  electrodes  deteriorate 
owing  to  both  chemical  attack  and  mechanical  disintegration. 

Schuckert  Cell. — This  electrolyser  does  not  employ  bi-polar 
electrodes,  and,  in  its  most  usual  form,  is  so  constructed  that  the 
liquors  reach  the  required  concentration  by  flowing  once  through  the 
apparatus.  By  means  of  vertical  partitions,  alternately  reaching 
from  the  top  nearly  to  the  bottom  of  the  cell,  and  from  the  bottom 
nearly  to  the  top,  a  trough  of  suitable  length  is  divided  into  a 
number  of  compartments  which  contain  alternately  electrodes  and 
cooling  coils.  Each  electrode  chamber  has  two  graphite  plate 
cathodes,  and  an  anode  of  thin  Pt-Ir  foil,  the  electrode  connections 
with  the  next  compartment  being  carefully  protected  by  being 

1  P.  345. 


332    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY  [CHAP. 


covered  over  with  earthenware  and  cement.  Current  is  led  in  and 
out  at  the  two  end  electrodes,  and  hence,  although  the  cell  does  not 
strictly  consist  of  a  series  of  bi-polar  electrodes,  it  nevertheless  suffers 
from  the  shunt  current  losses  inherent  in  such  a  system.  Each 
separate  compartment  absorbs  about  6  volts.  Through  this  series 
of  cells  the  brine  flows  in  a  zigzag  course,  the  solution  leaving  the 
electrolyser  having  a  NaCIO  concentration  depending  on  the  current, 
the  size  of  the  unit,  the  brine  concentration,  and  its  velocity  of  flow. 
To  counteract  cathodic  reduction,  a  little  sodium  resinate  is  added  to 
the  brine. 

Table  XL VII  shows  the  results  the  apparatus  can  furnish. 

TABLE   XLVII 

Strength              Grams  active  K.W.H.  Kilos,  of  salt 

of  brine  chlorine  per  kilo.  used  per  kilo, 

used  per  litre  active  chlorine        active  chlorine. 

10  per  cent.  10-12  5  10-10-5 

10                                        20  7  5-5-3 

15  10-12  4-5  15-16 

15                                        20  6  7-5-8 

Solutions  containing  20  -.       -  active  chlorine  are  generally  pre- 
pared, but  liquors  with  30  or  even  50 can  be  economically 

litre 

produced  if  power  is  relatively  cheap  compared  with  salt.  Electro- 
lysers  are  now  also  made  fitted  with  bi-polar  electrodes. 

Kellner   Cell :    Horizontal  Type.— The   most   efficient  electrolytic 
hypochlorite  apparatus  is,  however,  probably  that  devised  by  Kellner, 


Fia.  75.— Kellner  Cell.     Side  Elevation. 

using  horizontal  electrodes.  It  is  shown  diagrammatically  in  Figs. 
75  and  76.  A  long  cement  trough  is  divided  by  glass  walls  into  a  number 
of  electrolysis  chambers.  These  are  usually  arranged  terrace- wise,  so 
that  the  brine,  entering  at  the  uppermost,  circulates  through  the  series 
by  gravity,  passing  from  one  compartment  to  the  next  by  the  channels 
shown.  The  electrodes,  as  in  the  Kellner  electrolyser  already  described, 


XX.] 


HYPOCHLOKITES  AND  CHLORATES 


333 


are  of  Pt-Ir  network,  and  bi-polar.  The  anodic  half  lies  very  close  to 
the  bottom  of  the  cell,  the  cathodic  half  (where  gas  is  liberated)  some 
5  nim.  above  its  anode.  Connection  between  the  two  halves  of  the 
same  electrode  is  made  beneath  the  separating  glass  plates.  A  220- 
volt  60-ampere  unit  will  have  36  compartments,  each  requiring  21 


FIG.  76.— Kellner  Cell.     Plan. 

grams  platinum.  15  per  cent,  brine  is  usually  employed,  with  the 
addition  of  an  organic  sulphur  compound1  (and  if  necessary  some 
CaCl2),  to  counteract  cathodic  reduction.  It  passes  several  times 
through  the  apparatus  until  of  the  required  strength,  cooling  and  cir- 
culation being  effected  as  with  the  earlier  type  of  Kellner  electrolyser. 
The  temperature  is  kept  at  about  21°. 

This    cell  yields  very  favourable    results.      A  liquor    containing 

25-30    ^-         active    chlorine    can    be    produced    with    an    energy 
litre 

expenditure  of  6-6*3  K.W.H.   per  kilo.,  the  salt  used  being  only 


4-6  kilos,  per  kilo,  of  active  chlorine.     And  solutions  with  50 


grams 
litre 

active  chlorine  can  be  produced  at  9*3  K.W.H.  per  kilo.  Even 
without  the  addition  of  the  membrane-forming  substance,  the  results 

are  good.      Thus  a  liquor  with   20   -  active   chlorine   can   be 

litre 

produced  with  salt  and  energy  consumption  of  5  kilos,  and 
7  K.W.H.  respectively  per  kilo,  of  active  chlorine.  Kellner 
assumes,  to  account  for  these  results,  that  the  alkaline  solution 
formed  at  the  cathode  falls  away  from  it,  on  account  of  its  greater 
specific  gravity,  and  meets  with  an  ascending  layer  of  brine  saturated 
with  chlorine.  The  production  of  hypochlorite  thus  takes  place  in  a 

1  See  p.  323. 


334    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY  [CHAP. 

more  or  less  well-defined  region  some  way  removed  from  the  cathode, 
and,  with  the  stirring  due  to  the  hydrogen  also  largely  eliminated 
by  the  electrode  arrangement,  the  cathodic  reduction  of  CIO'  ions  is 
much  lessened.  This  explanation  is  not  improbable,  and  may  be 
accepted  in  default  of  a  more  certain  one. 

Finally,  brief  mention  will  be  made  of  the  Poplar  municipal  electro- 
lytic disinfectant  plant.1  The  electrolyte  contains  5  per  cent.  NaCl  and 
1  per  cent.  MgCl2,  and  flows  by  gravity  through  four  troughs,  each 
containing  ten  '  elements/  composed  of  two  zinc  plates  as  cathodes 
and  a  slate  slab  wound  with  platinum  wire  as  anode.  These  *  elements  ' 
are  connected  in  series  with  the  230- volt  mains,  each  taking  about 
5'6  volts.  The  current  is  16  amperes.  The  finished  liquors  contain 

about  4'5  ^      -  active  chlorine.    A   little  NaOH  solution  is  added  to 
litre 

neutralise  the  free  acid  resulting  from  the  precipitation  of  magnesia. 
The  power  consumption  is  7*2  K.W.H.  per  kilo,  of  active  chlorine,  the 
total  consumption  of  salt  (NaCl  +  MgCl2)  13' 5  kilos,  per  kilo.  The  poor 
energy  efficiency  is  due  to  the  low  NaCl  concentration,  to  the  irra- 
tional addition  of  MgCl2  (a  relic  of  the  old  Hermite  process),  to  the  high 
working  temperature  (30°-35°),  to  the  absence  of  chromate,  etc. 

Comparative. — The  following  Table  (XL VIII)  contains  typical  results 
yielded  by  the  above  electrolysers.  They  cannot  be  very  closely  com- 
pared, owing  to  the  varying  conditions,  but  nevertheless  give  an  idea  of 
the  relative  capabilities  of  the  different  types. 

TABLE   XLVIII 


Grams 

K.W.H. 

Kilos. 

. 

xSnne 

active 

per 

of  salt 

Type 

used 

chlorine 

kilo. 

per  kilo. 

Addition 

per 

active 

active 

litre 

chlorine 

chlorine 

Per  cent. 

Kellner    (vertical    elec- 

trodes) 

15 

12 

6-5 

— 

K,Cr04 

Haas-Oettel 

17 

12-3 

6-4 

14 



Schuckert 

15 

20 

6 

7-8-8 

Sodium  resinato 

Kellner  (horizontal  elec- 

trodes) 

15 

25 

6 

4-6 

Sulphur  compound 

The  great  advance  represented  by  the  later  Kellner  type  is  at  all  events 
clear.  That  the  Haas-Oettel  apparatus  works  without  any  addition 
agent  should  be  noted,  and  its  cheaper  first  cost  must  not  be  forgotten. 
Of  course,  if  dilute  bleaching  liquors  only  are  needed,  then  the  difference 


Trans.  Farad.  Soc.  2, 


xx.]       HYPOCHLORITES  AND  CHLOEATES       335 

between  the  various  types  largely  disappears,  and  particularly  the  use 
of  the  Haas-Oettel  apparatus,  with  its  lower  cell  voltage,  becomes  in- 
creasingly advantageous.  Its  electrodes  are  also  less  attacked  under 
those  conditions  than  usually. 

The  reversible  voltage  needed  to  convert  a  brine  solution  into  a 
hypochlorite  liquor  of  given  strength  has  been  calculated.1  If  we 
assume  as  a  final  product  a  2'5  n.  NaCl  +  0*25  n.  NaCIO  solution 

(14- 6  per  cent.   NaCl ;    17'7  -        -    active   chlorine),  this  reversible 

litre 

decomposition  voltage  works  out  at  1*68  volts.  (Thomson's  Rule 
gives  2*3  volts.)  The  production  of  one  kilo,  of  active  chlorine  needs 
therefore  a  minimum  of 

96540  x  1000  x  1-68 

35-5  x  3600  X  1000  ' 

and  the  efficiencies  obtained  in  practice  amount  to  only  about  20-25 
per  cent.  High  overvoltages  and  low  current  efficiencies  are  jointly 
responsible. 

Electrolytic  bleaching  liquors  have  considerable  advantages  over 
bleaching  powder  solutions  for  certain  kinds  of  work.  Cost  of  pro- 
duction is  not  one.  There  is  very  little  difference  for  dilute  solutions ; 
and  for  stronger  solutions  bleaching  powder  has  the  advantage.  Thus, 
for  paper-pulp  bleaching  or  for  sewage  treatment,  electrolytic  liquors 
can  seldom  be  economically  employed.  But  for  laundry  work,  fine 
textile  bleaching,  and  other  purposes  for  which  dilute  liquors  suffice, 
the  reverse  is  true.  Owing  to  the  presence  of  the  HC10  they  bleach 
more  quickly.  Subsequent  dipping  in  acid  is  hardly  necessary,  as 
there  are  no  insoluble  lime  salts  to  remove.  The  fabric  is  exposed  to 
the  action  of  neither  acid  nor  alkali.  And  the  electrolytic  liquors  can 
be  relied  on  far  more  for  constancy  of  composition  and  regularity  of 
action.  The  extreme  simplicity  and  ease  of  working  of  the  process  are 
also  of  importance — for  those  reasons  even  the  Poplar  installation 
has  commended  itself,  in  spite  of  its  low  electrochemical  efficiency. 

4.  Chlorates. — Theory 

We  have  seen  that  in  electrolysing  a  neutral  alkaline  chloride  solu- 
tion the  primary  products  are  chlorine  and  alkali,  and  that  from  them 
results  hypochlorite,  together  with  a  little  free  HC10  produced  by 
hydrolysis.  From  such  a  solution  chlorate  formation  is  possible  in 
two  ways,  chemically  through  oxidation  of  CIO'  ions  by  free  HC10,  and 
electrochemically  through  CIO'  discharge.  Under  the  conditions  so  far 
considered,  the  former  reaction  is  very  slow,  and  the  latter  furnishes 

1  Luther,  Zeitsch.  Elektrochem.  8,  601  (1902) ;  also  Abel,  Hypochlorite  und 
Elektrische  Bleiche. 


336    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY  [CHAP. 

a  maximum  current  efficiency  of  only  66  per  cent.,  together  with  a 
solution  containing  much  available  chlorine  as  hypochlorite.  Never- 
theless these  two  reactions  are  fundamental  in  technical  chlorate  pro- 
duction, and  we  must  discuss  how  they  can  be  modified  so  as  to  yield 
satisfactory  commercial  results. 

A  consideration  of  the  phenomena  of  electrolysis  of  alkaline  chlorides 
containing  free  alkali  or  acid  is  first  necessary.  The  electrolysis  of  a 
halide  solution  containing  free  alkali  is  qualitatively  identical  with 
the  electrolysis  of  a  neutral  solution,  but  quantitatively  yery  different. 
The  presence  of  the  alkali  increases  the  CIO'  concentration — particularly 
at  the  anode — at  the  expense  of  the  HC10  otherwise  present,  and  causes 
it  to  discharge  more  easily  than  in  a  neutral  solution.  Consequently 
the  formation  of  chlorate  and  the  evolution  of  oxygen  commence 
sooner  and  at  a  lower  hypochlorite  concentration  than  in  a  neutral 
solution.  If  more  alkali  be  added  the  effect  becomes  greater,  and  finally 
the  whole  current  is  engaged  in  producing  chlorate  and  liberating 
oxygen,  the  NaCIO  concentration  sinking  to  a  very  low  figure  indeed. 
(The  current  efficiency  of  the  chlorate  production  does  not,  of  course, 
exceed  66  per  cent.)  With  still  further  rising  alkali  content,  the 
oxygen  evolution  increases  and  the  rate  of  chlorate  falls.  This  is  due 
to  direct  OH'  discharge. 

Fig.  77  expresses  the  relations  existing  between  the  alkali  concen- 
tration and  the  quantities  of  hypochlorite  and  chlorate  produced  by  the 


Centigrams  of  Active  Oxygen. 

>  SSS&8832 

\ 

/ 

N! 

\ 

/ 

\ 

/ 

^ 

^^_ 

\ 

\ 

/ 

\ 

/ 

/ 

/ 

/ 

\ 

7 

\ 

/ 

\ 

X 

^  * 

^ 

_. 

0-5     1-0     1-5      2-0      2-5      3-0     3-5     4-0 

Cirarnf  NaOH  per  100  c.c. 
Dotted  curve— hypocblorlte.        Pull  curve -chlorate. 

FlG.   77. 

passage  of  a  given  quantity  of  electricity  through  a  20  per  cent.  NaCl 
solution  at  50.1  Strongly  alkaline  solutions  commence  by  giving  oxygen 
only,  particularly  with  a  platinised  platinum  anode  ;  but,  because  of 

1  Foerster  and  Muller,  Zeitsch.  Anorg.  Chem.  22, 


XX.] 


HYPOCHLORITES  AND  CHLORATES 


337 


the  progressive  increase  in  the  oxygen  overvoltage,  the  anodic  potential 
soon  rises  to  the  value  at  which  depolarised  Cl'  ions  are  discharged. 
A  low  anode  potential,  in  fact,  always  means  a  larger  fraction  of 
the  current  used  in  evolving  oxygen,  and  a  lower  chlorate  yield.  The 
equilibrium  hypochlorite  content,  on  the  other  hand,  is  greater,  a  larger 
concentration  being  necessary  for  discharge  at  the  low  polarisation. 
It  is  accordingly  found  that  better  chlorate  yields  and  lower  hypo- 
chlorite concentrations  are  obtained  at  a  polished  than  at  a  platinised 
platinum  electrode.  For  the  same  reason  an  increase  in  current  density 
lowers  the  hypochlorite  concentration.  These  points  are  illustrated  by 
Table  XLIX.1 

TABLE    XLIX 


Anodic 

Current 

Grams  of 

Grams  of 

Anode 

current 

evolving 

chlorate 

hypochlorite 

density 

oxygen 

oxygen 

oxygen 

Amp./  cm.2 

Per  cent. 

Platinised 

0-067 

75 

1-049 

0-087 

Smooth 

0-067 

58 

1-639 

0-0012 

Smooth 

0-017 

58 

1-586 

0-0032 

The  electrolyte  consisted  of  200  c.c.  of  n  .  NaOH  +  3'6  n  .  Nad  at  17°. 
A  rise  in  temperature,  on  the  other  hand,  increases  the  hypochlorite 
concentration,  owing  to  the  lessened  oxygen  overvoltage  and  the 
resulting  fall  of  anode  potential.  The  yield  of  chlorate  simultaneously 
decreases.  (See  Fig.  78.)  We  see  then  that  electrolysis  -at  a  low  tem- 
perature of  a  brine  solution  containing  1-1 '5  per  cent.  NaOH  (Fig.  77), 


10°     20°     30°     40°     50°     60°     70° 
FiG.  78. 


using  a  smooth  platinum  anode  and  a  fairly  high  current  density,  will 
yield  chlorate  with  a  66  per  cent,  current  efficiency  and  with  very 
little  formation  of  hypochlorite. 

Foerster  and  Miiller,  Zeitsck.  Elektrochem.  9,  203,  204  (1903). 


338    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY  [CHAP. 

The  electrolysis  of  an  alkaline  chloride  solution  which  contains  a 
little  free  acid  is  a  quite  different  process.  In  absence  of  acid,  the  first 
product  of  mixing  anolyte  and  catholyte  is  a  hypochlorite  solution 
containing  a  little  free  HC10.  If  now  a  little  free  mineral  acid  be 
added,  the  concentration  of  this  HC10  is  proportionately  very  much 
increased.  And  hence  also  the  velocity  of  the  reaction 

2HC10  +  CIO7  — >  CIO/  +  2C17  +  2IT  (v) 

The  hydrogen  ions  liberated  give  more  HC10,  and  the  process  continues, 
chlorate  being  formed  in  all  parts  of  the  electrolyte.  This  reaction  is 
not  very  rapid  at  room  temperatures.  But  if  the  electrolysis  be  carried 
out  at  70°,  its  velocity  is  enormously  increased,  and  under  those 
circumstances  will  play  a  far  larger  part  in  the  formation  of  chlorate 
than  does  the  CIO'  discharge.  If  cathodic  reduction  is  avoided  by 
addition  of  chromate,  and  if  CIO'  discharge  does  not  take  place,  then  a 
100  per  cent,  current  efficiency  should  be  possible,  as  at  no  stage  of  the 
process  is  oxygen  evolved. 

In  practice,  85-95  per  cent,  current  efficiencies  can  be  obtained 
using  smooth  platinum  anodes,  and  up  to  99  per  cent,  using  platinised 
platinum.  The  deficiency  is  due  to  CIO7  discharge,  with  its  accom- 
panying free  oxygen  production.  Reference  to  Fig.  69  will  explain  the 
difference  shown  by  the  two  kinds  of  anode,  resulting  from  the  higher 
potential  at  the  smooth  platinum.  The  free  HC10  necessary  can  be  pro- 
duced by  the  addition  of  NaHC03,  Na2Cr207  (instead  of  Na2Cr04)  or 
HF.  In  this  last  case  Foerster  and  Miiller x  have  shown  that  no  specific 
effect  of  the  F7  is  involved  as  the  patentee  asserted.  Best  of  all  is  the 
regulated  addition  of  dilute  HC1. 

5.  Chlorates— Technical 

In  accordance  with  these  two  distinct  reactions  resulting  in  chlorates, 
technical  chlorate  processes  are  divisible  into  two  classes — those  using 
an  alkaline  electrolyte  from  the  start  with  a  66  per  cent,  maximum 
possible  current  efficiency,  and  those  using  an  acid  electrolyte  with  a 
possible  100  per  cent,  efficiency. 

Early  Processes. — The  first  electrolytic  chlorate  processes  used  a 
neutral  KC1  solution,  and  employed  a  diaphragm.  Su,ch  were  the 
original  one  proposed  by  Hurter  and  that  of  Gall  and  Montlaur,  the 
first  makers  of  electrolytic  chlorate.  In  the  latter  case  the  liquors  were 
circulated  from  cathode  to  anode  in  order  to  avoid  reduction.  Although 
the  current  efficiency  was  only  25  per  cent.,  the  process  paid  well,  due  to 
its  extreme  simplicity  and  the  cheap  power  available.  Diaphragm 
cells  have  now  fallen  entirely  into  disuse,  cathodic  reduction  being  to  a 
great  extent  prevented  by  a  suitable  addition  to  the  electrolyte.  Most 

1  Zcitech.  Elektrochem.  10,  781  (1WI). 


xx.]       HYPOCHLORITES  AND  CHLORATES       339 

plants  also  have  recently  stopped  working  the  alkali  (Oettel)  process,  its 
slightly  simpler  character  not  compensating  for  its  low  electrochemical 
efficiency. 

Later  Processes. — Our  knowledge  of  the  actual  arrangement  and 
working  of  a  chlorate  plant  is  very  meagre,  but  the  main  outlines  are 
probably  as  follows.  The  cells  used  (of  cement  or  some  similar  material) 
are  rectangular  in  shape,  carefully  insulated,  and  arranged  in  terraces 
so  that  the  electrolyte  circulates  by  gravity.  Every  anode  is  placed 
between  two  cathodes  and  similar  electrodes  are  connected  in  parallel. 
The  largest  units  will  take  1000-1500  amperes.  The  anodes  are  of 
smooth  platinum  or  platinum-iridium  foil.  Platinised  platinum 
would  be  better  if  its  use  were  possible — the  voltage  would  be  reduced 
and  the  yield  increased.1  Unfortunately,  the  platinum  black  deposit 
is  too  loose.  It  has  been  shown,2  however,  that  if  platinised  platinum 
be  carefully  heated  and  converted  into  coherent  '  grey '  platinum,  it 
still  largely  retains  the  properties  of  the  original  platinum  black,  and 
the  more  so  the  lower  the  temperature  to  which  it  has  been  heated. 
Thus  under  certain  conditions  the  voltages  using  different  electrodes 
were  : — 

Smooth  platinum  3*  45  volts. 

Platinised  platinum  2' 88  volts. 

Grey  platinum  2' 95  volts. 

And  the  following  comparable  yields  of  chlorate  were  obtained  : — 

TABLE  L 

Black  Grey  Smooth 

59-2  60-3  58-4 

60-5  59-0  58-8 

69-6  69-0  64-0 

77-6  77-6  74-3 

74-2  78-0  75-2 

72-2  72-2  65-4 

Using  such  electrodes  (though  it  is  doubtful  for  how  long  they  would 
retain  their  special  qualities),  it  would  seem  possible  both  to  consider- 
ably reduce  the  voltage  and  to  get  rather  higher  yields.  It  should  be 
mentioned  that,  whereas  platinum  black  is  slightly  attacked  by  anodic 
chlorine,  grey  platinum,  like  smooth  platinum,  is  comparatively 
unaffected. 

As  cathode  material,  Acheson  graphite  can  be  used  with  advantage, 
and,  in  working  the  acid  process,  nickel,  copper,  brass,  etc.  (With  the 
alkaline  process  such  cathodes  should  not  be  employed.  The  small 
quantities  of  copper  or  nickel  salts  which  would  undoubtedly  enter  the 

1  Magnetite  would  behave  similarly  to  smooth  platinum. 
-  Geibel,  Zeitsch.  Elektrochem.  12,  817  (1906). 

z  2 


340    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

electrolyte  would  catalyse  the  decomposition  2  CIO'  --- >  2  Cl'  -f  02. 
In  the  acid  process  the  CIO'  concentration  is  always  very  small.)  Iron 
can  also  be  employed,  if  chromate  or  some  other  efficient  agent  be  used 
for  preventing  cathodic  reduction.  C103'  ions  are  reducible  by  hydrogen 
at  an  iron  cathode.  With  the  alkaline  process,  zinc  can  be  employed. 


FIG.  79.— Bi-polar  Electrode  for  Chlorate  Cell. 

Bi-polar  electrode  systems  have  also  been  used.  Corbin's  electrodes 
(Fig.  79)  consisted  of  thin  platinum  foil  (A)  in  the  middle  of  large 
ebonite  frames  (B).  These  were  placed  about  15  mm.  apart  in  a  long 
trough,  and  kept  exactly  parallel  by  suitable  guides  (GH).  The  position 
of  the  electrode  surface  in  the  middle  of  the  ebonite  frames  largely 
prevented  shunt  current  losses.  Current  was  led  in  and  out  at  the 
ends  of  the  trough  by  platinum-covered  metal  discs. 

The  electrolyte  initially  contains  about  25'pe/cent.  of  KC1  or  Nad. 
Small  quantities  of  lime  or  magnesia  salts  are  no  drawback — rather  an 
advantage.1  The  insoluble  bases  which  are  precipitated  leave  acid 
behind  which  accelerates  the  process.  Normally  the  electrolyte  is 
continually  treated  with  small  quantities  of  dilute  HC1.  The  chromate 

used  is  added  as  K2Cr04  f  1  -     -  J  or  K2Cr207.    Even  though  acid  is 

being  continually  added,  the  solution  is  always  yellow,  as  no  marked 
quantity  of  the  red  Cr207"  can  exist  at  the  H'  concentration  of  the 
HC10  present.  The  anodic  current  density  averages  10-20  amps./  dm.2. 
This  high  value  and  the  close  proximity  of  the  electrodes  keep  the 
liquors  at  a  working  temperature  of  70°,  at  which  the  reaction  producing 
chlorate  proceeds  very  rapidly.  The  voltage,  despite  the  hot  electrolyte, 
is  4-5-5*5  volts. 

If  KC103  is  being  prepared,  the  liquors  are  circulated  through  the 
cells  until  saturated  at  the  working  temperature,  drawn  off,  and  allowed 
to  crystallise.  The  product  is  recrystallised  and  is  then  very  pure.  The 
mother  liquor  is  re-saturated  with  KC1  and  returned  to  the  cells.  If 
NaC103  is  being  produced,  the  treatment  is  different,  owing  to  its  great 
solubility.  Fresh  salt  is  continually  added  until  the  chlorate  content 


1  Compare  their  effect  on  the  preparation  of  bleaching  liquors  (p.  327).J 


xx.]  HYPOCHLORITES  AND  CHLORATES  341 

has  reached  about  750  y--  ,  when  the  greater  part  will  crystallise 
out  on   cooling.      Or    the    liquors   are  withdrawn  when  containing 


500-600  NaClOa  and  120  Nad   and    evaporated.      The 

litre  litre 

NaCl  precipitates,  and  the  chlorate  crystallises  on  cooling.  There  is 
no  fear  of  perchlorate  formation  in  this  long-continued  electrolysis, 
provided  that  the  NaCl  concentration  is  kept  up. 

The  current  efficiency  for  either  KC103  or  NaC103  by  this  process 
is  about  90  per  cent.  Assuming  5'0  volts  to  be  required,  we  calculate 
that  1  ton  KC103  requires 

96540  X  6  X  10  X  1000  X  1000  X  5  = 
9  X  123  X  3600  X  1000 

Taking  Luther  and  Abel's  figure  of  1'43  volts1  as  the  theoretical 
voltage  required  to  produce  chlorate  from  chloride,  one  ton  KC103 
requires  a  minimum  of 

96540  X  6  X  1000  X  1000  X  1'43 


123  X  3600  X  1000 


=  1870  K.W.H. 


Technically,  therefore,  an  energy  efficiency  of  only  about  25  per  cent, 
is  obtained,  a  result  due  to  the  high  voltage  needed.  The  success  of 
these  electrolytic  methods,  which  have  very  largely  displaced  the  older 
chemical  processes,  is  essentially  due  to  their  great  simplicity.  One 
main  raw  material  only  is  necessary,  and  the  operation  is  completed  in 
a  few  stages. 


Literature 

Abel.     Hypochlorite  und  EleJclrische  BleicJie.     Theoretischer  Teil. 
Engelhardt.     Hypochlorite    und    Elektrische    Bleiche.     Technisch- 
konstruktiver  Teil. 

i  Zeitsch.  Elektrochem,  8,  601  (1902).     Hypochlorite  und  Ekktrische  Bleiche. 


CHAPTER   XXI 

ALKALI-CHLORIXE   CELLS 

1.  General  Theory 

IT  has  been  shown  above  that,  when  an  aqueous  alkaline  chloride 
solution  is  electrolysed,  the  chief  products  are  hydrogen  and  an  alkaline 
hydroxide  at  the  cathode,  chlorine  at  the  anode.  When  cathode 
and  anode  products  are  allowed  to  freely  intermix,  we  obtain,  depending 
on  the  conditions,  hypochlorite  or  chlorate.  In  this  chapter  will  be 
considered  the  methods  by  which  these  products — the  alkaline  solution 
and  the  chlorine — can  be  kept  apart  and  worked  up  separately. 

We  will  first  discuss  the  various  electrode  reactions  involved  and 
the  other  substances  which  can  thereby  arise,  and  how  the  formation 
of  these  various  products  is  affected  by  changes  in  electrode  material, 
temperature,  concentration  of  electrolyte,  current  density,  etc. 

Cathodic  Processes. — An  aqueous  brine  solution  will  be  taken  for 
consideration.  At  the  cathode  the  two  possible  primary  electrode 
reactions  are  H'  and  Na*  discharge,  and  we  have  already  seen  that 
normally  H*  ions  are  discharged  far  more  easily.  For  example,  from 
a  neutral  aqueous  solution  normal  with  respect  to  Na*  ions,  their 
discharge  requires  a  cathodic  potential  of  —  2 '71  volts,  that  of  H'  ions 
only  —  0'4  volt.  Under  certain  conditions  these  relations  can  be 
reversed.  If  the  electrolytic  solution  pressure  of  sodium  is  lowered  by 
its  being  discharged,  not  as  pure  metal,  but  as  an  alloy  with  the  cathode 
material,  or  if  the  cathode  metal  has  a  high  hydrogen  overvoltage,  then 
cathodic  sodium  deposition  is  no  longer  impossible. 

In  agreement  with  this,  it  is  found 1  that  at  cathodes  of  tin.  platinum, 
lead,  etc.,  using  high  current  densities  (with  correspondingly  high  hydro- 
gen overvoltages),  sodium  alloys  result.  These  subsequently  react 
with  tho  water  of  the  electrolyte  and  the  escaping  gas  disintegrates  the 
surface  of  the  cathode.  With  mercury,  a  sodium  amalgam  is  obtained, 
and  the  fact  of  its  being  liquid  (when  unsaturated)  renders  possible  the 
various  '  mercury '  alkali-chlorine  cells,  in  which  the  amalgam,  after 
removal  from  the  brine  solution,  is  decomposed  by  water.  If  again  the 

1  Sec  p.  122. 
342 


ALKALI-CHLORINE  CELLS  343 

electrolyte,  instead  of  being  a  neutral  brine,  has  a  very  high  value  for 

Na* 

the   ionic   ratio  -=— ,  primary  cathodic  sodium  formation  is  rendered 
H 

possible.  For  example,  metallic  sodium  can  be  directly  deposited  in 
globules  from  an  exceedingly  concentrated  NaOH  solution  at  room 
temperature. 

Except,  however,  in  the  mercury  cells,  hydrogen,  not  sodium,  is 
liberated,  and  it  is  important  that  the  cathode  used  should  have  a 
low  hydrogen  overvoltage.  Amongst  the  commoner  metals,  the 
overvoltage  at  lead  is  particularly  high,  very  near  the  figure  for  mercury. 
The  values  for  copper  and  nickel  are  less,  but  for  iron  still  lower, 
and  this  metal,  with  its  further  advantage  of  cheapness,  is  the 
one  most  frequently  employed.  At  technical  current  densities 
(1-10  amps./dm.2)  and  temperatures  (30°-900),  the  overvoltage 
apparently  varies  between  0*3-0*55  volt.1 

The  strength  of  the  brine  used  is  important  in  the  mercury  processes. 
If  the  Na"  content  becomes  low,  hydrogen  discharge  is  facilitated,  and 
the  current  efficiency  falls.  In  other  cells,  the  most  important  factor 
is  the  alkali  concentration  at  which  the  catholyte  is  drawn  off,  a  higher 
polarisation  being  necessary  to  liberate  hydrogen  from  strong  alkali 
solutions  than  from  weak.  The  last  important  factor  is  the  tempera- 
ture. Raising  it  lowers  the  hydrogen  overvoltage.  Thus,  with  iron, 
Sacerdoti  found  a  decrease  of  O'05-O'l  volt  on  increasing  the  tempera- 
ture from  24°  to  97°.  A  high  temperature  is  therefore  favourable  when 
hydrogen  is  the  primary  cathode  product,  but  unfavourable  when  the 
object  is  to  deposit  sodium,  as  in  the  mercury  processes.  There  it  will 
also  hasten  the  rate  of  decomposition  of  the  amalgam. 

Anodic  Processes. — At  the  anode  there  have  to  be  considered  very 
similar  reactions  to  those  already  discussed  in  the  previous  chapter,  but 
as  the  cathodic  OH'  ions  are  as  far  as  possible  kept  away  from  the 
electrode,  these  reactions  are  here  much  less  important.  As  before, 
the  equilibrium 

ci2 + H2o  ^=±  H*  +  cr  +  HCIO 

tends  to  be  set  up,  and  from  the  HCIO  formed  CIO'  ions  result.  These 
can  discharge  as  follows  : 

6C10'  +  3H20  +  80  — >  2C103'  +  4C1'  +  6IT  + f  02 
It  has  also  been  shown  that  free  HCIO  can  react  anodically  thus  : 
6HC10  +  3H20  +  60  — >  2C103'  +  4C1'  +  12H'  +  f  02. 
In  both  cases  oxygen  and  chlorate  are  produced,  and  the  solution 
becomes  acid.    Another  possible  reaction  is  OH'  discharge.     This  will 
occur  less  easily  the  more  acid  the  solution. 

Let  us  now  consider  the  anode  processes  during  the  electrolysis  of 
a  brine  solution  with  constant  current,  all  cathode  product^  being 

1  Sacerdoti,  ZeitscJi.  Elektrochem.  17,  473  (1911). 


344    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY  [CHAP. 

kept  apart,  and  no  further  salt  being  added  to  keep  up  the  concentration. 
At  first  chlorine  will  be  evolved  and  the  equilibrium  C12  +  H20  ^±  H" 
+  Cl'  +  HC10  will  set  in.  Gradually  the  Cl'  concentration  will  fall. 
As  that  of  the  dissolved  chlorine  remains  constant,  the  HC10  concen- 
tration will  increase  according  to  the  above  equation  and  the  mass- 
action  law  ;  and  further,  to  keep  the  current  constant,  a  higher  anodic 
polarisation  will  be  required.  As  a  result  of  these  two  factors,  anodic 
chlorate  formation  will  slowly  commence,  the  solution  will  become 
more  acid,  and  oxygen  will  be  evolved.  The  lower  the  Cl'  content 
falls,  the  larger  will  be  the  fraction  of  the  current  so  employed.  As  the 
very  mobile  H'  ions  will  rapidly  migrate  away  from  the  anode,  there 
will  never  be  a  large  accumulation  of  acid. 

Now,  as  the  concentration  of  dissolved  chlorine  in  the  electrolyte  is 
in  equilibrium  with  the  atmosphere  above  it,  the  increasing  percentage 
of  oxygen  in  the  anode  gases  will  effect  a  continuous  decrease  in  the 
chlorine,  and  hence  in  the  HC10  concentration  in  the  electrolyte. 
Gradually  this  effect  will  overpower  the  opposing  one  due  to  the 
decreasing  Cl'  concentration.  The  HC10  concentration,  and  hence  the 
fraction  of  the  current  engaged  in  chlorate  formation,  will  reach  a 
maximum,  and,  as  the  electrolyte  becomes  more  and  more  dilute,  this 
fraction  will  steadily  decrease.  This  critical  maximum  point  will  be 
reached  the  more  quickly  because,  as  the  anode  potential  rises,  OH7 
discharge  will  begin  to  be  appreciable,  and  the  chlorine  partial  pressure 
will  thus  fall  off  more  quickly  than  would  otherwise  be  the  case.  As 
the  electrolysis  proceeds,  this  last  process  will  become  increasingly 
important,  and  we  have  as  follows  : 

(a)  Chlorine  evolution — diminishes  continually  throughout. 

(6)  Chlorate  formation — increases  to  a  maximum  and  then  falls  off. 

(c)  Oxygen  evolution  due  to  OH'  discharge — increases  continually 
throughout. 

The  following  Tables,  LI  and  LII l  show  the  progressive  results  of 
halide  electrolysis  at  platinum  anodes.  The  first  holds  for  the  electro- 
lysis of  aqueous  KC1,  the  second  for  HC1.  The  two  sets  of  figures 
are  further  not  strictly  comparable,  owing  to  differences  in  current 
density,  etc. 

TABLE  LI 


Concentration  of  KC1 

Percentage  of  current 

•  •                        *   v. 

Concentration  of  free  acidj 
at  the  anode  after  the 

giving  oxygen     i]    g] 

experiment 

3-16-3-04    • 

0-09  per  cent. 

0-0001  N. 

1-96-1-92 

0-20 

0-0007 

1-47-1-42 

0-43 

0-0014] 

0-98-0-92 

1-20 

0-0024] 

0-48-0-43 

3-15 

0-005 

0-30-0-22 

6-3 

o-oi 

1  Foerster  and  Sonneborn.  Zeitach.  Elektrochem. 

6,  597  (WOO).      Haber  and 

Grinberg.  Zeitsch.  Anorg.  Chem.  16,  221  (1898). 


XXL]  ALKALI-CHLORINE  CELLS  345 

TABLE  L1I 

_,  Percentage  ol  current  Percentage  of  current 

Concentration  of  HC1  giying  £*£2>  oxygen  giving  oxygen  gas  «! 

I'O  n.  1-04  per  cent.  0'9  per  cent. 

0-33  6-54  9-7 

O'l  34-62  34-41 

0-033  26-50  53-6 

Anodes. — The  nature  of  the  anode  can  exert  a  two-fold  influence 
on  these  processes.  The  overvoltage  required  for  chlorine  or  oxygen 
discharge  may  vary,  and  secondly  it  is  very  important  as  to  whether 
the  anode  is  porous  or  not.  In  practice  the  materials  employed  are 
platinum,  carbon,  and  magnetite.  Platinum  is  always  used  smooth, 
never  platinised,  the  platinum  black  deposit  being  very  easily  rubbed 
off.  Carbon  anodes  at  present  in  use  are  almost  invariably  of  artificial 
graphite.  Both  magnetite  and  platinum  need  a  considerable  over- 
polarisation  for  Or  or  OH'  discharge,  graphite  electrodes  a  far  lesser  one. 
Consequently  the  formation  of  chlorate  will  commence  later  at  graphite 
than  at  the  other  electrodes  under  otherwise  equal  conditions.  On  the 
other  hand,  OH'  discharge  will  commence  earlier,  and  will  take  a  larger 
fraction  of  the  total  current. 

Considerably  more  important  is  the  influence  exerted  by  the  porous 
structure  of  the  carbon,  from  which  magnetite  and  platinum  are  free. 
The  electrolyte  penetrates  inside  at  the  beginning  of  the  operation, 
and  Cr  ions  are  discharged  in  the  ordinary  way.  But  whereas  the 
Cl"  content  at  the  boundary  of  contact  of  electrolyte  and  electrode  can 
continually  be  renewed,  this  is  much  more  difficult  for  the  solution  in 
the  pores,  which  therefore  becomes  depleted  far  more  quickly  than  the 
main  bulk  of  the  electrolyte.  Oxygen  evolution  commences  sooner,  and, 
except  with  a  very  non-porous  electrode,  the  oxygen  percentage  in  the 
gases  and  the  acid  content  of  the  anolyte  are  very  sensibly  increased. 
But  this  oxygen  does  not  entirely  appear  in  the  free  state.  It  attacks 
the  carbon  electrodes  and  burns  them  to  C02,  which  partly  dissolves  in 
the  electrolyte,  and  partly  enters  the  anode  gases.  The  destruction 
of  the  electrode  is  often  not  confined  to  loss  as  C02.  If  of  poor  quality 
carbon  it  will  partly  disintegrate,  and  the  dust  produced  will  be  found 
in  the  anode  liquors.  Graphite  also,  which  burns  away  much  less 
readily,1  has  a  tendency  to  '  dust/ 

An  increase  of  temperature  lowers  the  solubility  of  the  chlorine  in  the 
solution,  but  increases  its  degree  of  hydrolysis,  these  two  actions 
affecting  the  hypochlorite  concentration  in  opposite  senses.  Experi- 
mentally it  is  found  that  the  fraction  of  current  producing  chlorate 
decreases.  But,  on  the  other  hand,  the  OH'  discharge  overvoltage  de- 
creases. At  a  carbon  electrode  there  is  the  further  disadvantage  that 

1  Cf.t  however,  p.  152. 


346    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP 

the  production  of  C02  is  much  facilitated.  To  get  a  good  yield  of 
chlorine,  therefore,  the  temperature  should  be  low. 

Current  density  is  the  last  point  to  be  considered.  Using  platinum, 
the  high  cost  practically  necessitates  the  use  of  high  current  densities, 
though  these  of  course  involve  high  bath  voltages.  With  Fe304  the 
case  is  otherwise,  and  low  current  densities  are  advantageously  em- 
ployed. With  a  porous  carbon  anode,  on  the  other  hand,  the  current 
density  is  best  kept  high,  as  by  far  the  greater  part  of  the  electrolysis 
will  then  necessarily  take  place  on  the  surface  of  the  electrode.  The 
electrolysis  of  the  dilute  solution  in  the  interior  of  the  carbon  will 
thus  be  largely  avoided,  and  a  purer  anode  gas  will  result.  The  less 
porous  the  anode  and  the  lower  the  temperature,  the  lower  the  current 
density  can  be. 

The  best  conditions  for  the  production  of  a  pure  anode  gas  are 
then  :  (1)  anode  of  platinum  or  good  quality  carbon  ;  (2)  strong  brine  ; 
(3)  high  current  density ;  (4)  low  temperature.  In  practice,  of  course, 
it  is  often  impossible  to  entirely  keep  away  cathodic  alkali  from  the 
anode.  The  discharge  of  OH'  and  CIO'  ions  thus  takes  place  more 
readily,  and  the  formation  of  chlorate,  oxygen,  and  C02  becomes  more 
important. 

The  importance  of  carbon  anodes  in  brine  electrolysis  makes  some 
rapid  method  of  testing  their  efficiency  desirable.  A  determination  of 
their  porosity  might  be  thought  to  be  a  suitable  test.  In  fact,  porosity 
and  liability  to  attack  do  run  very  roughly  parallel,  as  the  following 
figures  show : 

Porosity  per  cent.  Relative  losses 

by  volume  in  weight  -3 

11  13 

13  24 

21  27 

22  37 

28  41 

The  test,  however,  is  insufficiently  conclusive,  and  a  direct  one  is 
better.  We  have  seen  that  the  anodic  processes  in  chlorate  and  in 
alkali-chlorine  cells  are  essentially  similar,  the  difference  being  quantita- 
tive only.  It  follows  that  a  good  test  for  an  anode  intended  for  the 
latter  purpose  is  to  use  it  for  the  electrolysis  of  a  brine  solution  without 
a  diaphragm  and  with  addition  of  K2Cr04.  The  more  nearly  it  behaves 
like  one  of  platinum,  in  respect  to  both  electrolytic  products  and  evolved 
gases,  the  better  it  will  serve  its  purpose.1  A  porous  carbon  anode 
intended  for  working  at  low  temperatures  is  materially  improved  by 
soaking  in  wax.  This  treatment  is  more  harmful  than  otherwise  at 
temperatures,  as  the  chlorine  will  attack  the  w;ix. 

1  Sj.riisser,  Zeihch,  Elektrochem.  7,  10!>2  (/.W/). 


xxi.]  ALKALI-CHLOKINE  CELLS  347 

Classification  of  Cells. — The  first  problem  in  the  design  of  alkali- 
chlorine  cells  is  the  keeping  apart  of  cathodic  alkali  and  anodic  chlorine. 
As  the  solubility  of  chlorine  in  the  strong  brine  used  is  low,  the  extent  of 
its  diffusion  to  the  cathode  compartment  will  also  be  low.  On  the 
other  hand,  the  alkali  produced  completely  dissolves,  and  its  passage 
towards  the  anode  is  determined  not  only  by  diffusion,  but  also  by 
migration  of  the  negative  OH'  ions.  The  problem,  therefore,  practically 
reduces  to  one  of  preventing  the  alkali  from  reaching  the  anolyte,  and 
the  means  adopted  for  this  purpose  afford  us  a  convenient  method 
of  classifying  the  different  kinds  of  cells.  The  usual  method  of  dividing 
them  into  diaphragm,  gravity,  and  mercury  cells  is  somewhat  empirical 
and  arbitrary,  and  the  following  classification  is  preferable  : 

(1)  Cells  in  which  the  OH'  ions  are  not  liberated  inside  the  electro- 
lysis chamber.     E.g.  all  mercury  processes. 

(2)  Cells  in  which  the  OH'  ions  liberated  at  the  cathode  are  prevented 
from  being  carried  to  the  anode  by  convection  or  mechanical  causes,  but 
in  which  no  attempt  is  made  to  counteract  their  electrical  migration. 
E.g.  Griesheim  process. 

(3)  Cells  in  which  all  the  causes  tending  to  carry  the  OH'  ions  formed 
at  the  cathode  towards  the  anode  are  more  or  less  counteracted.     E.g. 
Bell-jar  process  and  most  diaphragm  cells. 

2.  Mercury  Cells. 

In  these  cells  the  cathodic  process  is  not  H',  but  alkali  metallion 
discharge.  The  liberated  metal  dissolves  in  the  mercury  cathode,  and 
the  amalgam  is  removed  and  decomposed  elsewhere.  At  the  anode 
chlorine  is  evolved,  and,  as  the  OH'  concentration  is  very  low,  a  pure 
gas  results,  provided  that  the  anodes  (if  carbon)  are  of  good  quality, 
and  that  the  concentration  of  the  alkaline  halide  in  the  electrolyte  is 
kept  up.  The  theory  is  therefore  simple. 

Reversible  hydrogen  evolution  from  a  neutral  aqueous  solution 
requires  a  cathodic  potential  of  —  0'4  volt.  With  a  low  current  density — 
e.g.  0*1  amp./ dm.2 — at  a  mercury  cathode,  the  overvoltage  is  about 
I'O  volt.  The  deposition  of  metallic  sodium  from  a  w.Na*  solution  would 
require  a  cathodic  potential  of  —  2'7  volts,  and  the  formation  of  a 
saturated  sodium  amalgam  a  potential  of  at  least  —  1-8  volts.  (The 
figures  for  potassium  and  potassium  amalgam  are  very  similar.)  Under 
these  conditions,  hydrogen,  not  sodium,  would  be  liberated.  But  if 
the  current  density  is  raised,  and  with  it  the  hydrogen  overvoltage, 
making  H'  discharge  less  easy,  and  if  Na'  discharge  is  facilitated  by 
quickly  removing  the  mercury  and  producing  a  dilute  and  not  a  concen- 
trated amalgam,  sodium  is  the  substance  chiefly  deposited,  small 
quantities  only  of  hydrogen  being  liberated. 

In  practice,  at  the  current  density  and  temperature  used,  the 


348    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY  [CHAP. 

hydrogen  overvoltage  is  about  1/25  volts,  corresponding  to  a  necessary 
potential  of  —  T65  volts.  At  the  same  time,  by  producing  an  amalgam 
with  only  0*02  per  cent,  sodium  instead  of  a  saturated  one,  the  sodium 
potential  is  lowered  to  perhaps  about  •-  1/5  volts.  Electrolysis, 
therefore,  must  furnish  sodium,  not  hydrogen.  Using  KC1,  the 
necessary  polarisation  is  probably  a  little  higher. 

At  the  anode,  Cl'  discharge  determines  the  potential.  If  smooth 
platinum  anodes  are  used,  there  is  a  high  chlorine  overvoltage.  With 
technical  current  densities,  the  anodic  potential  is  probably  about 
+  2'1  volts,  making  the  total  drop  at  the  two  electrodes  3' 6  volts.1  To 
this  must  be  added  the  voltage  drop  in  the  electrolyte,  determined  by 
the  brine  concentration,  the  distance  apart  of  the  electrodes,  the  tem- 
perature, and  the  current  density.2  Current  density  must  be  high 
for  reasons  already  discussed,  and  it  is  advantageous  to  place  the 
electrodes  close  to  one  another  (which  raises  the  temperature)  and  to 
use  strong  brine.  In  practice  this  extra  voltage  amounts  to  perhaps 
1/4  volts,  making  a  total  bath  voltage  of  about  five  volts.  With 
carbon  anodes,  owing  to  the  lower  overvoltage,  this  total  seldom 
exceeds  4'  3-4* 5  volts,  except  with  exceptionally  high  current  densities. 

Sources  of  Loss. — Although  no  alkali  is  supposed  to  form  in  the 
electrolyser,  nevertheless  the  current  efficiencies  of  the  different  cells  do 
not  reach  100  per  cent.,  but  generally  vary  round  about  95  per  cent. 
There  are  several  reasons  for  this.  If  the  current  density  falls  too  low, 
if  the  brine  circulation  is  too  slow,  or  if  the  amalgam  becomes  too  con- 
centrated, H*  discharge  will  become  important.  In  good  practice  this 
does  not  exceed  1  per  cent,  of  the  total  current,  but  nevertheless  does 
take  place.  It  becomes  far  greater  if  carbon  anodes  (arranged  above 
the  cathodes)  are  used.  Firstly,  small  quantities  of  acid  are  produced 
at  them  which  facilitate  cathodic  H'  discharge  ;  and  secondly,  small 
particles  of  carbon,  falling  on  to  the  mercury,  permit  of  hydrogen  evolu- 
tion at  a  lower  overvoltage.  The  same  effect  will  result  if  the  electrolyte 
contains  metallic  impurities  which  are  easily  reduced  at  the  cathode 
to  a  form  which  will  not  amalgamate  with  the  mercury.3 

Besides  the  electrochemical  evolution  of  hydrogen,  there  is  its 
chemical  production  by  interaction  between  the  amalgam  and  the 
aqueous  brine.  This  reaction,  which  is  slow  at  room  temperature,  goes 
quickly  at  higher  temperatures,  and  the  electrolyser  must  be  kept 

1  Taussig  [Trans.  Farad.  Soc.  5,  258  (7.90.9)]  found  the  back  E.M.F.  for  brine 
electrolysis  between  a  mercury  cathode  and  a  platinum  anode  to  increase  from 
3'0  to  3'2  volts  as  the  sodium  concentration  in  the  amalgam  increased  from  0'002 
per  cent,  to  0'02  per  cent. 

2  Taussig  (Joe.  cit.)  has  made  a  detailed  investigation  of  the  relations  which 
hold  for  brine  electrolysis  between  voltage,  current  density,  concentration  of 
amalgam,  rate  of  brine  and  mercury  circulation,  electrode  distance,  and  tempera- 
ture.    The  reader  is  referred  to  the  original  paper  for  his  interesting  results. 

3  Compare j>p.  201,  209,  227,  282,  402. 


xxi.]  ALKALI-CHLORINE  CELLS  349 

below  60°  in  consequence.  Particles  of  carbon  or  metallic  substances 
floating  on  the  mercury  greatly  accelerate  the  change  by  facilitating  the 
evolution  of  hydrogen.  Cells  with  carbon  anodes  usually  give,  therefore, 
a  lower  cathodic  current  efficiency  and  more  hydrogen  than  cells  using 
platinum.  A  high  current  density  proportionately  reduces  these 
chemical  losses,1  as  also  does  rapid  removal  of  the  amalgam  from  the 
cell.  These  sources  of  loss  all  involve  formation  of  hydrogen  instead 
of  sodium,  and  further  produce  an  impure  anode  gas.  Dangerous 
explosions  have  occurred  as  a  result  of  this  hydrogen  content,  and  in 
any  case  there  is  always  the  possibility  of  subsequent  formation  of 
HC1,  which  will  give  trouble  in  the  bleach  chambers.  The  average 
hydrogen  content  in  the  anode  gases  from  cells  with  carbon  anodes  is 
2-3  per  cent.,  but  may  rise  to  5  per  cent,  with  bad  management.  The 
danger  of  explosions  is  then  considerable.  With  platinum  anodes  the 
amount  is  much  less,  seldom  exceeding  O5  per  cent. 

Another  source  of  loss  in  mercury  processes  is  cathodic  ionisation  of 
dissolved  chlorine  as  follows  :  JC12  -f-  Q  — >  Cl'.  This  reaction  takes 
place  more  easily  than  either  Na'  or  H'  discharge,  and,  as  anodes  are 
close  to  cathodes  and  there  is  no  diaphragm,  will  proceed  just  as  rapidly 
as  chlorine  can  diffuse  from  the  anode.  This  source  of  loss  affects  both 
anode  and  cathode  current  efficiencies,  and  amounts  to  perhaps  3  per 
cent.  We  can  therefore  say  that  the  average  current  efficiency  in  a 
mercury  cell  using  platinum  anodes  will  be  about  96-97  per  cent,  at 
both  electrodes.  The  anode  gases  will  contain  a  small  quantity  of 
hydrogen  and  oxygen.  With  carbon  anodes  95  per  cent,  will  be 
obtained.  The  anode  gases  will  contain  2-3  per  cent,  of  both  C02  and 
hydrogen. 

In  actual  practice  the  electrolyte  fed  in  contains  30  per  cent.  NaCl 
or  KC1.  This  is  reduced  to  20  per  cent,  during  its  rapid  passage 
through  the  cells,  and  is  then  re-saturated.  Impurities,  such  as  Na2S04,2 
calcium  and  iron  salts,  etc.,  must  be  removed.  The  cathodic  current 
density  varies  in  different  cells  between  5  and  25  amps./dm.2,  being 
high  for  reasons  already  discussed.  At  the  carbon  or  platinum  anodes, 
particularly  at  the  latter,  it  is  still  higher.  The  working  temperature 
is  about  50°,  kept  up  by  the  heating  effect  of  the  current.  A  higher 
temperature  saves  voltage,  but  considerably  lowers  the  current 
efficiency.  The  charged  mercury  must  be  moved  away  rapidly  from  the 
electrolyser  to  the  decomposing  vessel — otherwise  it  will  become  pasty 
and  unworkable,  and  current  losses  will  set  in.  The  alkali  metal 
content  is  usually  kept  below  0'02  per  cent.  The  methods  adopted 
for  the  removal  and  decomposition  of  the  amalgam  are  many,  and 
distinguish  the  various  types  of  cells  which  we  will  now  consider. 

Castner   Cell. — The  reaction  between    the   amalgam  and   water. 

1  Compare  pp.  30,  160,  361,  406. 

~  S04"  ions,  if  allowed  to  accumulate,  destroy  carbon  anodes. 


350    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

resulting  in  the  formation  of  caustic  alkali  and  hydrogen,  takes  place 
with  liberation  of  free  energy.  If,  therefore,  this  decomposition  could  be 
arranged  to  proceed  electrolytically,  the  amalgam  functioning  as  a  soluble 
anode  and  hydrogen  being  evolved  at  the  cathode,  a  primary  cell  of  a 
certain  definite  E.M.F.  would  result,  which,  if  coupled  against  the  brine 
decomposition  cell,  should  lower  the  resultant  voltage  required  for  the 
whole  electrolyser.  This  was  recognised  by  Castner,  who  designed 
for  that  purpose  his  well-known  ingenious  rocking  cell,1  which,  though 
now  nearly  obsolete,  merits  a  description  here. 

The  combined  cell  (Fig.  80)  consisted  of  a  large  slate  box,  four 
feet  square  and  six  inches  high,  divided  by  two  vertical  partitions  into 


FIG.  80.— Castner  Ceil. 

three  equal  compartments.  These  partitions  stopped  short  ,'/'  from 
the  bottom  of  the  cell,  and  there  was,  further,  a  shallow  groove  under- 
neath each.  The  mercury,  placed  on  the  bottom  of  the  cell,  could  thus 
pass  from  one  compartment  to  another.  In  the  two  outer  divisions 
the  mercury  was  cathode,  the  anodes  being  of  graphite  securely 
cemented  into  the  roof,  and  terminating  about  J"  from  the  mercury. 
The  roof  was  also  provided  with  chlorine  outlets,2  whilst  the  brine  entered 
and  left  the  cell  through  suitable  openings  in  the  sides.  In  the  middle 
compartment,  the  mercury  charged  with  alkali  metal  became  anode 
in  an  alkali  solution,  the  cathode  being  an  iron  grid.  The  mercury 
was  circulated  by  pivoting  one  end  of  the  cell  and  resting  the  other 
on  a  slowly  revolving  eccentric,  by  means  of  which  it  was  every  moment 
raised  and  lowered  through  a  distance  of  J".  The  mercury  thus  flowed 
backwards  and  forwards  between  the  anode  and  cathode  compart- 
ments, being  alternately  charged  with  and  depleted  of  alkali  metal. 
The  alkaline  liquors  were  continuously  withdrawn  at  a  concentration 
of  20  per  cent. 

As,  however,  the  cathodic  current  efficiency  in  the  brine  compartment 

*  Electrochem.  Ind.  1,  12  (V.uv). 

"  In  most  alkali-chlorine  cells  the  chlorine  is,  for  obvious  reasons,  sucked  off 
under  slightly  reduced  pressure. 


xxi.]  ALKALI-CHLOKINE  CELLS  351 

was  only  90-95  per  cent.,  it  proved  impossible  to  allow  the  whole 
current  passing  through  the  brine  cell  also  to  flow  through  the  alkali 
cell.  90-95  per  cent,  only  could  enter  the  electrolyte  from  the  anode 
as  Na'  ions.  The  remaining  5-10  per  cent,  was  necessarily  associated 
with  some  other  anode  reaction.  As  oxygen  overvoltage  at  mercury  is 
very  high,  and  as  the  Hg2"  concentration  in  such  an  alkaline  solution 
is  very  low,1  thus  favouring  the  ionisation  of  mercury,  it  was  found 
that  the  mercury  was  indeed  attacked,  a  black  deposit  of  Hg20  resulting. 
The  loss  of  mercury  made  it  necessary  to  avoid  this,  which  was  done  by 
connecting  a  suitable  resistance  in  shunt  with  the  middle  compartment, 
as  is  shown  in  the  diagram.  The  fraction  of  current  thus  shunted 
corresponded  to  the  current  efficiency  losses  in  the  brine  compartments, 
and  was  therefore  about  8-10  per  cent. 

Such  cells  contained  45  kilos,  of  mercury  and  carried  about  630 

amperes  (  14          -  ).      They    required    about    4'l-4'3    volts.      We 

should  have  expected  this  figure  to  have  been  lower,  but  the  gain  in 
voltage  secured  by  the  double  cell  is  almost  counterbalanced  by  the 
increased  liquid  resistance  opposing  the  current.  Their  complicated 
structure  and  the  small  size  of  the  units  are  further  disadvantages,  as 
they  need  (comparatively)  much  care  and  attention.  Le  Blanc  and 
Cantoni 2  have  made  a  laboratory  study  of  their  behaviour.  With 
30  per  cent.  KC1  at  40°,  using  a  current  density  of  10  amps./dm.2, 
they  got  a  93  per  cent,  current  efficiency.  The  total  voltage  varied 
from  5'1  to  3*7  volts,  depending  on  the  distance  apart  of  the  various 
electrodes. 

Kellner's  Arrangement. — Kellner's  device  for  bringing  about  the 
decomposition  of  the  amalgam  is  essentially  different.  Instead  of,  as 
in  the  original  idea  of  Castner,  causing  the  total  current  to  pass  through 
the  electrolyte  and  gaining  the  total  voltage  of  the  '  sodium-amalgam 
primary  cell/  or,  as  in  Castner's  technical  cell,  shunting  a  fraction 
only  of  the  current  through  a  metallic  conductor,  and  gaining  the 
greater  part  of  this  primary  cell  voltage,  no  attempt  is  made  to  utilise 
the  energy  of  formation  of  hydroxide  from  amalgam,  or,  in  other  words, 
the  whole  of  the  current  is  shunted  in  the  alkali  compartment.  The 
amalgam,  on  leaving  the  electrolysing  vessel,  is  simply  short-circuited 
with  a  conductor  at  which  hydrogen  overvoltage  is  low,  usually  iron, 
under  which  conditions  its  decomposition  takes  place  almost  instan- 
taneously. The  voltage  and  energy  efficiency  are  determined  then 
solely  by  the  considerations  discussed  on  pp.  347-349. 

Several  types  of  Kellner  cell  have  been  proposed,  differing  in  the 
method  of  circulation  of  mercury,  gravity  and  mechanical  means  being 
employed.  We  must  first  mention  the  combination  of  the  Kellner 

1  In  consequence  of  the  low  solubility  of  Hg20. 
-  Zeitech.  Elektrochem.  11,  611  (1905). 


352    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

caustic-compartment  connections  with  the  Castner  rocking  cell.  The 
cell  is  as  described  above,  only  that  instead  of  Castner's  shunt  circuit 
and  an  iron  cathode  dipping  in  an  alkaline  solution,  the  iron  dips 
directly  into  the  mercury. 

Solvay  Cell.1 — But  these  cells  still  suffered  from  the  defect  that 
workable  units  were  inconveniently  small.  Hence  stationary  cells  of 
large  size  were  developed,  through  which  the  mercury  steadily  flows, 
the  amalgam  being  decomposed  in  separate  vessels.  Such  cells  are 
those  of  the  Solvay  Co.  (Jemeppe,  Belgium),  and  of  the  Castner-Kellner 


FIG.  81.— Solvay  Cell. 

Co.  at  Weston  Point.  The  former  is  illustrated  diagrammatically  in 
Fig.  81.  The  electrolyser  consists  of  a  large,  slightly  tilted,  rectangular 
cement  trough,  through  which  continually  flow  streams  of  mercury  and 
of  brine,  the  latter  being  introduced  closely  above  the  surface  of  the 
former.  The  layer  of  mercury  is  a  thin  one,2  and  its  rate  of  flow  is 
chosen  so  that  an  amalgam  of  suitable  concentration  leaves  the  cell. 
The  current  enters  through  platinum  anodes  (of  interlaced  wire)  securely 
cemented  in  the  roof,  and  leaves  by  numerous  connections  passing 
through  the  bottom  of  the  cell.  The  anodes  are  only  10-15  mm. 
above  the  mercury  surface.  The  issuing  amalgam  flows  by  gravity 
into  a  separate  trough,  probably  of  iron,  where  it  is  decomposed  by 
a  counter-stream  of  water,  strong  alkaline  liquors  being  continuously 
drawn  off.  The  regenerated  mercury  is  returned  to  the  electrolysis 
tank  by  means  of  the  well-wheel  A.  The  units  are  large,  carrying 
10,000-15,000  amperes  at  the  high  current  density  (at  the  mercury)  of 
15-20  amps./dm.2.  Each  takes  about  five  volts.  The  advantages 
of  the  cell  lie  in  its  simple  construction,  the  large  size  of  the  units,  and 
the  small  amount  of  attention  required. 

The  present  cell  used  at  Weston  Point  is  very  similar  to  the  above. 
But  carbon  anodes  are  employed  with  lower  current  densities,  and 
consequently  lower  voltages,  and  the  mercury  is  circulated  by  an 
Archimedean  screw.  A  unit  takes  4,000  amperes. 

Whiting  Cell. — We  may  also  mention  the  Whiting  cell,3  the  feature 

1  Tram.  Farad.  Soc.  6,  258  (1910) 

2  8-10  amperes  pass  per  kilo,  of  mercury  in  the  whole  cell. 

3  Trans.  Amer.  Electrochem.  Soc.  17,  327  (Win). 


xxi.]  ALKALI-CHLOKINE  CELLS  353 

of  which  is  an  automatic  intermittent  removal  of  the  amalgam  from  the 
elect rolyser.  The  cell  consists  of  a  shallow  cement  box,  divided  by  a 
partition  into  electrolyser  and  amalgam  denuding  chamber.  The 
electrolyser  itself  is  subdivided  by  long  glass  partitions  into  five  com- 
partments, each  containing  its  own  graphite  anode,  and  provided  with 
a  separate  valve  through  which  the  charged  mercury  can  flow  out  into 
the  denuding  chamber.  These  valves  are  alternately  opened  and 
closed  by  means  of  cams  attached  to  a  slowly  revolving  horizontal  shaft. 
The  amalgam  rapidly  passes  out  into  the  alkaline  chamber  (being 
replaced  by  fresh  mercury  from  a  constant  level  supply)  and  flows 
down  by  gravity  through  a  series  of  channels  in  graphite  slabs,  where 
it  is  short-circuited  and  completely  decomposed,  pure  mercury  arriving 
at  the  bottom.  This  is  pumped  up  again  by  a  cup-wheel  to  the  level 
of  the  electrolyser. 

A  cell  six  feet  square  takes  1,200-1,400  amperes,  the  current  density 
at  mercury  and  graphite  being  about  11  amps./dm.2  The  working 
temperature  is  about  40°.  The  current  efficiency  is  90-95  per  cent. 
Four  volts  only  are  required.  The  chlorine  is  98  per  cent,  pure,  the 
residue  being  hydrogen.  The  anodes  are  only  very  slightly  attacked. 
20  per  cent.  NaOH  is  produced,  but  40  per  cent,  can  be  readily  made. 
A  cell  of  the  size  described  requires  about  170  kilos,  of  mercury,  7-8 
amperes  passing  per  kilo. 

Compressed-air  Type. — Of  cells  with  mechanical  circulation  of  the 
mercury,  the  most  important  is  perhaps  that  used  at  Jaice,  Bosnia, 
where  compressed  air  is  employed.1  The  large  shallow  cement  cell 
(Fig.  82)  contains  three  compartments  separated  by  partitions  similar 


0  1  Metres.  2  3 

FIG.  82.-^Taice  Cell. 

to  those  used  in  the  Castner  cell.  In  this  case  the  middle  one  forms 
the  anodic,  the  two  outer  ones  the  cathodic,  compartments.  The  anode 
compartment  is  not  completely  covered  over,  but  contains  six  large 
cement  hoods,  which  carry  the  anodes.  These  consist  of  pieces  of 
platinum  gauze  or  netting,  fused  into  glass  tubes,  and  making  contact 
with  external  leads  by  means  of  mercury  and  copper  wires.  Each 
electrode  weighs  one  gram — there  are  88  per  hood — 528  per  3,500-4,000 

1  See  Taussig,  loc.  cit. 

2  A 


354    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 


ampere  unit.  The  current  leaves  the  cell  by  the  cast-iron  grids  in  the 
cathode  compartment,  which  make  contact  with  the  mercury  and  effect 
the  decomposition  of  the  amalgam. 

For  the  circulation  of  the  mercury  the  cathode  compartments  are 
provided  with  troughs  at  their  outer  edges.  In  these  troughs,  which, 
of  course,  are  filled  with  mercury,  there  dip  a  number  of  iron  vessels, 
inverted  slightly  truncated  cones  in  shape,  connecting  below  with  the 
troughs,  and  above  with  an  air-pump.  When  the  pump  works,  air  is 
alternately  compressed  into  one  series  of  vessels  and  exhausted  from 
the  other,  and  the  mercury  is  thus  made  to  travel  backwards  and 
forwards  from  anode  to  cathode  compartment.  This  circulation  is 
very  efficient,  and  allows  of  the  cell  being  worked  with  high  current 
densities — viz.  15-20  amps./dm.2  at  the  mercury  (as  in  the  Solvay  cell). 

Rhodin  Cell. — In  this  cell,1  which  was  worked  for  a  short  time  at 
Sault  St.  Marie,  Canada,  the  hood  containing  the  anode  was  rapidly 
revolved  (30  times  per  minute)  and  the  charged  mercury  thus  driven 
centrifugally  to  the  outer  parts  of  the  cathode  vessel,  where  it  was 
decomposed. 

Wildermann  Cell. — In  conclusion,  the  Edser-Wildermann  cell2 
(founded  on  English  patents  18,958  [1898],  22,902  [1900],  and  9,803 
[1902])  must  be  considered.  The  transference  of  the  amalgam  from 

the  brine  to  the  alkali 


Bearing  for  stirring 
arrangement 


FIG.  83.— Wildermann  Cell. 


compartment  is  here 
effected  by  mechanical 
agitation.  Fig.  83  shows 
the  cell  diagrammatically. 
It  is  constructed  of  iron, 
lined  with  a  specially  re- 
sistive kind  of  ebonite, 
and  divided  into  a  circu- 
lar inner  compartment  and 
a  special  ring-shaped  outer 
compartment  by  means  of 
a  ring-shaped  vertical  dis- 
continuous partition,  consisting,  as  the  figure  shows,  of  a  number  of 
shallow  channels  arranged  one  above  the  other,  each  having  in 
section  the  shape  of  a  ship's  hull.  By  filling  these  channels  with 
mercury  to  the  necessary  depth,  the  partition  is  made  continuous, 
the  two  compartments  being  now  separated  from  one  another  by 
mercury  seals.  The  inner  compartment  is  the  electrolysis  cell  and  is 
filled  with  brine.  It  contains  graphite  anodes 3  and  a  stirring  arrange- 
ment3 by  which  the  mercury  in  the  different  channels  is  agitated. 

1  Jour.  Soc.  Chem.  Ind.  21,  449  (1<JOH). 

2  Taussig,  loc.  cit.    Also  in  part  private  communication. 

3  Not  shown  in  diagram. 


XXL]  ALKALI-CHLORINE  CELLS  355 

This  agitation  causes  the  amalgam  to  pass  over  to  the  outer  caustic 
compartment,  where  it  is  at  once  decomposed  by  the  agency  of  rods 
of  carbon  or  iron  contained  in  the  mercury  troughs.  20-23  per 
cent.  NaOH,  containing  at  the  most  0*2  per  cent.  NaCl,  is  continuously 
drawn  off. 

Very  high  current  densities  are  used  at  the  mercury.  Even  up  to 
60  amps./dm.2  can  be  employed  without  any  trouble  with  the  amalgam, 
so  efficient  is  the  circulation.  At  the  anodes  10  amps./dm.2  is  usual. 
The  quantities  of  hypochlorite  and  chlorate  formed  in  the  anode  liquors 
arc  very  low.  Further,  as  the  amalgam  is  rapidly  removed  from  the 
influence  of  any  particles  which  have  fallen  from  the  electrodes,  and 
as,  further,  the  agitation  renders  it  difficult  for  such  particles  to  settle 
on  the  surface  of  the  mercury,  the  current  efficiency  is  high — 97-98 
per  cent.  The  cell  takes  about  five  volts.  2,200-ampere  units  have 
been  constructed.  In  spite  of  the  apparent  complication  introduced  by 
the  stirrer,  the  cell  is  said  to  work  very  smoothly  and  to  require 
practically  no  repairs  or  renewals.  Thus  anodes  have  lasted  three 
years.  A  further  advantage  is  that  the  vertical  arrangement l  of  the 
mercury  and  the  high  current  density  lead  to  a  far  greater  compactness 
than  is  usual  with  mercury  cells. 

The  advantages  of  mercury  processes  lie  in  the  concentrated  caustic 
liquors  that  can  be  produced — 24  per  cent.  NaOH  can  easily  be  made — 
and  in  the  high  current  efficiencies.  Further,  the  working  current 
densities  are  high,  though  this  advantage  is  usually  neutralised  by  the 
horizontal  arrangement  of  the  mercury.  Evaporation  charges  are 
saved,  a  purer  product  (99  per  cent.  NaOH  -f  Na2C03  -f-  NaCl)  is 
got,  and  the  life  of  the  carbon  anodes  is  longer  than  in  other  processes. 
Against  that  must  be  put  the  higher  voltages  required  and  the  first 
cost  of  the  mercury.  The  latter  item  is  the  more  important.  The  actual 
losses  of  mercury  during  working  are  usually  very  small,  and  should 
not  exceed  2  per  cent,  in  a  year. 

3.  Diaphragm  Cells  with  Stationary  Electrolyte 

In  these  cells  the  OH'  ions  are  actually  liberated  in  the  electrolyser. 
The  disturbing  effects  of  convection  currents  and  gas  evolution,  which 
tend  to  bring  together  alkali  and  the  anodic  chlorine,  are  avoided. 
Diffusion  of  alkali  from  its  region  of  high  concentration  (catholyte) 
to  that  of  low  concentration  (anolyte)  is  also  partly  prevented.  But 
no  attempt  is  made  to  prevent  interaction  caused  by  electrical  migration 
of  OH'  ions  towards  the  anode.  The  most  important  cell  of  this  type  is 
the  Griesheim  Elektron  Cell,  which,  though  relatively  inefficient,  came 
into  use  early,  and  is  still  very  extensively  worked  on  the  Continent. 

The  means  used  to  eliminate  the  effects  of  convection  and  of  mass 

1  Cf.  p;  370. 

2  A  2 


356    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY     [CHAP. 

movements  of  the  electrolyte  is  the  employment  of  a  porous  diaphragm 
separating  anolyte  and  catholyte.  We  have  already1  discussed  the 
essentials  of  a  good  diaphragm  and  considered  some  of  the  numerous 
proposed  materials.  For  alkaline  chloride  electrolysis,  the  starting 
material  is  generally  asbestos  cloth,  the  necessary  fine  porous  structure 
being  attained  by  applying  some  suitable  powder.  In  the  Griesheim 
cell,  instead  of  asbestos,  cement  is  used. 

Now  a  porous  diaphragm  offers  no  selective  resistance  to  the  trans- 
port of  ions.  We  know  that  the  current  in  an  electrolyte  is  carried  by 
the  various  ions  present,  positive  and  negative,  and,  moreover,  in  pro- 
portions depending  on  their  relative  concentrations  and  on  their  ionic 
conductivities.  Hence  some  of  the  current  will  be  carried  in  the 
present  case  by  OH7  ions,  which,  passing  through  the  diaphragm  into 
the  anode  compartment,  will  react  with  the  chlorine  and  H'  ions  there 
present.  Thus  the  alkali  yield  will  be  lowered.  As  OH'  ions  have 
a  high  ionic  conductivity,2  they  will  carry  a  comparatively  large  fraction 
of  the  current,  and  the  more  so  the  greater  their  concentration  in 
the  catholyte — i.e.  the  stronger  the  alkali  made.  The  following  figures, 
got  with  20  per  cent.  KC1,  give  an  idea  of  the  current  efficiencies 
obtainable  : — 


TABLE  LIII 


Foerster  and  Jorre 


Winteler  4 


Concentration  of 
KOH  produced 

Current 
efficiency 

Concentration  of 
KOH  produced 

Current 
efficiency 

Per  cent. 
4-3 
8-3 
11-2 

Per  cent. 
78-4 
72-6 
68-4 

Per  cent. 
5-26 
8-03 
11-37 

Per  cent. 
851 

70-7 
68-9 

The  following  calculation  of  current  efficiency  in  a  diaphragm  process 
with  non-percolating  electrolyte  we  owe  to  Foerster  and  Jorre.5  It 
is  assumed  that  ordinary  diffusion  processes  play  no  part. 

At  any  moment  let  the  respective  molecular  concentrations  of  the 
alkaline  chloride  and  hydroxide  at  the  diaphragm  be  [CJ  and  [C2], 
their  degrees  of  dissociation  «j  and  a2,  and  their  limiting  molar 
conductivities  Aooi  and  A  002-  Let  KI  and  /c2  be  the  specific  con- 
ductivities which  the  electrolyte  would  have  if  chloride  or  hydroxide 
alone  were  respectively  dissolved.  With  two  such  solutions  of  high 
and  very  similar  degrees  of  dissociation  we  can  assume  that  the  con- 
ductivity of  the  mixed  solution  is  equal  to  the  sum  of  the  single  con- 
ductivities— i.e.  equal  to  ^  -f  KI.  Now  let  x  be  the  fraction  of  the  current 


i  P.  lr,l.  2  See  p.  07. 

<  Zeitech.  Elektrochcm.  5,  10(1898). 


Zeitech.  Anorg.  Chem.  28,  158 
'*  Loc.  cit. 


xxi.  1  ALKALI-CHLORINE  CELLS  357 

at  the  diaphragm  carried  by  the  hydroxide.     Then  1  —  x  is  the  fraction 
carried  by  the  chloride.      And  we  have  — 


1—  x        ATI        [Ci 

whence 

1  (a) 


j_ 


During  the  electrolysis  [C2]  continually  increases  and  [Ci]  continually 
decreases  owing  to  migration  of  Cl'  ions  into  the  anolyte.     Also  Aa>2 

is  much  greater  than  Aooi-     Therefore  ^—  -  —  1    °01  will  decrease  very 

aA 


rapidly  during  the  electrolysis,  and  hence  x  continually  increases,  the 
yield  of  alkali  at  any  moment  falling  off  as  the  electrolysis  proceeds. , 

Now  let  n  be  the  transport  number  for  OH'  in  NaOH — i.e.,  if  only  the 
NaOH  were  to  conduct,  a  fraction  n  of  the  total  current  would  be 
carried  by  the  OH'  ions.  But  in  reality  the  total  current  is  divided 
between  NaOH  and  NaCl,  a  fraction  x  being  carried  by  the  former. 
Hence  the  fraction  of  the  total  current  carried  by  the  OH'  ions  is  xn. 
Therefore,  whilst  one  equivalent  of  alkali  is  being  produced  at  the 
cathode,  xn  equivalents  pass  through  the  diaphragm  and  leave  the 
catholyte.  Then,  if  C.E.  is  the  current  efficiency  at  any  moment, 

C.E.  =  100(1  -  xn). 

We  can  simplify  the  expression  for  x  deduced  above  by  putting  at  =  a2, 
which  is  very  nearly  correct,  and  expressing  the  constant  ratio 
by  a.     We  finally  get 


OJt -II ^IXlOO  (6) 


n  and  a  both  being  constants.  We  see  that  the  current  efficiency 
depends  on  the  values  of  [CJ  and  [C2],  and,  in  most  practical  cases, 
where  the  change  in  [CJ  is  small  compared  with  the  change  in  [C2], 
is  essentially  a  function  of  [C2],  the  alkali  concentration.  It  is  an 
obvious  advantage  to  work  with  a  high  chloride  and  a  low  alkali  con- 
centration. In  practice,  however,  this  means  large  evaporation  charges, 

rc  i 

which  consequently  limit  the  increase  of  the  ratio  ^~.     The  following 


Table  (LIV)  contains  the  results  of  electrolysis  of  a  20  per  cent.  KC1 
solution,  using  a  Pukall  diaphragm  :  — 


358    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

TABLE  LIV 


Mean 

Gram-equivalents      Current  efficiency 

Period 

observed 

per  litre 

calculated 

efficiency 

[CJ 

[cy 

for  end  of 
period 

Per  cent. 

Per  cent. 

First  two  hours         .             88'06            2'382 

0-418 

81-3 

Second  two  hours     .             69*30            2*224 

0-754 

70-4 

Third  two  hours        .             66'50            2-096 

1-071 

62-6 

Fourth  two  hours     .             58-02            2-066 

1-331 

55-0 

The  mean  yield  calculated  from  the  above  instantaneous  yield  was  j 
71  per  cent.,  whilst  the  experiment  gave  69  per  cent.  The  agreement 
is  very  satisfactory. 

Other  formulae  have  been  deduced  by  Guye,1  which  allow  us  to 
follow  the  course  of  alkaline  chloride  electrolysis  with  a  porous 
diaphragm  and  non-percolating  electrolyte.  With  an  electrolyte  kept 
continually  saturated  with  the  alkaline  chloride,  and  an  initial  alkali 
concentration  of  zero,  the  following  formulae  hold  : 

C.E.  (instantaneous)  =  100  X  - 


«[C])|-  l] 


C.E.  (mean)  =  100  X 


C.E.  =  current  efficiency  :  a  is  a  constant  depending  on  the  temperature 
and  on  the  particular  chloride  used  :  [C]  is  the  molecular  concentration 
of  alkali  :  v  the  volume  of  the  catholyte  :  F  the  number  of  faradays 
passed  through  the  cell.  To  test  these  formulae,  a  brine  solution  kept 
saturated  was  electrolysed  at  40°  for  nearly  thirty  hours  with  1,150 
amperes,  a  under  these  conditions  was  found  to  be  2'50.  Rather  more 
complex  formula)  than  the  above  were  used,  as  the  initial  NaOH  con- 
centration was  not  zero,  but  26' 1  £rams.  Substituting  F  by  I .  t  (current 

litre 

X  time)  the  following  figures  were  got : — 


Time  after  beginning 

of  electrolysis 

0    hours 

6-8 

13-8 


TABLE  LV 

[C]  found 

0-653 

1-29 

1-94 

2-58 


29-6 


calculated 

1-35 

1-99 
2-62 
3-24 


Jour.  Chim.  Phys.  1,  121,  212  (1903). 


XXI.] 


ALKALI-CHLORINE  CELLS 


359 


For  the  whole  experiment,  C.E.  (mean)  was  found  to  be  56  per  cent, 
and  calculated  at  57  per  cent. 

For  the  case  when  the  concentration  of  the  chloride  is  not  constant, 
but  diminishes  in  proportion  as  the  alkali  content  rises,  Guye l  and 
Briner *  deduced : 

C.E.  (instantaneous)  =  1    t   _rr|l  X  100  (/) 


C.E.  (mean)  = 


X  100 


whence 


a  v 


(70 


(i) 


In  this  case  a  is  only  constant  during  any  particular  experiment  :  it 
varies  with  the  initial  chloride  concentration.  Briner  has  confirmed 
these  formulae,  using  a  brine  solution  under  conditions  for  which  a 
was  0'66.  He  obtained,  for  example  — 

TABLE  LVI 


F  x  96540 

C  =  40  [C] 
observed 

Weight  of  NaOH 
obtained  =  v  C 

Calculated  weight 
of  NaOH 

Coul. 

Grams  /litre 

Grams 

Grams 

25,000 

14-7 

9-0 

9-3 

50,000 

28-3 

17-0 

17-0 

75,000 

39-1 

23-7 

23-5 

100,000 

50-0 

29-8 

29'5 

125.000 

59-1 

35-6 

35-1 

150.000 

68-0 

40-7 

40-1 

Brochet3  has  carried  out  similar  experiments  with  potassium  and 
sodium  chlorides.  Some  of  his  results,  obtained  at  40°,  are  contained 
in  the  following  table  (for  NaCl),  and  in  Fig.  84 : — 


[C] 

0-120 

0-45 

0-835 

M7 

1-47 

1-76 

Loc.  cit. 


TABLE   LVII 

C.E.  (mean)  found 
Per  cent. 
96 
90 
83-5 
78 
73-5 
70-4 


C.E.  (mean)  calculated 
Percent. 

88 
82 
76 

72 
68-8 


2  Jour.  Chim.  Phys.  5,  398  (1907}. 
Bull  Soc.  Chim.  (iv.)  3,  532  (1908). 


360    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 


If  now,  returning  to  formula  (b),  we  substitute  the  values  for 
potassium  and  sodium  salts,  we  obtain — 

0-74 


C.K-(KOH)  —  '  1 


C.E. 


NnOI! 


X  100 


100 


& 


90 


80 


70 


V 


0          0-5          1-0         i-5         2-0         2-5 
Equivalent  Concentration  of  Alkali 

FIG.  84. 


Hence,  under  otherwise  equal  conditions,  the  yield  of  potash  should 
exceed  that  of  soda.     Fig.  84  shows  this  to  be  actually  so.     The  effect 

of  temperature  on  the  electrolysis 
can  also  be  readily  deduced  from 
the  same  formula.  In  Chap.  V  we 
have  seen  that  as  the  temperature 
rises  the  rates  of  migration  of  the 
different  monovalent  ions  tend  to 
become  equal.  Hence  n  and  a  in 
the  formula  approach  more  nearly 
the  respective  values  of  0'5  and 
TO  as  the  temperature  increases. 
Consequently  the  yield  of  alkali 
will  rise,  and  the  differences  shown 
by  the  different  chlorides  tend  to 
disappear.  It  must  not,  of  course, 
be  forgotten  that  the  increased 
rate  of  diffusion1  will  simultane- 
ously tend  to  lower  the  yield.  Equation  (b)  tells  us  nothing  about 
the  effect  of  current  density  on  the  yield,  but  from  formulae  (g),  (h), 
and  (k)  we  gather  that  the  latter  depends  solely  on  the  quantity 
of  electricity  passed  through,  and  should  be  independent  of  the  rate 
of  electrolysis — i.e.  the  current  density — if  migration  losses  are  the 
only  ones  to  be  considered. 

This  is  assumed  in  the  deduction  of  the  above  formulae,  which  only 
give  correct  results  as  far  as  the  supposition  is  true,  or  nearly  so.  As 
a  matter  of  fact,  a  certain  amount  of  diffusion  of  alkali  from  catholyte 
to  anolyte  must  necessarily  occur,  and  results  in  the  actually  obtained 
alkali  concentrations  being  less  than  the  calculated  ones.2  The  extent 
of  diffusion  depends  on  the  porosity  and  thickness  of  the  diaphragm, 

»  See  below. 

With  high  current  densities  at  the  diaphragm,  this  difference  may  be  neutralised 
by  another  phenomenon — that  of  endosmose,  which  tends  to  cause  the  electrolyte 
to  pass  bodily  through  the  diaphragm  in  the  direction  of  the  cathode.  This 
phenomenon  will  not  be  further  discussed  here,  as  the  part  it  plays  in  practice 
is  a  small  one. 


XXI.] 


ALKALI-CHLOKINE  CELLS 


361 


on  the  time,  on  the  temperature,  and  on  the  cathodic  concentration  of 
alkali. 

Diffusion  losses  will  be  less  with  a  thick  and  dense  diaphragm  and 
at  a  low  temperature.  On  the  other  hand,  such  conditions  necessitate 
a  higher  voltage,  and  these  various  facts  must  be  carefully  weighed 
against  one  another  when  deciding  on  working  conditions.  A  low 
catholytic  alkali  concentration  is  advantageous,  as  it  also  is  from  the 
point  of  view  of  migration  losses.  But  it  has  been  pointed  out  that 
working  thus  involves  large  evaporation  charges.  Finally,  diffusion 
losses  increase  with  time,  and  hence  by  using  a  high  current  density 
can  be  made  proportionately  less  important.1  Thus,  in  a  certain 
experiment  with  KC1,  the  current  efficiency  was  raised  from  59  per  cent, 
to  69  per  cent,  by  increasing  the  current  density  five-fold.  The  voltage, 
of  course,  simultaneously  rises.  From  the  work  of  Guye  and  Tardy 2  we 
gain  some  idea  of  the  magnitude  of  these  diffusion  losses.  The  following 
Table  (LVIII)  holds  for  different  Pukall  porous  clay  diaphragms  at  50°, 
using  a  25  per  cent,  brine  solution  in  both  chambers. 

TABLE   LVIII 


Grams  NaOH  per  litre 

Surface 

Per  cent. 

Dia- 
phragm 

of  dia- 
phragm in 
cm." 

volume 
occupied 
by  pores 

In  first 
chamber  at 
beginning  of 
experiment 

In  second 
chamber 
after  n  days 

n 

Grams 
NaOH 
diffused 

1 

57'76 

31-4 

162-11 

17-36            1-75      13-02 

2 

64 

29-2                  65-89 

6-02            2-8          6-32 

3 

64 

28-3                  65-89 

3-86            2-64        4-05 

2-99            1-66        3-14 

4 

64 

28-9                  65-89 

5-60            2-96        4-20 

3-51            1-68        2-63 

The  course  of  the  electrolysis  at  the  anode  has  already  been  discussed, 
and  little  needs  to  be  added.  In  cells  of  this  type  carbon  anodes 
were  first  used,  but  are  now  more  or  less  completely  replaced  by  anodes 
of  magnetite.  At  carbon,  as  we  have  seen,  OH'  ions  can  be  discharged 
under  the  prevailing  conditions.  Oxygen  is  evolved,  and  the  electro- 
lyte becomes  acid.  The  H'  ions  formed  will  migrate  cathode  wards,  and 
will  be  neutralised  by  the  OH'  ions  entering  the  anolyte.  If  a  strong 
caustic  solution  is  being  made,  this  OH'  influx  will  more  than  neutralise 
the  H*  ions  formed.  In  that  case  CIO'  ions  will  be  produced,  and  anodic 
chlorate  formation  will  commence.  But  if  a  weak  alkaline  solution 
only  is  being  made  the  anolyte  may  remain  permanently  acid.  OH' 

1  Cf.  pp.  30,  160,  349,  406. 

-  Jour.  Chim.  Phys.  2,  79  (1904).     See  also  p.  156. 


362    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

discharge  will  thus  be  hindered  and  the  chlorate  formation,  assuming 
a  high  working  temperature,  will  be  chiefly  chemical — 

CIO'  +  2HC10  — >  C103'  +  2C1'  +  2H\ 

An  acid  reaction  in  the  anolyte  is  better  with  carbon  electrodes,  as, 
at  the  high  temperatures  employed  (used  for  other  reasons),  the  liberated 
oxygen  largely  forms  C02,  occasioning  serious  anode  losses,  and  fur- 
nishing an  impure  chlorine.  Even  with  best  quality  carbons,  5-8  per 
cent.  C02  is  not  uncommon,  the  oxygen  amounting  to  perhaps  another 
10  per  cent.  This  fact  of  C02  formation  furnishes  an  important  addi- 
tional reason  for  making  dilute  alkali.  In  some  diaphragm  cells  it  is 
stated  that  HC1  is  continually  added  to  the  anolyte  to  avoid  this  CIO' 
and  OH'  discharge  as  far  as  possible.  The  oxygen  evolution  will  also 
depend,  as  has  been  pointed  out,  on  the  porosity  of  the  carbons  em- 
ployed, a  high  current  density  being  advantageous.  The  concentration 
of  the  dissolved  chloride  should  also  be  kept  high  for  the  same  reason. 
As  oxygen  formation  always  occurs  to  some  extent  even  if  the  cathodic 
alkali  is  carefully  kept  away  from  the  anode,  the  yield  of  chlorine  will 
never  quite  reach  that  of  the  caustic.  With  magnetite  anodes,  the 
conditions  are  more  favourable.  No  C02  is  formed,  the  anode  gas  thus 
being  of  far  better  quality.  Large  quantities  of  chlorate  are  produced, 
but  without  appreciably  deteriorating  the  electrodes. 

From  these  considerations  the  following  conclusions  can  be  drawn 
as  to  the  best  working  conditions  for  cells  of  this  type  : 

(1)  Anode.     Magnetite  is  better  than  carbon.     It  is  less  porous  and 
less  oxygen  will  be  evolved,  no  C02  will  be  formed,  and  a  purer  chlorine 
will  result.     Further,  a  stronger  caustic  can  be  made  and  a  higher 
working  temperature  used  without  the  electrode  being  destroyed. 

(2)  Brine  concentration.     Should  be  high,  both  current  and  energy 
efficiencies  being  thereby  increased. 

(3)  Alkali  concentration.     A  strong  alkali  is  only  obtained  at  the 
sacrifice  of  high  current  and  energy  efficiencies.     Weak  liquors  involve 
high  evaporation  charges,  a  large  plant,  etc.     These  considerations 
must  be  weighed  against  one  another,  local  power  and  fuel  considera- 
tions being  taken  into  account. 

(4)  Temperature.     This  should  be  high,  as  the  voltage  is  thereby 
lowered.     Alkali  losses  due  to  OH'  migration  are  also  lessened,  but 
those  caused  by  diffusion  are  increased.     With  magnetite,  the  purity 
of  the  chlorine  is  unaffected. 

(£>)  Current  density.  Should  be  high,  particularly  with  carbon 
anodes.  A  rather  higher  voltage  is  thereby  necessitated,  but  diffusion 
losses  arc  icndcrcd  less  important,  and  the  plant  gains  in  compactness. 

Griesheim  Cell— This  cell l  is  the  most  important  of  its  type  at 

issermann,  Ding.  Pobj.  Jour.  315,  No.  30  (WOO).      Lepsius,  Chem.  Zeit. 
33,  1MMM/W).      Milliter  [ /;/,    /•:/, ktrocJiemischen    Vcrf<il,rni,  v.,1.    ii.  p.   166  (1911)} 


XXI.] 


ALKALI-CHLORINE  CELLS 


363 


present  used,  being  worked  to  the  total  extent  of  about  33,000  H.P. 
(two-thirds  of  it  in  Germany).  Both  NaOH  and  KOH  are  made.  It 
consists  (Figs.  85  and  86)  of  a  rectangular  iron  box,  steam- jacketed, 
covered  externally  with  some  non-conducting  material  (heat),  and 


\  = 

£t 

= 

-7  - 

:i= 

tt 

i= 

1     

= 

it 

i- 

-r 

it 

r  - 

f= 

±t 

= 

-^ 

= 

it 

=7 

:--J 

I 

n 

-- 

- 

~ 

-  - 

" 

- 

-  - 

•   - 



_   _. 



_ 

/       Metres 
FIG.  85.— Griesheim  Cell. 


Front  Elevation. 


Steam 


carefully  insulated  electrically  from  the  ground.  In  it  are  supported 
cement  boxes  (1  cm.  thick),  which  act  as  diaphragms  and  contain  the 
anodes.  They  are  prepared  by  the  method  worked  out  by  Breuer, 
Matthes,  and  Weber.  The  cement  is  made  up  with  a  brine  solution 
containing  HC1,  and,  after 


setting,  the  diaphragms  are 
soaked  in  water.  The  salt, 
which  has  crystallised,  is 
washed  out,  as  are  also  the  steam 
more  soluble  constituents  of 
the  cement  which  have  been 
dissolved  by  the  acid,  and 
the  result  is  a  very  porous 
diaphragm  which  offers  but 
little  resistance  to  the  pass- 
age of  the  current,  and  is 
very  durable  against  the 
chemical  action  of  the  alkali, 
chlorine,  etc. 

The  anodes  are  of  mag- 
netite, and  prepared  accord- 
ing to  Speketer's  patent. 
Decopperised  burnt  pyrites,  which  is  chiefly  Fe203,  is  fused  in  the 

has  described  cells  arranged  somewhat  differently,  the  anode  chambers  being 
suspended  in  the  cathode  compartment,  the  heating  being  effected  by  steam  coils  in 
the  electrolyte,  etc. 


FIG.  86,— Griesheim  Cell.     End  Elevation. 


364   .PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

electric  furnace,  volatile  constituents  being  thereby  removed,  and  cast. 
To  the  molten  mass  before  solidifying  fresh  powdered  Fe203  is  added. 
This  converts  the  excess  of  FeO  which  has  formed  in  the  furnace 
into  Fe304,  and  the  result  is  a  homogeneous  mass  of  the  latter.  If  this 
precaution  is  omitted,  the  FeO  crystallises  out  separately,  and  the  anode 
becomes  mechanically  unstable.  As  it  is,  great  care  is  needed  to  pro- 
duce large  castings.  The  resulting  cylindrical  anodes l  are  one-fifth  the 
price  of  graphite  and  have  a  life  of  two  years,  even  under  the  unfavour- 
able electrochemical  conditions  of  the  Griesheim  cell.  And  of  course 
they  give  a  C02  free  chlorine  gas. 

As  disadvantages  must  be  mentioned  their  low  conductivity  and 
high  chlorine  overvoltage,2  both  of  which  facts  render  impossible  the 
use  of  high  current  densities.  They  are  securely  cemented  into  the  lids 
of  the  anode  boxes,  which  are  further  provided  with  exits  for  the 
chlorine  and  with  inlets  for  replenishing  the  anolyte  with  the  alkaline 
chloride.  This  is  introduced  as  solid  through  tubes  leading  down 
almost  to  the  bottom  of  the  boxes.  Pure  salt  is  used,  got  from  the 
alkali  concentration  pans.  The  walls  of  the  outer  trough  act  as 
cathode,  and,  in  addition,  a  piece  of  sheet  iron  is  hung  between  each 
pair  of  anode  cells.  The  cathode  chamber,  into  which  crude  brine 
is  fed,  is  suitably  covered  in,  and  provided  with  hydrogen  exits. 
AH  anocles  and  all  cathodes  in  each  cell  are  connected  in  parallel. 
The  largest  unit  constructed  takes  2200-2500  amperes. 

The  working  temperature  is  90°,  and  with  a  diaphragm  current 
density  of  1-2  amps./dm.2,  3' 6  volts  are  needed  per  cell  when  carbon 
anodes  are  used,  4  volts  with  magnetite  anodes.  The  alkaline 
chloride  solution  is  originally  about  3N.  in  the  cathode  chamber 

(l70  &~  NaCl ;  220  g^mS  KClY  the  crude  salt  being  used.  The 
\  litre  litre  / 

anode  boxes  are  kept  saturated.  When  making  2N.  alkali 
A  grams  Na  grams  R  \  efficiency  is 

\         litre  litre  / 

about  70  per  cent. ;  but  when,  as  more  frequently  happens,  the 
liquors  are  only  of  half  this  strength,  or  little  more,  it  rises  to 
80  per  cent.  The  caustic  liquors  are  drawn  off  intermittently  when 
of  the  right  strength.  On  evaporation,  solid  caustic  alkali  with 
1-2  per  cent,  salt  results.  If  KOH,  it  is  sometimes  merely  con- 
centrated to  a  strength  of  50  per  cent,  (weight)  and  sold  directly 
to  soapmakers.  Chlorate  is  formed  in  the  anode  compartment, 
and,  if  KC103,  crystallises  out,  and  is  from  time  to  time  removed. 
Of  recent  years,  owing  to  the  competition  of  more  efficient  processes, 

1  In  the  diagrams  carbon  anodes  are  represented,  and  are  still  used  in  some  of 
the  Griesheim  process  works. 

2  See  p.  ];-,:{. 


xxi.]  ALKALI-CHLOKINE  CELLS  365 

this  by-product  lias  assumed  considerable  importance,  and  in  some 
factories  makes  the  difference  between  economic  and  uneconomic 
working. 

Very  imperfect  electrocheinically,  the  Griesheim  cell  owes  its  present 
large  application  to  its  simplicity  and  cheapness,  and  to  the  large 
amount  of  capital  sunk  in  it.  As  at  present  worked,  it  can  give  a 
chlorine  of  good  quality,  has  no  diaphragm  troubles,  and  is  easy  to  run. 
For  example,  the  re-saturation  of  the  chlorinated  anolyte  with  halide, 
usually  a  troublesome  operation,  is  very  simply  done,  and  the  cathodic 
brine  needs  no  preliminary  purification.  Owing  to  its  working  tempera- 
ture, the  voltage  is  low.  Against  these  advantages  must  be  set  its  low 
current  efficiency,  the  weak  alkali  made,  and  the  small  capacity  of  the 
units  for  their  size,  due  to  the  low  current  density. 

Outhenin-Chalandre  Cell.  —  One  other  cell  of  this  type  should  be 
briefly  mentioned  —  viz.  the  Outhenin-Chalandre,1  operated  at  works  in 
France,  Italy,  and  Spain.  The  box  constituting  the  cell  is  divided 
by  two  vertical  partitions  into  three  divisions,  the  outer  ones  containing 
the  catholyte,  the  inner  one  being  the  anode  chamber.  The  diaphragms 
take  the  form  of  cylindrical  tubes  of  unglazed  porcelain,  cemented  into 
the  dividing  partitions  of  the  cell,  traversing  the  anode  chamber,  and, 
being  open  at  both  ends,  connecting  the  two  cathode  compartments. 
Each  tube  contains  an  iron  rod  as  cathode,  these  being  all  connected 
together.  To  facilitate  the  escape  of  hydrogen,  the  battery  of  tubes  is 
sloped  somewhat  upwards.  The  anodes  consist  of  Acheson  graphite 
plates.  A  1400-ampere  unit  contains  108  cathodes  arranged  in  six  rows 
one  above  the  other,  and  19  anodes. 

With  a  freshly  cleaned  cell,  the  initial  voltage  is  3'5  volts  ; 
this  rises  slowly  to  4  volts,  when  the  cell  is  cleaned  again.  As 
a  K.W.  day  is  said  to  produce  6'  7  kilos.  NaOH  when  the  apparatus 
is  in  good  condition,  it  follows  that  the  cathodic  current  efficiency  is 

6*7  X  96540  X  3-5  X  100  _, 

-  —  A(\        =66  per   cent.     The   anode   compartment 

X    4:0 


is  charged  with  concentrated  brine,  whilst  at  the  commencement 
dilute  alkali,  later  water,  is  passed  through  the  cathode  chambers. 
The  concentration  of  alkali  made  is  unknown.  To  avoid  the 
destructive  effect  on  the  anodes  of  OH'  ions  entering  the  electro- 
lyte, dilute  HC1  is  continually  added  to  the  latter.  This  lengthens 
the  life  of  the  anodes,  but  lowers  the  current  efficiency.  The  cells 
are  cleaned  out  fortnightly.  These  results  are  less  favourable  than 
those  given  by  the  Griesheim  cell,  whilst  the  apparatus  is  compli- 
cated and  needs  much  attention  and  labour.  A  1400-ampere  cell 
requiring  108  porcelain  tubes  as  diaphragms  can  hardly  be  regarded  as 
satisfactory. 

1  Brocket,  La  Soude  Electrolytique,  p.  103  (1909). 


366    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

4.  Cells  with  Moving  Electrolyte 

The  characteristics  of  these  cells  are  : — (a)  alkali  and  hydrogen  are 
formed  in  the  electrolysis  chamber  ;  (6)  the  effects  of  both  convection 
and  ionic  migration  in  causing  losses  of  alkali  and  chlorine  are  counter- 
acted. In  all  cases  there  is  a  flow  of  liquid  so  arranged  as  to  oppose 
the  diffusion  of  alkali  and  migration  of  OH7  ions  anodewards.  In  most 
cases  diaphragms  are  also  used. 

Theory. — Consider  the  electrolysis  of  a  strong  brine  solution  taking 
place  in  a  vessel  of  constant  cross-section  between  anode  and  cathode, 
the  electrolyte  being  stationary.  At  the  cathode,  hydrogen  and 
alkali  appear  ;  at  the  anode,  chlorine,  with  small  quantities  of  its  hydro- 
lysis products  with  water,  and,  if  the  electrode  be  of  carbon,  a  certain 
amount  of  acid  resulting  from  OH'  discharge.  Suppose  the  hydrogen 
and  chlorine  to  pass  off  without  disturbing  the  neighbouring  parts  of 
the  electrolyte.  Then,  owing  to  molecular  diffusion  and  OH'  migration, 
alkali  will  gradually  stream  out  from  the  cathode  through  the  solution 
towards  the  anode,  and  acid  and  dissolved  chlorine  will  similarly  spread 
out  from  anode  towards  cathode. 

Fig.  87  shows  diagrammatical ly  how  the  concentrations  of  these 
substances  will  vary  at  different  distances  along  the  electrolyser  and  at 


Cathode 


Anode 


FIG.  87. 


different  stages  of  the  electrolysis  (represented  by  the  different  curves). 
At  first  the  concentrations  of  both  alkali  and  [acid  -f  chlorine]  fall 
practically  to  zero  at  points  not  far  removed  from  their  respective  elec- 
trodes. As  the  electrolysis  proceeds,  these  points  approach  one  another 
until  (curve  4)  they  coincide.  Further  electrolysis  now  results  in 
interaction  at  the  layer  of  contact  between  these  alkaline  and  acid 
zones,  giving  water  and  CIO'  ions.  There  results  an  alkaline  zone 
with  a  sharp  boundary  of  a  definite  concentration,  then  a  narrow 
neutral  zone,  then  a  zone  containing  acid  and  chlorine,  whose  total 


xxi.]  ALKALI-CHLORINE  CELLS  367 

equivalent  concentration  at  its  boundary  is  equal  to  the  alkali  concen- 
tration at  the  other  side  of  the  neutral  layer.  The  more  quietly  the 
electrolysis  proceeds,  the  more  sharply  these  boundaries  will  be  denned 
and  the  narrower  will  be  the  neutral  zone. 

As  now  the  amount  of  alkali  proceeding  anodewards  far  more 
than  suffices  to  react  with  the  chlorine  +  acid  travelling  in  the  opposite 
direction,1  the  alkaline  boundary  will  have  a  resultant  motion  towards 
the  anode,  as  is  shown  in  the  figure.  We  see,  however  (supposing  the 
different  curves  to  hold  for  equal  intervals  of  time),  that  this  velocity 
is  less  than  the  velocity  of  the  alkaline  layer  before  it  encountered 
the  acid  layer  ;  and  it  is,  moreover,  plain  that  if  acid  were  produced  in 
greater  quantities  at  the  anode  than  the  diagram  assumes,  this  velocity 
would  be  still  further  reduced.  The  nearer  the  alkali  approaches  the 
anode,  the  higher  is  its  concentration  in  the  limiting  layer. 

Finally,  when  the  alkaline  zone  actually  reaches  the  anode,  where 
gaseous  chlorine  is  being  liberated,  there  will  be  strong  interaction 
between  the  latter  and  the  OH'  ions,  and  the  cathodic  current  effi- 
ciency, which  hitherto  has  only  fallen  below  100  per  cent,  by  reason  of 
the  small  quantities  of  acid  and  dissolved  chlorine  coming  from  the 
anode,  will  drop  very  considerably.  To  avoid  this,  the  movement  of 
the  alkaline  layer  anodewards  is  checked  in  all  cells  of  this  type  by  a 
counter -movement  of  the  electrolyte  as  a  whole  from  the  anode  towards 
the  cathode  ;  and  in  order  that  this  counter-movement  shall  keep  down 
the  alkali  losses  to  the  amounts  neutralised  by  the  chlorine,  etc.,  con- 
tained in  the  liquors  flowing  from  the  anode,  it  is  evident  that  the 
electrolyte  must  move  with  a  velocity  not  less  than  the  OH'  ionic  velocity 
under  the  voltage  gradient  prevailing  at  the  boundary.  Under  these 
conditions  a  steady  state  is  soon  reached,  a  brine  solution  containing 
a  constant  content  of  acid  and  chlorine  passing  into  the  alkaline  zone 
at  the  boundary  layer  (which  has  a  definite  position  in  the  apparatus 
and  a  definite  concentration),  and  an  alkaline  solution  of  definite 
concentration  streaming  away  regularly  from  the  cathode. 

Now,  the  current  density  being  assumed  constant  throughout  the 
apparatus,  it  is  clear  that  the  voltage  gradient  at  any  point  will  depend 
on  the  conductivity  of  the  electrolyte  at  that  point.  In  particular,  the 
higher  the  concentration  of  the  alkali  in  the  boundary  layer,  the  higher 
is  the  conductivity,  the  lower  the  voltage  gradient,  the  lower  the  OH' 
velocity,  and  the  smaller  the  rate  of  counterflow  of  electrolyte  needs  to 
be.2  We  will  first  calculate  for  the  above  treated  ideal  case,  in  which 

1  The  solubility  of  chlorine  in  strong  halide  solutions  is  very  low. 

-  When  using  carbon  anodes,  which  produce  appreciable  quantities  of  acid  in 
the  anolyte,  the  alkali  concentration  at  the  boundary  exceeds  that  present  when 
platinum  anodes,  which  produce  but  little  acid,  are  used.  Hence  the  necessary 
rate  of  counterflow  of  electrolyte  is  less  with  carbon  than  with  platinum  anodes, 
and,  for  the  same  current  density,  stronger  solutions  at  a  lower  current  efficiency  can 
be  made. 


368    PKINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

the  alkali  concentration  gradient  is  undisturbed  by  gas  evolution. 
Let  the  alkaline  cathodic  liquors  be  n  normal.     Let  a  be  the  cross- 

section  of  the  apparatus  in  cm.2,  I  the  current,  and  hence  -  the  current 

a 

density.     In  one  second  -  —  equivalents  of  alkali  are  produced,  and 


1000  1 
hence  the  number  of  c.c.  drawn  off  per  second  is  .     The  velocity 

of    flow    of    the    electrolyte    through    the    apparatus    is    therefore 

1000  I 

-  cm./sec.     Suppose  now  the  current  efficiency  to  be  c.     Then 

in  one  second  —         -  X  ---------    equivalents  of  [acid  -j-  chlorine]  enter 

the  alkali   boundary.     These    are   dissolved  in  -  -  litres,  their 

96540  n 

equivalent  concentration  being  -          -  X  n.     This,  therefore,  is  also 

1UU 

the  lowest  alkali  concentration  possible  at  a  stationary  boundary 
under  the  experimental  conditions,  and  the  OH'  velocity  under  these 
same  conditions  gives  an  upper  limit  to  the  rate  of  flow  of  the 
electrolyte  necessary  to  prevent  alkali  from  reaching  the  anode. 

A  lower  limit  is  given  by  the  OH'  velocity  when  we  assume  that 
the  alkali  concentration  gradient,  owing  to  the  mixing  action  of  the 
hydrogen  gas,  is  non-existent,  and  that  the  alkali  concentration  at  the 
boundary  is  equal  to  that  in  the  cathode  liquors  drawn  off.  All 
technical  cases  will  lie  between  these  two  extremes.1 

The  following  examples  will  illustrate  the  above  deductions  :— 

In  the  Finlay  cell,2  2n.NaOH  is  made  with  a  98  per  cent,  current  efficiency,  the 
current  density  being  0*02  amps,  /cm.-  The  linear  velocity  of  the  brine  towards  the 

1000  x  0'02 
cathode  must,  therefore,  be  about  "ggg7Q~y~fl  T  =  0'000104   cm./sec.     Assuming 

an  alkali  concentration  gradient  [n  the  catholyte  to  be  possible,  the  brine  at  the 

100  —  98 
boundary  will  be  about  —  IAA       x  2  =  0'04n.      The  specific  conductivity  at  18°of 

recip.  ohms 
the  original  brine  used  is  about  0'2  ---  ^  —  »  and  of  the  final  catholyte  (2u.NaOU 

recip.  ohms 
-}-NaCl)  about  0'4  —          .,—   -.     We  can  suppose  the  conductivity  of  the  solution  at 

recip.  ohms 
present  in  question  to  be  *c  =  0'21  -      —  -  -  .     The  current  density  being  0*02 

0'02 
amps.  /cm.2,  the  voltage  gradient  is  0-21=0'095  volt/  cm.,  and  the  OH'  velocity 

under  a  potential  gradient  of    1  volt  /cm.  being    0*0018  cm./sec.,  the  present 

foe  also  Brochet,  Butt.  tfor.  Chim.  (iv.)  3,  1057  (11)08). 
2  See  p.  380. 


xxi.]  ALKALI-CHLORINE  CELLS  369 

0-02x0-0018 
velocity  is  Q-,          =  0*00017  cm./ sec.,  considerably  greater  than  the  rate  of 

flow    of   electrolyte.     If,  however,  we   assume    that    the  concentration  at   the 
boundary  is  identical  with  that  in  the  catholyte,  we  have  a  conductivity  of  0'4 
recip.   ohms  0'02 
IT —  ,   a  voltage  gradient   of   -^    volt /cm.,   and    an    OH7    velocity   of 

0'02 

^rj-x  0*0018 =0-00009  cm. /sec.,  appreciably  lower  than  the  rate  of  flow  of  liquid. 

As  a  matter  of  fact,  this  last  condition  corresponds  very  closely  to  the  conditions 
in  the  actual  cell,  owing  to  the  lively  hydrogen  evolution. 

Similarly  Foerster1  states  that  the  belljar  process2  can  furnish  2'5  n.  NaOH 
with  a  90  per  cent,  current  efficiency  at  a  current  density  of  0"04  amp. /cm.2 
Calculating  as  before,  we  get  0'000166  cm.  /sec.  as  the  rate  of  movement  of  the 
electrolyte.  Assuming  for  the  moment  the  alkaline  gradient  to  be  perfect,  the 
alkali  concentration  at  the  boundary  being  therefore  0'25n.,  we  get  0'00029 
cm. /sec.  as  the  approximate  velocity  of  the  OH'  ions,  considerably  exceeding 
the  rate  of  counterflow  of  the  electrolyte.  In  order  that  this  velocity  may  be 
brought  down  to  0'000166  cm. /sec.  we  must  have  a  solution  at  the  boundary 

0-04  x  0-0018             recip.  ohms 
with  K  =     Q.QQQiee     :     °'43 ^"3 »  which  corresponds  pretty  closely  to  the 

conductivity  of  the  final  cathode  liquors. 

Advantages  of  Diaphragms. — We  have  mentioned  that  most  cells 
of  the  counter- current  type  are  provided  with  a  diaphragm  between 
anode  and  cathode,  usually  situated  immediately  in  the  neighbourhood 
of  the  latter.  This  diaphragm  eliminates  convection  effects  and 
mechanical  disturbances,  enables  anode  and  cathode  to  be  placed  near 
•to  one  another,  and  far  higher  current  densities  to  be  used  than 
would  otherwise  be  possible  without  great  losses  through  interaction 
between  the  alkali  and  the  chlorine.  It  is  thus  a  great  constructional 
advantage.  Further,  by  allowing  the  hydrogen  evolution  to  destroy 
the  alkali  concentration  gradient  in  the  catholyte  without  simul- 
taneously destroying  the  alkali  boundary  itself,  the  use  of  a  diaphragm 
renders  it  possible  to  work  with  a  higher  alkali  concentration  at  that 
boundary,  corresponding  to  a  lower  OH'  velocity,  a  slower  current  of 
electrolyte,  and  a  more  concentrated  alkaline  cathodic  product. 

Disadvantages  of  Diaphragms. —The  accompanying  disadvantages 
are  the  increased  resistance  offered  to  passage  of  current  and  passage 
of  liquid.  On  the  one  hand,  an  additional  voltage  is  necessitated,  which 
may  ultimately  outweigh  any  gain  resulting  from  the  more  compact 
structure  of  the  apparatus.  On  the  other  hand,  the  f  rictional  resistance 
in  the  diaphragm  capillaries  necessitates  an  increased  hydrostatic 
pressure  on  the  anode  side,  in  order  that  the  electrolyte  shall  be  forced 
through  at  a  rate  sufficient  to  overcome  the  high  OH'  velocity  (high 
because  of  the  steep  voltage  gradient  in  the  diaphragm).  If  then  a  cell 
were  working  at  constant  current  density  and  with  the  flow  of  brine 
regulated  by  a  constant  hydrostatic  pressure  difference,  and  supposing 

1  Zeitsch.  Angew.  Chem.  23,  1375  (1910).  2  See  p.  371. 

2  B 


370    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

this  rate  of  flow  to  only  just  suffice  to  keep  back  the  OH7  ions,  then  the 
substitution  of  a  more  finely  porous  diaphragm  for  the  one  normally 
used  might  easily,  by  allowing  the  OH'  ions  to  pass  through,  markedly 
lower  the  current  efficiency. 

It  is  also  important  that  the  diaphragm  should  be  of  perfectly 
uniform  structure,  as  the  frictional  resistance  to  passage  of  liquid  is 
inversely  proportional  to  the  fourth  power  of  the  radius  of  the  capillary 
pores.  Hence,  whilst  in  those  parts  of  the  diaphragm  with  pores  of 
comparatively  large  cross-section  the  velocity  of  the  electrolyte  might 
suffice  to  counteract  the  motion  of  the  OH7  ions,  this  would  not  be  so  in 
the  more  finely  porous  parts,  and  either  losses  would  ensue  or  the  brine 
velocity  would  have  to  be  increased,  thus  giving  a  weaker  alkali  than 
would  be  the  case  with  a  diaphragm  of  uniform  structure. 

Horizontal  and  Vertical  Diaphragms. — The  diaphragm  can  be 
arranged  either  horizontally  (the  cathode  being  beneath  it,  the  brine 
percolating  through  from  above,  and  the  heavier  alkaline  liquors 
passing  away  underneath)  or  vertically  ;  the  respective  advantages  and 
disadvantages  of  the  two  arrangements  have  been  discussed  by  Billiter.1 
Vertical  diaphragms  allow  of  a  more  accessible  cell  construction,  can 
be  readily  changed,  etc.,  and,  for  the  same  power  consumption,  give  a 
far  more  compact  cell.  Further,  impurities  settle  on  the  bottom  of 
the  cell  instead  of  on  the  diaphragm.  Horizontal  diaphragms  have, 
however,  two  advantages.  If  for  any  reason  (e.g.  irregular  structure) 
alkali  does  actually  pass  through,  being  heavier  than  the  brine  solution, 
it  will  remain  as  a  layer  covering  the  upper  side  of  the  diaphragm^ 
provided  that  there  are  no  mechanical  disturbances  due  to  evolved 
chlorine  (and  always  presupposing,  of  course,  a  sufficiently  rapid  flow  of  1 
electrolyte).  With  vertical  diaphragms  this  is  not  so.  Any  alkali 
passing  through  will  fall  away  from  the  diaphragm  and  mix  and  react 
with  the  anolyte,  the  more  so  because  the  flow  of  brine  will  be  less  I 
regular  and  uniform  than  with  horizontal  diaphragms. 

As  in  all  these  counter-current  processes  the  unavoidable    losses 
are  due  to  the  dissolved  chlorine  and  acid  coming  from  the  anode,  all  ; 
further  losses  being  due  to  such  mechanical  mixing,  it  follows  that,  1 
other  things  being  equal,  cells  with  horizontal  diaphragms  must  give  ' 
better  yields  than  those  with  vertical  diaphragms,  and  particularly 
when  making  stronger  alkali.     The  second  point  is  that,  whereas  one 
side  of  a  vertical  diaphragm  is  practically  always  in  contact  with  an 
acid  chlorine  solution,  with  horizontal  diaphragms,  as  we  have  seen,  1 
this  solution  can  often  be  alkaline.     As  it  is  still  far  more  easy  to 
prepare  diaphragms  chemically  resistant  against  alkali  than  against 
acid,  the  horizontal  arrangement  has  here  a  manifest  advantage. 

Of  other  conditions  which  affect  the  working  of  these  counter- 

1  Die  Elektrochemischen  Verfahren,  vol.  ii.  p.  203  (1911). 


XXI.] 


ALKALI-CHLORINE  CELLS 


371 


current  cells,  we  have  already  discussed  the  differences  shown  by 
carbon  and  platinum  anodes.  The  brine  used  should  be  concentrated. 
Apart  from  reasons  previously  considered,  a  concentrated  brine  means 
a  higher  conductivity,  and  a  lower  voltage  gradient  and  OH'  velocity 
at  constant  current  density,  and  hence  the  possibility  of  making  stronger 
alkaline  liquors.  A  high  current  density,  apart  from  the  increased 
voltage,  means  a  more  rapid  flow  of  brine  or  else  a  more  concentrated 
alkali  at  a  lower  current  efficiency.  The  increased  mechanical  disturb- 
ances must  also  be  reckoned  with.  Working  with  a  definite  alkali 
concentration,  the  best  current  efficiencies  will  be  given  with  a  high 
current  density  and  correspondingly  rapid  brine  velocities,  as  convection 
and  mechanical  disturbances  are  then  rendered  less  important. 

A  high  temperature  does  not  appreciably  affect  the  actual  OH' 
velocity.  It  increases,  of  course,  the  velocity  under  a  constant  potential 
gradient,  but  as  it  decreases  the  potential  gradient  in  the  electrolyte 
in  nearly  the  same  ratio,  the  resultant  ionic  velocity  is  hardly  changed. 
There  is  in  fact  a  slight  decrease,  and  hence,  convection  and  diffusion 
effects  apart,  a  counter-current  cell  working  without  a  diaphragm 
should  do  rather  better  at  high  temperatures  than  at  low.  There  is, 
further,  the  fact  that  the  solubility  of  chlorine  decreases  with  rise  of 
temperature,  and  hence  also  the  '  unavoidable  '  losses.  With  dia- 
phragms the  advantage  is  greater,  as  not  only  are  the  increased  con- 
vection and  diffusion  effects  largely  avoided,  but  also  the  capillary 
resistance  of  the  diaphragm  to  the  passage  of  liquid  is  much  decreased. 

Bell  jar  Cell. — There  is  only  one  cell  of  the  counter-current  type 
working  technically  without  any  kind  of  diaphragm l — the  well-known 


r.f-  L  G- 


\  Caustic 
I  Liquors 


FIG.  88.— Belljar  Cell. 

belljar  or  gravity  cell  (worked  to  a  total  extent  of  about  6,000  H.P., 
of  which  half  is  at  the  original  works  at  Aussig,  Bohemia).  It  is  shown 
diagrammatically  in  Fig.  88.  A  belljar  A  of  non-conducting  and  non- 
porous  material  is  inverted  in  a  vessel  B,  containing  an  alkaline  chloride 
solution.  Outside  and  around  the  base  of  A  is  a  ring-shaped  cathode. 

1  Cf.  p.  373. 

2  B  2 


372    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

A  contains  the  anode  C,  and  is  further  provided  with  a  chlorine  exit  and 
an  inlet  D  for  fresh  brine.  The  outer  vessel  has  an  overflow  for  the 
caustic  liquors. 

From  what  has  already  been  said,  the  working  of  the  cell  will  readily 
be  understood.     At  the  cathode  alkali  is  formed,  and,  in  consequence  of 
its  specific  gravity  and  the  mixing  due  to  the  hydrogen  evolution,  dis- 
tributes itself  fairly  regularly  throughout  the  whole  of  the  cathode 
vessel  B.     By  virtue  of  OH7  migration  it  tends  to  ascend  the  bell  jar, 
but  is  prevented  by  means  of  the  downward  directed  counter-current 
of  brine  coming  from  the  anode,  a  sharp  alkali  boundary  and  a  neutral 
layer  forming  at  some  level  above  FG  and  below  C.     The  position  of  I 
this  boundary  and  the  alkali  concentration  prevailing  at  it  are  deter- 
mined by  the  various  factors  already  discussed.  At  E,  alkali  of  a  constant ! 
strength  is  continually  drawn  off.     It  contains  a  certain  amount  of  I 
hypochlorite  and  chlorate,  some  of  which  is  decomposed  during  the] 
subsequent  concentration  and  fusion,  but  part  of  which  is  found  in  the] 
final  product. 

In  practice  the  belljars  are  of  small  size,  rectangular,  and  of  con-j 
siderably  greater  length  than  breadth.  They  are  constructed  of  sheet 
iron  (serving  as  cathode),  lined  inside  with  cement.  The  anodes  are  on 
graphite  and  are  bored  with  a  large  number  of  small  vertical  holes 
through  which  the  chlorine  can  stream  away.  They  are  placed  hori- 1 
zontally  a  few  cm.  above  the  lower  edge  of  the  bell  jar,  the  section  of 
which  they  almost  completely  fill.  Their  life  is  very  long.  The  brine  I 
enters  through  a  hollow  duct  in  the  anode  support,  and  is  evenly] 
distributed  by  two  horizontal  glass  tubes  pierced  with  fine  holes  over] 
the  whole  surface  of  the  anolyte.  The  bell  jar  has  three  exit  pipes 
above.  One  leads  to  the  chlorine  main  ;  the  others  communicate  with 
the  two  adjacent  belljars  to  ensure  equality  of  pressure  throughout  the! 
system.  Twenty-five  such  vessels  are  supported  in  a  large  cement  tanJ 
serving  as  cathode  compartment,  and  provided  with  an  overflow  for  the! 
causticised  brine. 

Of  actual  working  data  we  have  few.  We  know,  indeed,  that  the 
belljars  are  of  small  size,  taking  each  only  20-30  amperes,  and  thaq 
the  process  requires  very  careful  attention  to  avoid  mixing  of  the  liquors! 
and  consequent  losses.  With  larger  belljars,  owing  to  flow  of  liquid 
and  current  density  varying  in  different  parts,  it  is  practically  impossible 
to  obtain  a  satisfactory  alkali  boundary.  From  the  laboratory  results! 
of  Adolph l  and  Steiner 2  we  conclude  that  in  all  probability  2n.  alkali  is] 
made  with  a  current  efficiency  of  85  per  cent,  and  employing  four  voltsj 
the  temperature  being  30°,  and  the  current  density  in  the  be  11  jar  not] 
linn  2  amps./dm.2  According  to  Foerster3  indeed,  4  amps./dm.^ 

1  Zeitsch.  Elektrochem.  7,  r>81  (1001) ;  10,  44<>  (1M-I). 
-  Zeitsch.  Elektrochem.  10,  317  (Mi-l). 
:i  Ziit*rh.  Angew.  Chem.  23,  137f>  (1910). 


XXI.] 


ALKALI-CHLORINE  CELLS 


373 


can  be  used  continuously  on  a  laboratory  scale,  giving  2'5n.  NaOH 
at  4' 4-4*5  volts  and  90  per  cent,  current  efficiency,  the  chlorine 
bcini:  99  per  cent.  pure.  Sw.NaOH  and  97'5  per  cent,  pure  chlorine 
can  be  obtained  with  80  per  cent,  current  efficiency.  Temperatures  of 
45°  can  be  worked  at  without  trouble.  These  results  would  indicate 
that  not  convection  currents,  but  mechanical  disturbances  due  to 
evolving  chlorine,  limit  in  practice  the  current  density  and  size  of 
the  belljar.  These  limitations  are  decidedly  the  weakest  point  of 
the  process. 

Billiter  Belljar  Cell l  (Billiter-Leykam  Cell). — The  gravity  process 


FIG.  89.— BilJiter  Belljar  Cell. 

has  been  modified  by  Billiter,  wrho  has  constructed  a  cell  which,  though 
carrying  large  currents,  contains  one  belljar  only,  and  in  which,  by 
decreasing  the  distance  between  anode  and  cathode,  and  by  working 


Brine, 


Asbestos 
tube 


Cathode 


FIG.  90.— Billiter  Belljar  Cell. 

at  a  high  temperature,  the  voltage  has  been  considerably  reduced. 

Figs.  89  and  90  show  diagrammatically  two  cross-sections  of  this  cell 

1  English  Patent  11693  (1910).     Also  private  communication  from  Dr.  Billiter. 


374    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

at  right  angles  to  one  another.  A  large  rectangular  cement  box  A  is 
inverted  in  an  outer  trough  B.  As  in  the  belljar  process,  A  is  provided 
with  a  chlorine  exit  and  entrances  for  brine,  and  the  large  horizontally 
placed  graphite  anodes,  pierced  through  with  numerous  holes,  are  care- 
fully cemented  into  it.  The  cathodes  consist  of  a  number  of  pieces  of 
T-iron  CC,  enclosed  in  suitably  braced  porous  asbestos  tubes  to  catch 
the  hydrogen  and  to  prevent  it  from  disturbing  the  main  electrolyte,  and 
arranged  almost  horizontally  and  parallel  to  one  another  immediately 
under  the  lower  edge  of  A.  Some  90  per  cent,  of  the  section  of  the 
belljar  is  occupied  by  these  asbestos  tubes.  The  heating  can  be 
effected  by  steam  pipes  arranged  in  a  manner  at  present  kept  secret, 
but  this  is  not  necessary.  The  best  working  temperature  is  80°-90°. 
Brine  is  fed  into  A  and  the  alkaline  liquors  drawn  off  from  B  through 
the  pipe  D.  As  the  cell  is  fed  automatically,  it  requires  little  attention, 
and  is  found  to  work  very  smoothly  and  regularly  in  practice. 

The  current  density  can  be  varied  within  wide  limits — 2*5  to 
16  amps./dm.2  at  the  anodes.  The  most  convenient  value  is  5-7 
amps./dm.2  In  the  belljar  it  is  some  15  per  cent,  smaller.  During  a 
two  months'  practically  continuous  test  on  a  250  ampere  unit,  making 
liquors  containing  up  to  12-15  per  cent.  NaOH  and  16  per  cent.  NaCl, 
the  voltage  averaged  3*1-3*2  volts,  and  the  cathodic  current  efficiency 
92-93  per  cent.  0*45  kilo.  NaOH  and  0'4  kilo,  chlorine  were  produced 
per  K.W.H.  On  a  technical  scale  much  larger  units  will  ultimately 
be  constructed,  taking  up  to  4,000  amperes.  The  first  installation 
consists  of  1,250-ampere  cells. 

These  figures  are  more  favourable  in  all  respects  (concentration  of 
alkali,  current  efficiency,  voltage)  than  those  given  by  the  belljar 
process.  The  immediate  cause  of  the  improved  results  is  that,  owing 
to  its  direction  not  being  changed  at  the  lower  edge  of  the  belljar,  the 
velocity  of  the  brine  through  the  latter  is  practically  constant  at  all 
points.  This  makes  it  possible  to  use  large  units  and  to  work  at  high 
temperatures  without  any  danger  of  serious  current  efficiency  losses, 
such  as  would  arise  in  the  ordinary  belljar  process,  with  its  unequal 
brine  velocities  in  the  middle  and  at  the  edges  of  the  belljar  and  its  much 
greater  liability  to  disturbance  by  convection  effects.  This  regular 
and  directed  flow  of  brine  is  in  turn  made  possible  by  the  position  of  the 
hooded  cathodes  underneath  the  belljar.  The  asbestos  hoods  must,  of 
course,  from  one  point  of  view  be  regarded  as  diaphragms,  as  all  the 
current  necessarily  passes  through  them  ;  but  as  they  in  no  way  affect 
the  alkali  boundary  layer  or  separate  anolyte  and  catholyte,  it  is  some- 
what straining  matters  to  do  so.  They  are,  of  course,  of  as  open  a 
structure  as  is  consistent  with  completely  retaining  the  hydrogen. 
inucd  electrolysis  hardly  affects  them  in  any  way.  Impurities 
in  the  brine  do  not  settle  on  them,  but  pass  away  with  the  cathode 
liquors. 


XXI.] 


ALKALI-CHLORINE    CELLS 


375 


Billiter  Diaphragm  Cell1  (Billiter-Siemens  Cell).  — Of  cells  with 
horizontal  diaphragms  the  only  one  needing  a  description  here  was 
also  designed  by  BiUiter  (previous  to  the  modified  belljar  cell  just  con- 
sidered). Fig.  91  shows  the  cell  diagrammatically  in  section.  In  a 
large  cement-lined  iron  trough — the  cathode  chamber — are  inverted  one 
or  more  cement  or  slate  boxes  closed  underneath  by  diaphragms  which 
are  almost  in  contact  with  the  bottom  of  the  outer  vessel.  These 


\ 


Chlorine 


J)iafhragm!    uxthode      Itiaphragi 

FIG.  91.— Billiter  Diaphragm  Cell. 

'  belljars '  carry  large  [horizontal  graphite  anodes,  exits  for  chlorine, 
inlets  for  feeding  in  brine  (not  shown),  and  earthenware  pipes,  the 
horizontal  limbs  of  which  are  immersed  in  the  brine  above  the  anodes 
as  shown.  Through  these  circulates  a  suitable  heating  liquid  which 
serves  to  keep  the  cell  at  its  high  working  temperature. 

The  diaphragm  closing  each  anode  compartment  consists  of  woven 
asbestos  cloth,  resting  on  an  iron- wire  network  (the  cathode)  and  care- 
fully cemented  all  round  its  edges  to  the  belljar.  It  is  slightly  sloped  as 
shown,  to  allow  of  escape  of  hydrogen.  Its  upper  (anode)  surface  is 
uniformly  covered  with  a  mixture  of  asbestos  wool  and  BaS04  which 
forms  the  active  part  of  the  diaphragm,  the  asbestos  cloth  merely  serving 
as  a  support  and  to  eliminate  all  disturbing  effects  caused  by  the 
hydrogen  evolution.  Such  a  diaphragm,  provided  that  the  BaS04 
mixture  is  uniformly  made  up  and  applied,  is  of  excellent  quality. 
The  fine  insoluble  powder,  while  offering  an  adequate  resistance  to 
convection  and  diffusion,  has  no  great  electrical  resistance,  and  gives 
a  diaphragm  of  very  uniform  properties  at  all  points. 

The  current  density  employed  is  4-6  amps./dm.2,  and  at  the 
working  temperature  of  85°-90°  the  bath  voltage  is  3'4r-4'0  volts. 

Starting  with  concentrated  brine  2  (300  ~      - ) ,  3  n.  (12  per  cent.)  and 

\          litre  / 

1  Zeitsch.  Angew.  Chem.  23,  1072,  1375  (1910). 

-  As  in  other  diaphragm  percolation  cells  (e.g.  Townsend,  Finlay),  the  brine 
must  first  be  purified  by  removing  iron  and  magnesium  salts.  Otherwise  these  will 
subsequently  be  precipitated  by  the  cathodic  alkali  in  the  pores  of  the  diaphragm, 


376    PKINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

4  n.  (16  per  cent.)  NaOH  can  be  made  at  95  per  cent,  and  90  per  cent, 
current  efficiency  respectively.  Quite  similarly  18-23  per  cent.  KOH 
can  be  produced.  The  chlorine  is  very  pure,  containing  <1'5  per  cent. 
C02  even  with  the  strongest  alkaline  liquors,  and  for  2-3  n.  alkali  merely 
traces.  Oxygen  is  absent.  The  life  of  the  anodes  is  consequently 
very  long.  Provided  that  the  brine  does  not  contain  iron  or  magnesia 
salts  to  any  extent,  the  same  is  true  for  the  diaphragms.  Otherwise  their 
porosity  gradually  diminishes  with  time.  This  is,  of  course,  one  of  the 
disadvantages  of  the  horizontal  arrangement.  A  convenient  capacity 
for  each  cement  belljar  has  proved  to  be  about  500  amperes.  Several  of 
these  are  combined  together  in  practice  in  one  cathode  trough  to  give 
a  2,000-3,000-ampere  cell. 

A  detailed  laboratory  study  of  the  Billiter  cell  has  been  published  by 
Miihlhaus.1  He  first  investigated  the  diaphragm,  and  found  that,  to 
obtain  constant  results,  the  BaS04  used  must  be  in  equilibrium  with  the 
strong  caustic  liquors  with  which  it  is  surrounded  in  practice.  It  is 
best  prepared  by  precipitation  in  the  hot.  At  current  densities  exceeding 
6-7  amps./ dm.2  it  deteriorates.  At  first  Miihlhaus  worked  at  room 
temperature.  Using  30  per  cent,  brine,  he  got  current  efficiencies 
varying  from  98-99  per  cent,  for  3  n.  NaOH  to  75  per  cent,  for 
6  n.  NaOH.  The  chlorine  varied  correspondingly  in  purity  between 
99'5  per  cent,  and  93  per  cent.  3  n.  NaOH  was  practically  free  from, 
chlorate.  4  n.  NaOH  gave  about  1  per  cent.  NaC103  in  the  solid  product, 
6  n.  NaOH  gave  7'2  per  cent.  NaC103.  Using  more  dilute  brine,  lower 
current  efficiencies  were  obtained.  From  the  point  of  view  of  current 
efficiency  and  purity  of  liquors,  a  high  current  density  and  a  rapid  brine 
velocity  are  better  than  a  small  current  and  a  small  velocity.  The  best 
working  conditions  at  room  temperature  are  4  amps. /dm.2  and  30  per 
cent,  brine,  making  4  n.  NaOH. 

Working  at  higher  temperatures  (70°-80°)  the  cathodic  product  was 
purer,  the  current  efficiency  somewhat  increased,  and  the  voltage 
reduced  by  0*5  volt.  Here  again  it  is  most  advantageous  to  make 
4  n.  NaOH,  which  is  possible  with  a  99  per  cent,  current  efficiency. 
The  product  is  practically  free  from  chlorate.  These  favourable  results 
arise  partly  from  the  increased  conductivity  of  the  electrolyte,  partly 
from  the  decrease  of  frictional  resistance  in  the  diaphragm. 

Hargreaves-Bird  Cell. — Cells  with  vertical  diaphragms  will  now  be 
considered.  The  oldest  is  the  Hargreaves-Bird  cell,  worked  by  the 
Electrolytic  Alkali  Co.,  Middlewich.  In  one  form  (Fig.  92)  it  consists  of 
an  iron  box  lined  with  cement  (length  10',  width  2',  height  5'),  divided 
vertically  along  its  length  into  three  compartments.  The  middle  one 

and  thus  lower  the  efficiency  of  the  latter.     These  substances  are  best  removed  by 
treatment    uith  the  necessary  quantity  of  alkali,  being  then  allowed  to  settle. 
Excess  »if  alkali  must  be  can-fulK  avoided. 
l>««ertation  (Dresden,   1<»I1). 


XXL] 


ALKALI-CHLORINE  CELLS 


377 


FIG.  92. — Hargr  eaves -Bird 
Cell. 


contains  the  anodes,  the  outer  ones  the  cathodes.  The  separating 
partitions  are  of  asbestos  or  an  asbestos-cement  composition  of  some 
kind,  and  serve  as  diaphragms.  The  cathodes  consist  of  copper  gauze 
spread  over  the  whole  outer  surface  of  each  diaphragm  and  thus  sup- 
porting the  same.  In  presence  of  the  brine, 
iron  would  be  too  strongly  attacked  by  the 
C02  which,  together  with  steam,  is  led  into 
the  cathode  compartments.  The  copper 
gauze  in  turn  is  suitably  braced  against 
the  outer  cell  walls.  The  anodes1  are 
of  carbon,  probably  graphite,  and  are 
cemented  in  through  the  roof  of  the  anode 
chamber,  which  also  contains  an  exit  for 
the  chlorine. 

The  brine  enters  at  the  bottom  of  the 
anode  compartment  and  leaves  by  an  over- 
flow at  the  top.  Part  of  it  percolates 
through  the  diaphragms  and  is  subjected 
to  electrolysis  at  the  copper  gauze.  Owing 
to  the  C02  which  dissolves  in  the  percolat- 
ing liquid,  H'  discharge  is  much  facilitated, 
and  for  the  same  reason  very  few  OH'  ions 
result.  The  solution  drawn  off  from  the 

cathode  chamber  contains  essentially,  therefore,  a  mixture  of  NaCl 
and  Na,C03,  which  are  separated  by  crystallisation.  In  practice 
saturated  brine  is  fed  into  the  anode  chamber  and  the  greater  portion 
of  it  transformed  at  one  operation  into  carbonate.  We  can  sup- 
pose about  3  n.  (16  per  cent.)  carbonate  solutions  to  result.  Very 
little  is  known  definitely  of  other  working  details.  A  cell  of  the  size 
described  takes  about  2,000  amperes  at  a  diaphragm  current  density  of 
about  2  amps./dm.2,  and  a  somewhat  greater  one  at  both  anode  and 
cathode.  The  current  efficiency  is  probably  85-90  per  cent.,  and  the 
voltage  should  not  exceed  3*5-4  volts  at  the  working  temperature  of 
85°.  These  figures  are  for  cells  with  satisfactorily  working  diaphragms. 
Under  contrary  conditions  both  current  efficiency  and  voltage  will  l?e 
much  less  favourable. 

Indeed,  the  Hargreaves-Bird  cell  seems  open  to  some  practical 
objections  arising  from  the  diaphragms.  These  diaphragms  must 
be  fairly  thick  or  dense,  or  else  the  hydrostatic  flow  through  them  at 
85°  would  be  too  rapid,  and  would  not  allow  of  making  concentrated 
carbonate  liquors.  This  is  at  once  a  disadvantage  from  the  point  of 
view  of  voltage.  Further,  the  rate  of  percolation  of  brine  cannot  be 
satisfactorily  regulated — in  fact,  to  materially  change  it  without 
seriously  diminishing  the  efficiency  of  the  cell  would  probably 
1  See  p.  153  and  Fig.  40. 


378    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 


necessitate  the  substitution  of  a  new  diaphragm  more  or  less  porous 
than  the  old  one.  Thirdly,  the  hydrostatic  pressure  forcing  the  brine 
through  varies  enormously  at  different  levels  of  the  diaphragm, 
increasing  continuously,  of  course,  towards  the  bottom.  This  means 
that,  even  with  a  diaphragm  of  perfectly  uniform  structure,  a  uniform 
flow  of  brine  is  impossible  ;  and  the  effect  of  all  irregularities  in 
structure  will  be  greatly  magnified.  Again,  the  diaphragms  are  subjected 
to  the  action  of  a  hot  chlorine  solution  on  one  side  and  to  that  of 
steam  and  a  hot  weakly-alkaline  solution  on  the  other.  Asbestos 
diaphragms  are  almost  certain  to  frequently  break  down  when  thus 
treated.  If  caustic  alkali  were  made,  matters  would  be  still  worse. 

Other  advantages,  of  course,  are  gained  by  passing  in  C02.  The 
voltage  is  lowered  owing  to  the  facilitated  H'  discharge,  and  the 
current  losses  are  much  diminished,  as  only  a  few  OH'  ions  can 
appear,  and  /C(V,  and  /HC03/  are  far  less  than  /OH/.  The  great  disadvan- 
tage is  the  production  of  cheap  soda  crystals  and  soda  ash  instead  of 
comparatively  high-priced  caustic  alkali.1 

Finally,  the  efficiency  of  the  diaphragm  determines  essentially  the 
cell  voltage  also.  If  it  is  working  well  the  voltage  will  be  low,  owing 
to  the  low  OH'  concentration  at  the  cathode, 
to  the  high  conductance  of  the  electrolyte  and 
the  absence  of  overvoltage  effects  at  the  high 
temperature,  and  to  the  low  current  density. 
But  if  the  diaphragm  is  inordinately  thick  or 
dense,  or  becomes  pulped  or  choked  during 
working,  it  will  entirely  alter  the  voltage 
relations. 

Townsend  Cell. — A  cell  somewhat  similar 
in  construction  to  the  above,  but  working 
under  very  different  conditions,  is  the  Towns- 
end  cell2  (Figs.  93  and  94),  which  has  been 
successfully  operated  for  some  years  by  the 
Hooker  Electrochemical  Co.  at  Niagara.  The 
centre  compartment,  A,  again  forms  the  anode 
chamber.  Its  two  ends  and  bottom  are  con- 
structed of  cement  and  form  a  shallow  U  (Fig. 
94  a).  In  the  thickness  of  the  cement  are  several 
channels  as  shown,  which  serve  to  introduce 
and  draw  off  the  brine,  to  lead  away  the 
chlorine,  and  for  cleaning  purposes.  The  sides 
of  the  anode  compartment  are  formed  by 
asbestos  diaphragms,  B,  on  the  outside  of 
which  press  the  iron  grids,  C,  which  serve  as  cathodes.  These  grids 


1  i',.  1)3.--   To\\nsend 
Cell. 


1  The  Electrolytic  Alkali  Co.  has  also  att  mi  pi  «•<  I  the  manufacture  of  NaHCO3. 

2  Electrochem.  Ind.  5,  209  (1907) ;  7,  313  (1909). 


XXL] 


ALKALI-CHLORINE  CELLS 


379 


simply  form  one  face  of  the  iron  side-pieces  (Fig.  94  6)  which  constitute 
the  two  cathode  compartments,  and  which  are  securely  bolted  into 
position.  The  graphite  anodes,  D,  are  cemented  in  through  the  roof 
of  the  anode  compartment,  of  which  they  occupy  the  greater  part  of 
the  space.  The  whole  construction  of  the  cell  is  very  simple,  and  it 
can  be  rapidly  dismantled  or  set  up. 


(a,) 


FIG.  94.— Townsend  Cell. 

The  cathode  compartments  have  hydrogen  exits  and  adjustable 
swan-necks  at  the  bottom  for  drawing  off  the  causticised  brine.  In 
order  to  rapidly  remove  this  product  from  the  region  of  the  cathode, 
they  are  partly  filled  with  a  liquid  indifferent  to  and  immiscible  with^ 
caustic  alkali.  This  device  constitutes  the  novel  feature  of  the  cell. 
Kerosene  oil  is  the  liquid  used  in  practice.  In  running  the  cell,  the 
relative  levels  of  the  anodic  brine  and  the  cathodic  oil  are  regulated 
by  means  of  the  adjustable  overflow  E  in  the  concrete  wall.  For  rapid 
percolation  E  is  raised,  and  vice  versa.  The  brine  thus  passing  through 
the  diaphragm  is  charged  with  alkali  at  the  cathodes.  But  it  does  not 
remain  in  the  neighbourhood  of  cathode  or  diaphragm.  Owing  to 
contact  with  the  paraffin,  the  drops  of  electrolyte  at  once  become 
spherical,  and  are  whirled  away  by  the  hydrogen  gas  into  the  main 
bulk  of  the  oil,  subsequently  falling  to  the  bottom  of  the  cathode  com- 
partment, collecting  in  the  pocket  F,  and  siphoning  off.  In  this  way 
losses  due  to  OH'  migration  are  to  a  great  extent  avoided,  and  very  high 
current  efficiencies,  together  with  high  strength  caustic  liquors,  result. 
The  same  is  true  of  the  anodic  chlorine.  As  CIO'  production  is  almost 
excluded,  very  little  oxygen  or  C02  is  evolved,  and  a  pure  anode  gas 
at  a  high  current  efficiency  is  obtained. 

Another  essential  improvement  on  the  Hargreaves-Bird  cell  effected 
by  this  oil  filling  must  be  mentioned.  The  hydrostatic  pressure  on  the 
two  sides  of  the  diaphragm  is  thereby  largely  equalised.  A  thinner 
diaphragm  can  consequently  be  used,  which  means  a  gain  in  voltage  ; 
and  a  percolation  of  brine  is  attained  which  is  practically  uniform  at 
all  parts  of  the  diaphragm.  This  is  of  great  importance. 


380    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

The  removal  of  the  alkali  is  so  effective  that  very  high  current 
densities  giving  concentrated  alkali  can  be  united  with  excellent  current 
efficiencies.  A  2,500-ampere-unit  measures  8'  X  3'  X  I ',  and  takes 
15  amps./dm.2  at  the  diaphragm.  Such  a  current  density  far  exceeds 
anything  possible  with  other  diaphragm  cells,  and  in  fact  approaches 
the  values  reached  with  modern  types  of  mercury  cells  with  the  most 
efficient  mercury  circulation.  Under  these  conditions  the  Townsend 
cell  gives  a  current  efficiency  pf  90-95  per  cent.,  producing  15  per  cent. 
NaOH,  which  still  contains  some  20  per  cent.  NaCl.  4'5-5  volts  per  cell 
are  necessary  at  the  above  current  density.  But,  working  with  figures 
still  as  high  as  10  amps./dm.2,  this  can  be  reduced  to  4-4*2  volts. 
Saturated  brine  is  used  in  the  anode  compartment,  and  the  total  distance 
between  anode  and  cathode  hardly  exceeds  1  cm.  The  great  uprusli 
of  chlorine  gas  effectually  circulates  the  brine  and  prevents  any  local 
depletion  at  the  anode.  The  working  temperature  is  probably  about 
50°-60°,  kept  up  by  joule  heat. 

The  diaphragm  is,  as  usual,  one  of  the  most  important  parts  of  1  lie 
cell.  It  is  made  according  to  Baekeland's  patent,  and  consists  of 
woven  asbestos  cloth  painted  with  a  mixture  of  asbestos  fibre,  Fe203, 
and  colloidal  ferric  hydroxide.  It  appears  to  act  very  well,  and  only 
needs  a  monthly  scraping  and  repainting,  the  operation  being  very 
simple.  The  crude  brine  must,  however,  be  purified.  In  practice, 
after  removal  of  the  iron  by  adding  alkali  and  then  settling,  it  is 
made  slightly  acid  before  being  fed  into  the  .cell.  The  anodes  are 
only  very  slowly  attacked  during  the  electrolysis.  This  is  due  to  the 
very  high  current  densities  used  and  to  the  practical  absence  of 
hypochlorites  in  the  anolyte.  The  value  of  (HC10  +  HC103)  can  fall 

as  low  as  O'l  .     The  loss  of  kerosene  is  negligible, 

litre 

Other  diaphragm  cells  of  simple  construction  are  at  work,  particu- 
larly in  the  U.S.A.,  their  output  being  consumed  by  the  maker.  Paper 
and  pulp  mills  more  particularly  use  them,  the  chlorine  being  the 
product  required.  The  Macdonald  cell,1  though  obviously  not  very 
efficient  electrochemically,  seems  to  answer  such  purposes  well. 

Finlay  Cell.2— Finally  must  be  considered  this  ingenious  cell,  which, 
though  so  far  technically  operated  on  a  small  scale  only,  gives  results 
which  in  some  respects  exceed  those  furnished  by  any  other  cell.  In- 
stead of  the  flow  of  electrolyte  taking  place  from  the  anode  through  a 
diaphragm  to  the  cathode,  two  diaphragms  are  placed  between  anode 
and  cathode,  and  the  electrolyte,  introduced  between  these,  percolates 
through  them  both  towards  the  respective  electrodes.  Thus  not  only 
is  OH'  migration  from  cathode  to  anode  more  or  less  counteracted,  but 

1  Electrochem.  Ind.  5,  43,  2:.!»  (I'.ul). 

K.I'.   17K)  (MM),   1(5853,   17492  (/.W7).     Also  private  communication  from 
Mr.  Finlay. 


XXI.] 


ALKALI-CHLORINE  CELLS 


381 


Jlydrogen, 


Chlorine 


also  H'  migration  and  chlorine  diffusion  from  anode  towards  cathode. 
In  practice,  interaction  between  chlorine  and  OH7  ions  is  probably 
almost  completely  eliminated  ;  but  the  rate  of  flow  will  be  insufficient 
to  overcome  the  velocity  of  H*  migration,  and  neutralisation  of  OH' 
ions  will  take  place  in  or  near  the  cathode  diaphragm. 

Fig.  95  shows  diagrammatically  the  principle  of  the  cell.  A  is  the 
anode  ;  B  the  cathode  ;  C  the  cathode  diaphragm  ;  D  the  anode 
diaphragm  ;  E  the  cathode  chamber  ;  F  the  anode  chamber  ;  G  the 
intermediate  chamber.  The  supply 
of  brine  enters  G  under  a  constant 
adjustable  head,  of  which  the  level 
is  indicated  by  a  gauge  attached  to 
the  cell.  The  liberated  hydrogen 
leaves  at  K,  the  chlorine  at  L.  The 
cathode  and  anode  effluents  are 
drawn  off  by  the  adjustable  swan- 
necks  M  and  N.  By  varying  the 
level  difference  HM,  the  rate  of  per- 
colation of  the  brine  through  C, 
and  hence,  at  constant  current, 
the  alkalinity  of  the  catholyte,  can 
be  regulated.  A  similar  statement 
holds  for  the  anolyte. 

In  order,  for  technical  purposes, 
to    reduce    the    distance    between 

anode  and  cathode  as  far  as  possible,  and  at  the  same  time  to  produce  a 
unit  capable  of  giving  a  large  output  for  its  size,  the  cell  .is  constructed 
somewhat  similarly  to  a  filter-press.     The  cathodes  are  of  sheet  iron 
mounted  in  shallow  wooden  frames,  and  both  sides  are  active.     Over 
these  frames  are  stretched  the  cathode  diaphragms.     Under  the  work- 
ing conditions  of   the  cell    (comparatively   weak 
alkali,    low    temperature,    and    almost    complete 
absence  of    CIO'  ions)  cloth    acts  very  satisfac- 
torily.     The     narrow    vertical    spaces    bounded 
by   cathodes,   wooden    frames,    and    diaphragms 
form  the   cathode  compartments.     On  the  other 
side  of  each  cathode  diaphragm  is  put  a  '  separa- 
tor/ a  rectangular  frame  of  waxed  cardboard  T\/' 
thick.     On  this  in  turn  is  laid  an  anode  diaphragm 
of  about  the  same   thickness,  consisting  of  well- 
woven  asbestos  cloth,  treated  with  some  indiffer- 
ent  powder    in    order    to   decrease    its    porosity 
and  make  its  structure  more  regular.1      Into  the  compartment  thus 
formed  brine  is  fed.     The  anodes  consist  of  vertical  Acheson  graphite 
1  Compare  pp.  375,  380. 


FIG.  95.— Finlay  Cell. 


Cement 


/-Copper 


I.'Ii;    <)6.— Finlay  Cell. 
Anode  Compartment. 


382    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

plates  sealed  into  a  copper  trough  by  means  of  lead  (Fig.  96).  This 
trough  forms  the  top  of  an  anode  frame  (1J"  thick),  the  other  three 
sides  being  of  teak.  The  copper  is  protected  inside  from  the  action  of 
the  chlorine  by  a  cement  coating.  The  anode  frame  is  placed  between 
two  anode  diaphragms,  thus  constituting  an  anode  compartment,  and 
there  follow  successively  (a)  separator,  (b)  cathode  diaphragm,  (c) 
cathode,  and  so  on  until  a  convenient  size  of  unit  is  reached,  when 
the  whole  is  bolted  together  between  two  stout  teak  end-frames. 

All  like  electrodes  are  connected  in  parallel.  In  order  to  collect 
the  products  with  ease  and  to  ensure  that  the  level  of  liquid  in  similar 
compartments  is  the  same,  it  becomes  necessary  to  connect  together 
all  the  anode  compartments,  all  the  cathode  compartments,  and  all 
the  middle  brine  compartments.  This  is  done  by  perforating  the 
frames  and  the  borders  of  the  diaphragms  and  separators  with 
suitable  holes,  so  placed  that,  when  the  cell  is  put  together,  con- 
tinuous ducts  are  formed  by  which  the  brine  can  be  distributed  into 
all  the  middle  compartments,  and  the  cathode  and  anode  products 
separately  led  off.  These  channels  are  put  into  connection  with  their 
respective  compartments  by  slots  cut  through  the  separators  or  frames. 
It  should  be  mentioned  that  the  edges  or  borders  of  the  diaphragms 
are  soaked  in  paraffin  wax  to  prevent  leakage  of  liquid  outwards. 

Thus  described,  the  structure  of  the  Finlay  cell  appears  com- 
plicated. But  in  reality  it  is  not  so.  Its  different  parts  are  cheap, 
it  is  quickly  put  together  and  dismantled,  and  the  whole  is  so 
bolted  together  that  leakage  from  the  numerous  joints  is  practically 
eliminated.  The  feeding  in  of  the  brine  and  the  drawing  off  of  the 
catholyte  are  effected  automatically,  and  the  cell  needs  practically  no 
attention  when  working.  The  life  of  both  diaphragms  and  anodes  is 
very  long.  A  standard  1,000-ampere  unit  measures  3'  X  2J'  X  2',  in- 
cluding the  heavy  wooden  end-frames,  but  not  including  space  taken 
by  supports,  leads,  etc.  A  2,000-ampere  unit  is  5'  X  2J'  X  2'.  The 
cell  is  therefore  very  compact. 

The  best  working  current  density  at  diaphragms  and  cathodes  is 
2  amps./dm.2  Each  cathode  then  takes  100  amperes.  Under  these 
conditions,  making  8  per  cent.  NaOH  from  purified  brine  containing 
originally  30  per  cent.  NaCl,  a  cathodic  current  efficiency  of  98-99 
per  cent,  results,  the  cell  voltage  being  only  2'9  volts,  or,  including  the 
drop  in  leads  and  connections,  3'0  volts.  This  result  depends,  of  course, 
on  the  very  low  current  density  and  the  close  proximity  of  the  electrodes. 
If  the  rate  of  flow  be  decreased,  a  stronger  liquor  (12-15  per  cent.  NaOH) 
can  be  made,  but  the  current  efficiencies  fall  to  85-90  per  cent.  The 
voltage  remains  almost  unaltered.  If  current  density  and  rate  of  flow 
be  doubled,  the  current  efficiency  increases  slightly,  and  the  voltage 
rises  to  3'4-3'5  volts.  The  working  temperature  is  about  30°-35°,  kept 
up  by  joule  heat.  The  chlorine  obtained  is  very  pure  (99'4  per  cent.), 


XXI.] 


ALKALI-CHLORINE  CELLS 


383 


corresponding  to  the  high  cathodic  current  efficiency.  It  contains 
traces  only  (O05  per  cent.)  of  C02  ^and  small  quantities  of  oxygen. 
Under  normal  working  conditions,  the  anode  liquors  are  nearly  free 
from  bound  active  chlorine.1 

A  laboratory  investigation  of  the  Finlay  cell  has  been  carried  out  by 
Donnan,  Barker,  and  Hill.2  The  results  obtained  did  not  quite  equal 
those  given  by  a  technical  unit.  The  current  efficiencies  were  from 
1-5  per  cent,  lower,  and  the  voltage  averaged  3'4  volts.  The  discre- 
pancies are  due  to  the  somewhat  unsatisfactory  behaviour  of  the  Bern- 
feld  diaphragms  used,  and  to  certain  slight  constructional  differences 
inherent  in  the  small  size  (20  amperes)  of  the  cell  employed. 

5.  Comparative 

In  order  to  compare  the  energy  efficiencies  of  the  various  processes 
considered,  it  is  necessary  to  know  the  theoretical  decomposition  voltage 
of  a  NaCl  solution,  or  rather  the  E.M.F.  of  the  cell  H2  j  \^  |  NaCl  |  C12. 
And  this  E.M.F. ,  of  course,  varies  somewhat  with  the  NaOH  and  NaCl 
concentrations.  Polarisation  measurements  made  by  different  experi- 
menters vary  between  2'23-2'45  volts.  The  Helmholtz-Thomson  rule 
gives  2'3  volts,  and  it  is  perhaps  best  to  use  this  conventional  value  for 
purposes  of  calculation.  Then  we  can  make  up  a  comparative  table  of 
electrochemical  data,  which  holds  for  those  conditions  under  which  the 
cell  in  question  is  normally  worked. 

TABLE  LIX 


Cell 

Normality 
of  alkali 

Cathodic 
current 
efficiency 

Voltage 

Energy 
efficiency 

K.W.H. 

per  kilo. 
NaOH. 

Per  cent. 

Volts 

Per  cent. 

Castner  (rocking  cell) 

5  n. 

92 

4-2 

50 

3-1 

Kellner  (platinum  anodes) 

5-6  n. 

97 

5-0 

45 

3-4 

Kellner  (carbon  anodes) 

5-6  n. 

95 

4'5 

49 

3-1 

Whiting 

5  n. 

92 

4-0 

53 

2-9 

Wildermann 

5-6  n. 

97 

5-0 

45 

3-4 

Griesheim  (carbon  anodes) 

1-2  n. 

70-80 

3'6 

45-51 

3-0-3-4 

Griesheim  (magnetite  anodes) 

1-2  n. 

70-80 

4-0 

40-46 

3-3-3-8 

Outhenin-Chalandre 

(2  n.  ?) 

66 

3-7 

41 

3-7 

Belljar 

2  n. 

85 

4-0 

49 

3-1 

Billiter  '  membrane  ' 

3  n. 

92 

3-1 

68 

2-3 

Billiter  '  diaphragm  ' 

3-4  n. 

95 

3-7 

59 

2-6 

Hargreaves-Bird 

3n  Na^CO;} 

85 

3-7 

— 

— 

To  wnsend 

4  n. 

94 

4'8 

45 

3-4 

Finlay 

2  n. 

98 

3-0 

75 

2-0 

1  The  dissolved  gaseous  chlorine  must  of  course  be  removed  if  the  spent 
anode  liquors  are  re-saturated,  and  again  fed  into  the  middle  cell  compartments. 
-  Trans.  Farad.  Soc.  5,  49  (1909). 


384    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

It  has  been  repeatedly  made  clear  that  electrochemical  data  are  not 
the  only  points  to  be  considered  in  a  comparison  of  different  kinds  of 
cells.  Nevertheless  the  preceding  table  seems  to  show  that  in  localities 
where  power  is  dear  the  Finlay  cell  will  have  a  certain  advantage  over 
all  others,  in  spite  of  the  fact  that  it  is  only  designed  to  make  dilute 
alkali  solutions.  This  is,  of  course,  its  weak  point.  But  even  when 
making  stronger  alkali  (3-4  n.)  it  is  still  a  very  good  cell,  giving  almost 
the  same  results  electrochemically  as  the  Billiter  '  membrane  '  cell. 
Under  those  conditions  it  would  have  the  advantage  of  being  far  more 
compact  than  the  latter,  but  the  disadvantage  of  the  possibility  of; 
diaphragm  troubles.  In  particular  the  simple  cloth  cathode  dia- 
phragms could  probably  not  be  used. 

Where,  on  the  other  hand,  power  is  very  cheap,  recent  types  of 
mercury  cells  are  undoubtedly  the  most  suitable.  They  furnish  very 
strong  caustic  liquors  almost  free  from  chloride,  and  give  a  pure  anode 
gas.  But  their  first  cost  is  high.  The  Townsend  cell  is  free  from  thisj 
defect,  and,  moreover,  works  at  high  current  densities  and  gives  strong 
alkaline  liquors.  These,  of  course,  contain  much  salt.  In  districts  with 
moderately  cheap  power  the  two  Billiter  cells  should  find  large  applica- 
tion. The  diaphragm  cell  furnishes  results  similar  to  those  of  the 
Townsend  cell  as  far  as  current  efficiency  and  concentration  of  alkali! 
are  concerned,  whilst  the  voltage  is  much  lower.  The  *  membrane  ' ; 
cell  gives  a  somewhat  weaker  alkali,  but  at  a  still  better  energy  effi- 
ciency. Its  low  voltage  closely  approximates  to  that  of  the  Finlay  cell.] 
Both  these  cells  unfortunately  demand  a  large  floor  space,  but  needj 
very  little  attention  when  working.  The  other  cells  described  compare ; 
rather  unfavourably  with  the  above.  Their  energy  efficiency  is  low  or] 
they  make  weak  alkali,1  and  require  Constant  attention  during  working. 

The  Chlorine  Question. — The  future  of  the  electrolytic  alkali  industry 
depends  essentially  on  fresh  outlets  being  found  for  the  chlorine.2  The 
demand  for  bleaching  powder  is,  as  matters  stand,  actually  decreasing,  i 
This  is  partly  due  to  the  growing  number  of  small  electrolytic  bleaching 
liquor  installations  now  in  operation.  Further,  the  methods  of  using] 
bleaching  powder  solutions  now  generally  employed  are  more  rational 
and  efficient  than  the  old  methods,  and  the  consumption  of  bleach  is 
consequently  less.  Many  large  pulp  and  paper  works,  moreover,  now 
make  their  own  bleach,  often  using  cells  such  as  the  Macdonald  cell.s| 
Finally,  the  introduction  of  electrolytic  alkali  processes  means  the  pro-] 
duction  of  free  chlorine  where  the  Leblanc  process  gives  HC1. 

The  discovery  of  fresh  uses  for  chlorine  is  therefore  imperative,  ;m<l 
the  following  methods  of  utilisation,  other  than  bleach  production,  may 
be  mentioned 4 : 

1.  Combination  with  cathodic  hydrogen  to  form  HC1,   which  is 

1  OrNasCOa.  -  See  e.g.  Askenasy,  Zeit»ch.  Eltktrochem.  17,  075  (I! HI). 

3  800  p.  8  4  See  also  r//<-/«.  hid.  31,  404 


XXL]  ALKALI-CHLORINE  CELLS  385 

readily  carried  out.  This  is  chemically  a  step  backward,  but  there 
are  conditions  under  which  it  is  a  gain  economically.  And  if  a  gas  cell 
which  would  work  at  technical  current  densities  could  be  designed, 
using  the  chlorine  as  soluble  cathode,  the  hydrogen  as  soluble  anode,  the 
problem  would  be  finally  solved.1 

2.  Liquefaction  of  the  chlorine.     This  is  increasingly  carried  out  in 
Germany  and  U.S.A.,  the  chlorine  being  used  by  small  customers.     Its 
application  in  metallurgy  has  also  been  suggested. 

3.  Detinning.     The  Goldschmidt  method  of  detinning  tin-scrap  by 
the  action  of  dry  chlorine  will  probably  completely  replace  the  electro- 
lytic method 2  in  the  future,  and  should  form  a  valuable  outlet. 

4.  Production  of  substances  used  in  the  artificial  dye  and  indigo  in- 
dustries.    Such  are  CH2C1  .  COOH  (e.g.  the  Badische  Anilin  und  Soda 
Fabrik  uses  a  large  part  of  the  chlorine  from  its  Griesheim  cells  for  this 
purpose),  C6H5C1,  C6H3(N02)2C1,  C6H5CH2C1,  C6H5CHC12,  etc. 

5.  Production  of  solvents  such  as  CC14,  S2C12,  chlorides  of  ethylene 
and  acetylene,  etc.     The  Weston  Chemical  Co.  utilises  some  of  the 
Castner-Kellner  Co/s  chlorine  for  this  purpose,  using  the  process  of 
Askenasy  and  Mugdan.3 

6.  Other  organic  compounds,  such  as  chloroform,  chloral,  etc. 
Methods  of  utilising  the  hydrogen  are  also  important.     At  present 

most  of  it  is  lost.  But,  besides  the  formation  of  HC1  mentioned,  there 
are  other  possibilities.  Its  use  has  been  suggested  (in  the  Griesheim 
cells  which  work  at  80°)  to  reduce  nitrobenzene  to  aniline,  the  nitro- 
benzene being  simply  fed  into  the  cathode  compartment,  copper 
cathodes  being  employed.  Some  can  be  compressed  and  used  for 
filling  balloons  and  air-ships,  or  in  the  oxyhydrogen  blowpipe.  And 
it  can  be  mixed  with  the  producer  gas  and  burnt  in  the  gas-engines  if 
the  latter  are  used  for  power  raising.  Finally,  two  catalytic  reactions 
must  be  mentioned,  viz.  the  reduction  of  nitrogen  by  hydrogen  to 
ammonia,4  using  osmium  as  a  catalyst,  and  the  reduction  of  unsaturated 
fatty  acids  and  glycerides  to  the  saturated  products,  the  catalyst  being 
reduced  nickel.  Both  processes  have  lately  attracted  much  interest. 
To  avoid  '  poisoning '  of  the  catalyst  a  very  pure  gas  is  probably  needed, 
and  this  pure  gas  electrolytic  alkali-chlorine  cells  are  particularly 
adapted  to  furnish. 

Literature 

Billiter.  Die  eleJctrochemischen  Verfahren  der  chemischen  Gross- 
Industrie,  vol.  ii. 

Brochet.     La  Soude  Electrolytique. 

Lucion.  Elektrolytische  Alkalichlaridzerlegung  mit  flussigen  Metall- 
kathoden. 

1  See  p.  219.  2  See  p.  288. 

s  See  Zeitsch.  Elektrochem.  15,  773  (WOO).  4  See  p.  479. 

2  o 


CHAPTER  XXII 

OTHER  ELECTROLYTIC  PROCESSES 

IN  this  chapter  will  be  discussed  the  technical  electrolytic  preparation 
of  several  less  important  substances  not,  so  far,  considered.  We  shall 
deal  with 

Hydrogen  and  oxygen.  Potassium  ferricyanide. 

White  lead  ;    chrome  yellow  etc.     Permanganates. 
Bromine  and  bromates.  Perchlorates. 

Bromoform  and  iodoform.  Persulphates. 

Chromic  acid.  Hydrogen  peroxide. 

Anthraquinone.  Hyposulphites. 

Hydrogen  and  Oxygen. — Where  one  only  of  these  gases  is  needed, 
its  preparation  by  electrolysis  is  usually  uneconomical.     Hydrogen 
is  produced  in  large  quantities  in  electrolytic  alkali  works,  and  can  be 
manufactured  very  cheaply  from  water-gas,  whilst  oxygen  is  best  ob- 
tained by  fractional  distillation  of  liquid  air.     But  where  both  gases  are 
required,  and  particularly  for  the  oxyhydrogen  flame,  used  extensively 
in  platinum  working  and  in  some  kinds  of  lead  burning,  electrolysis 
is  the  most  convenient  method  of  preparation.     Technical  electro- 
lysers  employ  solutions  of  either  H2S04  or  of  KOH  or  NaOH.     In  the 
first  case  they  are  constructed  of  lead,  and  of  iron  when  using  alkali. 
The  voltage  of  each  cell  is  composed  of  the  reversible  decomposition 
voltage  (in  this  case  T23  volts  at  room  temperature),  the  oxygen  and 
hydrogen  overvoltages,  and  the  voltage  necessary  to  overcome  the! 
electrolytic  resistance.     The  alkaline  solutions  used  (10-25  per  cent.)! 
have  a  lower  conductivity  than  the  20-30  per  cent.  H2S04  employed. 
On  the  other  hand,  hydrogen  and  oxygen  overvoltages  at  lead  are  con-  I 
siderably  greater  than  at  iron.     So  on  the  whole  electrolysers  using! 
alkaline  solutions  have  a  lower  working  voltage  than  those  using  acid,  j 
The  current  efficiency  invariably  approaches  100  per  cent.     The  purity  j 
of  the  gases  is  usually  97-99  per  cent.     Distilled  water  must  be  added  j 
to  replace  that  electrolysed,  otherwise  the  vessels  will  be  ultimately  I 
attacked. 

386 


HYDROGEN  AND  OXYGEN 


387 


The  only  well-known  electrolyser  using  H2S04  is  that  of  Schoop.1 
It  consists  of  a  cylindrical  lead-lined  vat,  some  feet  high,  containing 
a  number  of  electrodes  vertically  placed.  These  electrodes  (Fig.  97) 
take  the  form  of  long  tubes,  filled  with  fine 
lead  wire  to  increase  the  active  surface,  and 
perforated  round  the  bottom  to  allow  entrance 
to  electrolyte  and  current.  Each  electrode  is 
surrounded  by  a  cylindrical  hood  of  non-con- 
ducting material,  open  underneath  and  per- 
forated round  the  bottom  for  the  passage  of 
the  current,  but  closed  round  the  top  of  the 
electrode.  This  joint  is  further  protected  by 
a  water-seal.  The  gas  generated  inside  the 
hood  streams  out  through  the  top  of  the  tube. 
By  regulating  the  taps  and  the  level  of  the 
electrolyte  in  the  outer  vessel,  the  gases  can  be 
delivered  at  varying  pressures.  One  such  elec- 
trolyser may  contain  perhaps  four  oxygen  and 
eight  hydrogen  electrodes  connected  in  parallel, 
and  will  take  150-200  amperes  at  3'3-3'6 
volts.  Pb02  is  continually  formed  at  the  anodes, 
and  the  oxygen  usually  contains  ozone.  A  cubic 
metre  of  mixed  gases  requires  about  5'9  K.W.H. 

The  Schmidt 2  (Oerlikon)  electrolyser  uses  an 
alkaline  electrolyte,  and  in  construction  re- 
sembles a  filter-press  and  the  Finlay  alkali- 
chlorine  cell,  to  which  the  reader  is  referred.8 

The  electrodes  are  of  iron,  lightly  corrugated  ;  the  diaphragms  of 
asbestos,  with  rubber  packing  round  the  edges.  Instead,  however, 
as  in  the  Finlay  cell,  connecting  all  like  electrodes  in  parallel  and 
passing  through  large  currents  at  small  voltage,  the  electrodes  are 
bipolar,  as  in  the  series  system  of  copper-refining  and  the  Kellner 
bleaching-liquor  electrolysers.  A  high  voltage  is  impressed  on  the 
two  end  electrodes,  and  the  same  current  passes  through  all  the 
chambers.  In  the  present  case,  shunt  current  losses 4  are  very  small. 
The  current  efficiency  is  practically  100  per  cent.,  and  the  voltage 
per  compartment  in  the  latest  form  only  about  2'3  volts.  A  cubic 
metre  of  mixed  gases  requires  3'7  K.W.H. 

The  Garuti  apparatus  is  largely  used  in  Europe.  An  inverted  iron 
box  is  divided  into  a  number  of  narrow  compartments  by  vertical  iron 
diaphragms,5  and  suspended  in  an  outer  wooden  tank,  lined  with  iron. 

1  Electrochem.  2nd.  1,  297  (1903). 

2  Zeitsch.  Elektrochem.  7,  296  (1900).  3  P.  380.  4  Pp.  259,  327,  392. 
5  The  lower  half  of  the  diaphragms  can  be  perforated  to  allow  of  passage  of 

electrolyte  without  fear  of  any  mixing  of  gases. 

2  c  2 


a 

(scus  exit 
^-Waterseal 

ct«x 

—  Holes 

^falter  level 
of  electrolyte 

*—Hoo& 

—  Lead  tube 

~ 

"Dinner  level 
of  electrolyte 

1 

.  9 
:iec 

7.  —  Schoop 
jtrolyser. 

388    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

The  compartments  are  provided  alternately  with  iron  anodes  and 
cathodes,  connected  in  parallel  and  suitably  insulated,  and  all  anode 
and  all  cathode  chambers  are  connected  with  their  respective  common 
gas  reservoirs.  The  pressure  at  which  the  gases  are  evolved  is  readily 
regulated,  just  as  in  the  Schoop  apparatus.  Strong  KOH  is  the  elec- 
trolyte, and  the  voltage  is  kept  below  3'4  volts.  If  it  exceeds  this,  the 
iron  diaphragms  may  act  as  bipolar  electrodes  and  give  an  explosive 
gaseous  mixture.  The  normal  figure  is  2*5  volts,  and  a  cubic  metre  of 
electrolytic  gas  requires  4*1  K.W.H. 

The  last  electrolyser  we  will  describe  is  that  of  Schuckert.1  An 
iron  trough  of  fifty  litres  capacity  is  divided  into  a  number  of  compart- 
ments by  vertical  partitions  of  good  insulating  material,  extending 
from  the  top  three-quarters  of  the  way  down  the  cell.  These  contain 
alternately  iron  anodes  and  cathodes.  Iron  hoods  suspended  between 
the  insulating  partitions  carry  off  the  gases.  The  electrolyte  is  15-20 
per  cent.  KOH,  and  a  unit  of  this  size  takes  600  amperes  at  2*5-3  volts. 
The  heat  produced  warms  up  the  cell,  which  is  well  lagged,  to  about 
70°.  The  diaphragms  are  stable,  but  the  iron  anodes,  as  in  other  alkaline 
electrolysers,  must  occasionally  be  renewed,  owing  to  traces  of  chlorides, 
etc.,  present,  which  render  them  non-passive. 

White  lead  :  chrome  yellow  :  lead  peroxide  :  cuprous  oxide.— The 
preparation  of  these  four  substances  is  considered  together,  as  it  depends 
in  each  case  on  the  same  principle,  due  to  Luckow.  Lorenz 2  showed 
that,  if  an  alkali  salt  is  electrolysed  between  cathode  and  a  soluble  anode, 
the  cathodic  alkali  reacts  with  the  salt  dissolved  at  the  anode  to  form 
hydroxide  of  the  anode  metal,  of  excellent  quality,  which  can  be  readily 
washed  free  from  adsorbed  salts.  The  reason  is  that  no  excess  of  alkali 
is  employed,  the  alkali  and  the  anodic  salt  being  produced  in  equivalent 
quantities.  Thus,  if  Na2S04  be  the  electrolyte,  copper  the  anode,  and 
the  electrolysis  be  carried  out  at  90°,  CuO  is  produced. 

Suppose   now  the   electrolyte   is   a   salt   whose   anion    forms    an 
insoluble  salt  with  the  anodic  metal — e.g.  Na2S04  and  lead.     Then,  on 
electrolysis,  Pb"  ions  will  dissolve,  and  PbS04  will  be  precipitated — 
but   in   the    form    of    a    dense    adherent   layer  on    the    anode.     If 
now  the   electrolyte   contains  only  a   small  proportion    of   Na2S04,   I 
the  great  mass  of  dissolved  salt  (e.g.  NaC103)  giving  no  precipitate   , 
with    solutions  containing  lead,   then  on  passing   a   current   PbS04 
will   still  be   formed  as  before,   but  no  longer  as  adherent    anodic   • 
crusts.     The  surface  of  the  metal  will  remain  perfectly  clean  and   I 
metallic,  the  PbS04  streaming  away  from  it  as  a  fine  precipitate. 
The  reason  is  that,  when  the  small  number  of  SO/  ions  in  the  neigh- 
bourhood of  the   anode  are  removed  as  PbS04,  others  can  only  be 

1  Elfctrochem.  Ind.  1,  579  (1903).      Elektrochem.  Zeitsch.  14,  230,  248  (1908). 
'  Z'itsch.  Anorg.  Chem.  12,  436 


XXIL]  WHITE  LEAD,  ETC.  389 

furnished  by  diffusion  or  by  ionic  migration.  As  the  greater  part  of 
the  current  is  carried  by  the  anion  (€103')  which  is  present  in  excess, 
the  SO/  ions  are  removed  before  they  reach  the  anode  by  the  excess 
of  Pb"  ions  streaming  away,  and  the  PbS04  is  precipitated  at  a  short 
distance  from  the  electrode,  a  distance  increasing  if  the  concentration 
of  the  *  precipitating  '  ion  is  reduced. 

Luckow  worked  out  similar  methods  for  the  preparation  of  many 
insoluble  salts,  including  those  mentioned  at  the  beginning  of  the 
section.  For  white  lead  l  he  recommended  a  1'5  per  cent,  solution  of 
a  mixture  of-  9  parts  NaC103  :  1  part  Na2C03,  electrolysed  between  a 
pure  lead  anode  and  a  hard  lead  cathode,  C02  being  passed  in  as  required. 
The  anodic  current  density  was  about  O24  amp. /dm.2,  the  bath  voltage 
being  1-4  volts.  He  obtained  3'5-4  kilos,  white  lead  per  K.W.H.  For 
chrome  yellow  (PbCrOJ  the  same  conditions  and  the  same  electrolyte 
were  employed,  with  the  exception  of  the  substitution  of  Na2C03  by 
Na2Cr04,  chromic  acid  being  constantly  added  to  replace  that  removed 
in  the  precipitate.  The  electrolyte  for  Pb02  was  a  1*5  per  cent,  solution 
of  a  mixture  of  99'5  parts  Na2S04  +  0*5  part  NaC103,  slightly  acidified 
with  H2S04.  2*8  volts  were  required.  Cu20  requires  copper  electrodes 
and  an  electrolyte  of  NaCl,2  with  a  small  quantity  of  alkali. 

Of  these  substances  the  author  believes  that  small  quantities  of 
Cu20  and  white  lead  only  are  now  technically  prepared.  For  the  latter 
substance  the  temperature  is  kept  below  50°,  and  the  bath  requires  about 
two  volts.  Instead  of  leading  in  pure  C02,  a  gas  diluted  with  air  is 
employed,  as  it  gives  a  precipitate  of  the  required  basicity.  Unfor- 
tunately, a  pigment  with  the  great  '  covering  power  '  of  Dutch  process 
white  lead  has  not  yet  been  prepared. 

The  Luckow  processes  have  been  studied  by  Le  Blanc  and  Bin- 
schnedler,3  Isenburg,4  Burgess  and  Hambuechen,5  Miller,6  and  Gillett.7 
The  necessity  has  been  shown  of  employing  very  low  concentrations  of 
the  precipitating  salt  and  low  current  densities  if  precipitates,  and  not 
crusts,  are  to  result.  At  the  same  time,  as  would  be  expected,  and  as 
work  on  Cu20  actually  showed,  the  precipitated  particles  are  finer  the 
higher  the  concentration  and  the  greater  the  current  density — i.e.  the 
more  quickly  the  precipitate  is  formed.  These  two  opposing  sets  of 
conditions  render  it  impossible  to  produce  particles  beyond  a  certain 
degree  of  fineness,  which  explains  why  electrolytic  white  lead  has  not 
been  an  unqualified  success,  as  *  covering  power  '  and  fineness  of  division 
are  intimately  connected. 

1  Zeitsch.  Elektrochem.  9,  797  (1903). 

2  In  which  copper  dissolves  anodically  as  Cu   ions. 
:<  Zeitsch.  Elektrochem.  8,  255  (1902). 

4  Zeitsch.  Elektrochem.  9,  275  (1903). 

?  Trans.  Amer.  Electrochem.  Soc.  3,  299  (1903). 

!  Jour.  Phys.  Chem.  13,  256  (1909). 

'  Jour.  Phys.  Chem.  13,  332  (1909). 


390    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

Bromine  and  Bromates.  —  The  chief  sources  of  the  world's  bromine 
are  the  Stassfurt  salt  deposits  in  Europe  and  certain  brine  deposits 
in  America.  The  residual  liquors  obtained  from  these  contain  small 
quantities  of  bromine  as  MgBr2.  This  bromine  is  usually  recovered 
by  means  of  chlorine,  but  electrolysis  is  sometimes  employed.  Bromates 
are  readily  prepared  by  electrolysing  alkaline  bromide  solutions,  and 
find  some  small  application  as  oxidising  agents,  etc. 

The  electrolysis  of  alkaline  bromides  has  been  thoroughly  studied  by 
Pauli,1  Kretzschmar,2  Boericke,3  and  Foerster  and  Yamasaki.4  The 
value  of  E.P.j,,^^  Br,  is  +  1*098  volts  at  25°.  If  a  neutral  alkaline 
bromide  solution  be  electrolysed  with  a  platinised  platinum  anode 
free  from  oxygen,  Br'  discharge  takes  place  reversibly  as  soon  as 
this  potential  value  is  exceeded.  Traces  of  oxygen  are  simultaneously 
produced  (the  reversible  OH'  discharge  potential  in  a  neutral  solution 
is  +  0*82  volt).  This  oxygen  dissolves  in  the  anode,  perhaps  forming 
some  platinum  oxide,  and  renders  the  Br'  discharge  irreversible,  a  small 
and  increasing  overvoltage  being  now  necessary.  In  alkaline  solution, 
in  spite  of  the  depolarisation  of  the  bromine  discharge  effected  by  the 
OH'  ions,  the  formation  of  anodic  oxygen  is  so  facilitated  that  the 
condition  of  the  electrode  is  quickly  altered,  a  considerable  overvoltage 
being  associated  with  Br'  discharge.  With  smooth  platinum,  oxygen 
is  immediately  evolved,  and  in  turn  brings  the  bromine  overvoltage 
into  play,  an  excess  polarisation  of  about  0*33  volt  in  neutral  solution 
being  finally  required. 

The  anodically  produced  bromine  is,  like  chlorine,  partly  hydrolysed 
as  follows  :  Br2  -f  H20  ^±  HBrO  +  H'  -f  Br',  but  much  less  than 
was  the  case  with  chlorine.5  If  now  the  cathodic  OH'  ions  are  kept 
separated  from  the  bromine  by  a  diaphragm  or  other  means,  the 
bromine  can  be  continuously  produced  and  finally  distilled  off.  If, 
however,  the  OH'  ions  gain  access  to  the  anode,  the  above  hydro- 
lysis, now  better  written 


continues,  and  excess  of  OH'  ions  will  lead  to  the  neutralisation  of  the 
HBrO  thus  :— 

HBrO  +  OH'  ^±  H20  +  BrO'. 

Still  the  hydrolysis  of  the  bromine  and  the  neutralisation  of  the  acid 
are  both  very  incomplete,  HBrO  and  bromine  existing  side  by  side 
in  the  same  solution  with  OH'  ions.  C103'  formation  was  shown6  to  take 

Zeitsch.  Elektrochem.  3,  474  (/.v.'y7). 

Zeitech.  Elektrochem.  10,  802  (mi}. 

Zeitsch.  Elektrochem.  11,  57  (1905). 

Zeitsch.  Elektrochem.  16,  321  (MM). 

<  f.  page  319.  fl  Pp.  320,  323. 


xxii.]  BROMINE  391 

place  in  two  ways — in  alkaline  solution  by  CIO'  discharge  (primary 
formation),  and  in  acid  solution  by  the  chemical  interaction  of  HC10 
and  CIO'  ions  (secondary  formation). 

In  the  present  case  the  production  of  bromates  under  both  conditions 
results  from  a  secondary  reaction.  In  acid  or  neutral  solution  the 
equation  is  similar  to  that  with  chlorates — 

L'HBrO  +  BrO'  — >  Br03'  +  2H'  +  2Br'. 

This  reaction l  proceeds  100  times  more  quickly  than  the  corresponding 
chlorate  reaction,  and  anodic  bromate  formation  consequently  takes 
place  in  slightly  acid  solution  with  great  ease,  giving  a  very  high  yield, 
particularly  at  higher  temperatures.  In  alkaline  solution  the  formation 
of  bromates  is  the  result  of  direct  oxidation  of  BrO'  ions  by  anodic 
oxygen — BrO'  +  20  — >  Br03'.  But,  as  this  reaction  proceeds  more 
slowly  than  the  first,  there  are  consequently  oxygen  losses,  and,  the 
voltage  being  also  higher,  the  method  is  not  used. 

The  crude  bromide  liquors  contain  large  quantities  of  MgCl2  and, 

in  America,  NaCl.     The  ratio   -~  varies  between        -  and  

[1/LJ  1  1 

Fig.  98  contains  current  anode-potential  curves  taken  by  Bose2  in 
0-965  n.  HC1  containing  varying 
amounts  of  KBr.  The  first 
three  curves  rise  sharply,  corre- 
sponding to  Br'  discharge.  Curve 
IV  shows  an  initial  rise  (Br'  dis- 
charge), then  a  horizontal  part, 
followed  by  a  second  rise,  which 
corresponds  to  Cl'  discharge. 
Curve  V  is  practically  that  of 


M    flip  I'nfliioriPA  r»f  tTiphrnmidp          0         0-2       0-4       0-6       0-8       1-0       1-2       1-4 
1,  me  ]  Anode  Potential  in  Volts 

being      imperceptible.        Using   KBr  Succe33ively  i.0,o-i,o-oi,o-ooi,o-oooi  normal. 
these  results,  we  conclude  that  Fio.  98. 

solutions  with  a  ratio  -—  >  O'Ol  will  give  only  bromine  on  anodic 
[C1J 

polarisation,  whilst  solutions  with  a  ratio  - — -  of  O'OOl  will  furnish 
^  [Cl] 

pure  bromine  at  low  current  densities.  There  is  consequently  no  fear 
of  chlorine  being  liberated  under  technical  conditions  if  the  anodic 
current  density  be  kept  fairly  low. 

Of  technical  bromine  cells,  we  may  first  briefly  describe  that  of 
Wimsche,  formerly  worked  at  Westeregeln.  A  unit  consisted  of  a  rect- 
angular cement  bath,  through  the  cover  of  which  entered  the  electrodes. 
The  cathodes  were  enclosed  in  cylindrical  clay  diaphragm  cells.  They 

1  See  p.  320.  2  Zeitsch.  Elektrochem.  5,  159  (1899). 


392    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

were  originally  carbon  tubes,  connected  with  a  system  of  channels 
embedded  in  the  cover  of  the  bath.  The  electrolyte  flowed  from  cell 
to  cell  through  these  channels,  entering  each  cell  through  the  cathode. 
Later,  copper  cathodes  were  used,  two  for  each  cell,  the  electrolyte 
being  forced  to  flow  past  both  of  them  by  means  of  a  vertical  wooden 
partition  dividing  the  cell  into  two  parts.  The  anodes  were  of  carbon, 
cemented  into  the  roof  of  the  bath.  The  electrodes  were  connected 
in  parallel.  The  liquors,  heated  to  80°,  flowed  through  the  anode 
compartment,  were  charged  with  bromine,  and  passed  out.  After 
being  suitably  freed  from  bromine,  they  entered  the  cathode  system. 
There  they  became  charged  with  Mg(HO)2.  This  mostly  remained 
dissolved,  owing  to  the  high  content  of  MgCl2.  The  current  density, 
however,  had  to  be  kept  low,  and  the  liquors  were  very  rapidly  circu- 
lated. Using  a  current  density  (at  both  electrodes)  of  T15  amps./dm.2 
and  3*4  volts,  the  yield  of  pure  redistilled  bromine  corresponded  to  a 
current  efficiency  of  68-70  per  cent.  The  material  losses  were  small. 

The  process  was  eventually  abandoned  on  account  of  its  complica- 
tions (clay  cells,  etc.)  and  because  the  separation  of  magnesia  inside 
the  cathode  compartments  could  not  be  entirely  avoided.  The  only 

process  now  working  in 
Europe  is  that  of  Kossuth. 
His  apparatus  is  essentially 
simpler,  which  accounts  for 
its  success.  Fig.  99  shows 
the  electrolyser  in  plan.  A 
long  open  cement  trough  con- 


FIG  99 — Kossuth  Bromine  Cell.  tains    a    number   of    parallel 

vertical  carbon  plates  resting 

on  the  bottom  of  the  bath  and  rising  some  distance  above  the  level 
of  the  liquid.  They  are  so  arranged  that  the  electrolyte,  which  flows 
rapidly  through,  is  forced  to  take  a  zigzag  path,  passing  between  the 
whole  series  of  plates.  These  plates  are  bipolar  electrodes,  the  two 
end  ones  being  connected  to  the  source  of  current.  During  its  passage 
through  the  electrolyser  the  liquor  is  charged  with  bromine  and  magnesia. 
The  latter  is  largely  precipitated,  and  under  those  conditions  does  not 
readily  react  with  the  bromine.  The  temperature  is  about  60°. 

Using  100  amperes  and  3-3'5  volts  between  each  pair  of  plates, 
corresponding  to  about  100  volts  for  a  cell  containing  thirty  electrodes, 
a  mean  current  efficiency  of  40  per  cent,  is  obtained.  The  losses  as 
hypobromite  and  bromate  do  not  exceed  2-4  per  cent.,  but  much 
bromine  is  reduced  back  again  to  Br'  ions  at  the  cathodes,  and  the 
shunt  current  losses  are  considerable.  Some  2'7-3'0  K.W.H.  are  neces- 
sary per  kilo,  of  bromine.  In  spite  of  the  low  current  efficiency,  the 
simplicity  of  the  Kossuth  cell  and  the  ease  with  which  it  can  be  cleaned 
have  led  to  its  being  preferred  over  forms  employing  diaphragms. 


xxii.]  BROMOFORM  AND  IODOFORM  393 

Of  the  important  Dow  cell,  used  in  U.S.A.,  nothing  is  generally 
known  beyond  the  fact  that  diaphragms  are  used.  This  is  necessary 
because  of  the  high  NaCl  content  of  the  electrolyte,  which  would 
otherwise  lead  to  a  considerable  bromate  formation. 

Bromates. — Of  the  actual  technical  preparation  of  bromates  nothing 
is  known.  The  best  conditions  are  undoubtedly  the  electrolysis  at 

40°-50°  of  a  fairly  strong  bromide  solution  containing  perhaps  2  — 

litre 

K2Cr207.     This  imparts  the  acidity  necessary  for  a  rapid  production  of 

bromate  according  to  the  equation  2HBrO  -f  BrO' >  Br03'  +  2H' 

+  2Br',  and  provides  the  cathode  membrane  which  prevents  the 
reduction  of  hypobromite  and  bromate.1  As  anode,  smooth  platinum 
must  be  used.  Carbon  or  platinised  platinum  would  tend  to  dis- 
integrate owing  to  the  crystallisation  of  the  sparingly  soluble  bromates. 
As  cathode,  graphite  can  be  employed.  Working  with  an  anodic 
current  density  of  10-15  amps./dm.2,  the  current  efficiency  should  be 
nearly  100  per  cent. 

Bromoform  and  lodoform. — These  substances,  bromoform  slightly, 
iodoform  largely  used  in  medicine,  can  be  chemically  prepared  by  the 
interaction  of  alkali  and  either  alcohol  or  acetone,  with  bromine  or 
iodine  respectively.  Thus  iodoform  is  readily  made  by  adding  iodine 
at  60°-70°  to  an  aqueous  alcohol  solution  containing  some  Na2C03. 
The  equation  is 

CH3.CH2.OH  +  8Na2C03  +  5I2  +  2H20  — >  CHI3  +  9NaHC03+  7NaL 

Only  30  per  cent,  of  the  iodine,  however,  appears  as  iodoform,  the 
greater  part  being  converted  into  Nal.  If  now  the  iodine  be  anodically 
produced  in  the  system  itself  as  needed,  the  losses  of  this  expensive 
substance  are  avoided,  whilst  the  method  of  preparation  is  simpler 
and  the  product  purer.  Electrochemical  methods  are  now  almost 
exclusively  used  for  producing  these  substances. 

If  a  solution  of  an  alkaline  iodide  or  bromide  containing  a  little  alkali 
and  some  alcohol  or  acetone  be  electrolysed,  the  first  step  is  the  libera- 
tion of  hydrogen  and  alkali  at  the  cathode  and  of  halogen  at  the 
anode.  The  halogen  is  immediately  partially  hydrolysed,  giving 
hypohalogenous  acid  and  some  alkaline  hypohalogenite. 

R2  +  Na2C03  +  H20  — >  HRO  +  NaR  +  NaHC03 
HRO  +  Na2C03  — *  NaRO  +  NaHC03. 

But  in  both  cases  a  considerable  proportion  (greater  with  iodine)  of 
the  free  halogen  remains  unattacked.  The  next  step  is  the  substitution 
of  the  hydrogen  of  the  organic  substance  by  the  free  halogen. 

CH3.CH2.OH  +  3R2  — >  CR3.CH  ,.OH  -f-  3HR 
CH3.CO.CH3  +  3R2 *  CR3.CO.CH3  +  3HR. 

1  Cf.  p.  322.     C1O3'  ions  are  not  cathodically  reduced,  except  at  iron. 


394    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

These  derivatives  then  react  further.  With  alcohol  *  the  hypo- 
halogenous  acid  present  causes  further  oxidation,  with  acetone  there 
is  simply  a  hydrolysis. 

CR3.CH2.OH.  +  2HRO  —  >  CHR3  +  2HR  +  C02  +  H20 
CR3.CO.CH3  +  H20  —  >CHR3  +  CH3.CO.OH. 


The  various  acids  formed  are  neutralised  by  the  excess  of  alkali  present. 
Summing  up  all  these  reactions,  including  the  first  electrolysis,  we 
obtain  — 

CH3.CH2.OH  +  3NaR  —  >  Na2C03  +  CHR3  +  NaOH+  5H2 

+  3H20 

CH3.CO.CH3  +  3NaR  -  >CH3.CO.ONa  +  2NaOH  +  CHR3  +  3H2. 
+  3H20 

Using  alcohol,  five  molecules,  using  acetone,  three  molecules  of 
hydrogen  are  liberated,  corresponding  to  the  passage  of  ten  and  six 
faradays  respectively.  The  greater  consumption  with  alcohol  is  due 
to  the  necessity  of  liberating  halogen  to  effect  (indirectly)  the  oxidation 
of  part  of  the  molecule  to  C02.  In  both  cases  alkali  is  eventually 
produced,  more  being  formed  using  acetone.  Certain  by-reactions 
diminish  the  current  efficiency  in  practice.  There  is  cathodic  reduc- 
tion of  hypohalogenous  acid  and  of  free  halogen  ;  also  the  reaction 
2HRO  +  RO'  ---^R03'  +  2H'  -f  2R'  is  liable  to  occur.  The  former 
loss  can  be  reduced  by  employing  a  suitable  diaphragm,  the  latter  by 
suitably  regulating  the  alkalinity  of  the  solution.  Excess  of  either 
alkali  or  acid  favours  the  formation  of  bromate  or  iodate. 

Bromoform  is  best  prepared  according  to  Miiller  and  Loebe.2  They 
employ  a  solution  containing  55  grs.  KBr,  20  c.c.  acetone,  and  O3  gr. 
K2Cr04  (to  prevent  cathodic  reduction)  in  140  c.c.  of  water.  No  alkali 
needs  to  be  added  ;  indeed  a  strong  C02  stream  must  be  continually 
passed  in.  Working  at  25°  with  an  anodic  current  density  of  about 
0'07  amps./cm.2  at  a  smooth  platinum  anode  and  without  a  diaphragm, 
a  90  per  cent,  current  efficiency  is  obtained.  The  losses  are  due  to 
cathodic  reduction,  oxidation  of  acetone,  bromate  formation,  and,  to 
a  slight  extent,  oxygen  liberation. 

For  iodoform,  either  alcohol  or  acetone  can  be  employed.  The  former 
is  better  in  spite  of  the  smaller  yield  per  coulomb,  as  the  method  is 
simpler.  Working  with  acetone  (B.P.  56°)  at  the  favourable  tempera- 
ture of  60°-70°  is  difficult,  and  the  large  quantity  of  alkali  liberated  is 
also  a  complication.  Further,  the  reaction  does  not  proceed  as  smoothly 
as  with  alcohol.  To  avoid  cathodic  reduction,  Elbs  and  Herz  3  used  a 
diaphragm,  but  the  best  and  simplest  way  of  preparing  iodoform  from 

i  If  R  is  bromine,  the  reaction  with  alcohol  does  not  proceed  smoothly. 
'  Zeitach.  Elektrochem.  10,  409  (1904). 
3  Zeitsch.  Elektrochem.  4,  113  (18H7). 


xxii.]  ANTHRAQUINONE  395 

alcohol  was  worked  out  by  Foerster  and  Meves.1  They  employ  an 
electrolyte  containing  per  litre  50  grams  Na2C03,  170  grams  KI,  and 
100  grams  96  per  cent,  alcohol.  The  anode  is  smooth  platinum,  the 
cathode  lead,  surrounded  by  a  simple  diaphragm  of  parchment  paper. 
Working  at  60°-70°  with  an  anodic  current  density  of  1-2  amps./dm.2 

and  a  voltage  of  2-2'5  volts,  about  1'32  —          -  CHI3  can  be  produced, 

amp.  hr. 

corresponding  to  a  current  efficiency  of  90  per  cent.  The  losses  are  due 
to  iodate  formation  and  cathodic  reduction.  The  beautifully  crystalline 
product,  when  washed,  is  very  pure.  Fresh  alcohol  and  KI  must  be 
continually  added,  and  the  alkalinity  regulated  by  a  slow  stream  of  C02. 
The  (Na2C03  +  K2C03)  content  of  the  bath  must  also  be  kept  at  the 
right  value. 

The  electrolytic  production  of  iodoform  from  acetone  has  been 
studied  by  Abbott,2  Teeple,3  and  Roush.4  Abbott,  working  at  75°, 
added  the  acetone  in  small  quantities  at  a  time,  and-  obtained  a 
current  efficiency  of  60  per  cent.  Teeple  worked  at  room  tem- 
perature and  obtained  good  current  efficiencies.  But  he  neutralised 
the  liberated  alkali  with  iodine  instead  of  acid.  His  method  was 
therefore  partly  chemical,  partly  electrochemical.  Roush  obtained 
satisfactory  results  by  using  two  cathodes,  one  enclosed  in  a 
diaphragm  cell,  and  thus  regulating  the  amount  of  alkali  which 
mixed  with  the  main  electrolyte.  But  all  these  methods  introduce 
complications,  and  the  use  of  alcohol  is  preferable. 

Anthraquinone  and  the  Regeneration  of  Chromic  Acid. — The 
oxidation  of  anthracene  to  anthraquinone,  the  basis  of  alizarin,  is  often 
carried  out  electrochemically.  If  anthracene  be  suspended  in  H2S04 
and  the  mixture  used  as  anolyte,  employing  a  platinum  anode,  a 
certain  amount  of  oxidation  occurs,  but  accompanied  by  much  libera- 
tion of  oxygen  and  by  a  high  anode  potential.  The  depolarisation  of 
the  oxygen  discharge  by  the  anthracene  is  slow.  To  render  it  rapid, 
a  catalyst  must  be  added.  Two  are  employed,  eerie  sulphate  [Ce(S04)2] 
and  chromic  acid,  their  mode  of  action  being  perfectly  simple.  They 
oxidise  the  anthracene  directly  and  smoothly  to  anthraquinone — 

CUH10  +  30  — -+  C14H802  +  H20, 

being  reduced  in  the  process  to  Ce2(S04)3  or  Cr2(S04)3  respectively.  By 
the  anodic  action  of  the  current,  the  original  oxidising  salts  are 
regenerated,  and  the  reaction  proceeds. 

Both  furnish  an  equally  good  product,  but  there  is  one  difference. 
Ce(S04)2  oxidises  far  more  quickly  than  H2Cr207.  The  anthracene  can 

1  Zeitsch.  Elettrochem.  4,  268  (1897). 

-  Jour.  Phys.  Chem.  7,  84  (1903). 

1  Jour.  Amer.  Chem.  Soc.  28,  170  (1904). 

4  Trans.  Amer.  Electrochem.  Soc.  8,  281  (1905). 


396    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY 

be  directly  added  to  the  electrolyser  in  which  the  Ce(S04)2  is  regenerated. 
The  oxidation  is  so  rapid  that,  as  long  as  anthracene  is  present,  there 
is  no  possibility  of  Ce(S04)2  being  cathodically  reduced  to  Ce2(S04)3,  thus 
causing  current  losses.  (No  diaphragm  is  used.)  H2Cr207,  on  the  other 
hand,  reacts  much  less  quickly.  The  oxidation  must  be  carried  out  in  a 
suitable  vat,  the  spent  liquors  drawn  off,  electrolytically  regenerated 
elsewhere,  and  returned  to  the  oxidising  vessel. 

The  Ce(S04),  process  is  carried  out  by  the  Farbwerke  vorm.  Meister, 
Lucius  und  Briining,  according  to  a  patent  of  Moest.  The  electrolyser 
is  a  lead  -lined  vessel  which  acts  as  anode,  the  electrolyte  20  per  cent. 
H2S04  with  2  per  cent.  Ce(S04)2.  The  temperature  is  raised  from  70°- 
to  100°  during  the  operation,  and  the  liquors  are  well  agitated.  Using 
an  anodic  current  density  of  5  amps./dm.2  and  2'8-3'5  volts,  the 
current  efficiency  is  practically  100  per  cent.,  the  product  being  very 
pure.  Ce(S04)2  is  yellow,  Ce2(S04)3l  colourless.  The  rapidity  of 
depolarisation  may  be  gauged  from  the  fact  that  the  liquors  remain 
colourless  till  the  finish  of  the  oxidation,  which  is  indeed  announced 
by  this  change.  Fontana  and  Perkin1  have  studied  this  reaction 
(experimenting  also  with  chromium  and  manganese  salts  as  cata- 
lytes).  They  showed  that  the  use  of  a  diaphragm  makes  little 
difference.  Using  an  anodic  current  density  1  amp.  /dm.2  at  70°-80° 
and  2'5-3'5  volts,  they  obtained  an  80  per  cent,  current  efficiency. 

The  electrolytic  regeneration  of  H2Cr207  has  received  much  atten- 
tion. Darmstadter  wished  to  add  the  anthracene  directly  to  the. 
electrolyser,  as  is  the  case  with  Ce(S04)2,  but,  as  Le  Blanc  pointed  out, 
cathodic  reduction  would  cause  considerable  current  losses.  According 
to  the  Farbwerke  Hochst  patent,  the  best  solution  for  the  oxidation 


contains  100  g™niS  Cr203  and  350  ?*amS  H2S04,  and  a  cur  rent  "density 
litre  litre 

at  the  lead  anode  of  3  amps./dm.2  is  recommended.  Regelsberger  2 
showed  that  this  anode  really  functions  as  a  Pb02  anode.  An  oxidation 
current  efficiency  of  about  100  per  cent,  can  be  obtained  with  it, 
whereas  no  oxidation  takes  place  at  platinum. 

Le  Blanc  3  studied  the  subject  thoroughly,  employing  the  diaphragm 
described  on  p.  155  and  working  according  to  the  conditions  used  by 
the  Hochst  dye-works.  At  50°  and  using  3*5  volts,  the  current  effi- 
ciency was  70-90  per  cent.  The  equation  involved  in  the  oxidation 
of  anthracene  by  H2Cr207  and  in  the  anodic  electrolytic  regeneration 
of  the  latter  is 

H2Cr207  +  3H2S04  =  Cr2(S04)3  +  4H20  +  30. 
the  oxygen  being  in  the  one  case  given  up  to  the  anthracene,  in  the 

1  Electrochem.  Ind.  2,  240  (Mil). 

2  Zcitorti.  WrktrorJifim.   6,  308  (1900). 

3  Zeitsch.  Elektrochem.  7,  290  (1900). 


XXII.] 


CHROMIC  ACID  REGENERATION 


397 


other  produced  electrolytically.  A  complete  cycle  therefore  leaves  the 
H2S04  content  of  the  solution  unaffected.  But  during  the  electrolysis 
S04"  ions  wander  continuously  from  the  catholyte  to  the  anolyte, 
increasing  the  acidity.  If  therefore,  using  H2S04  as  catholyte,  as  Le 
Blanc  first  did,  the  same  solution  is  continually  used  and  regenerated 
anodically,  its  H2S04  content  will  gradually  rise  until  it  is  useless.  To 
avoid  this,  Le  Blanc  employed  the  spent  liquor  itself  as  catholyte, 
causing  it  to  circulate  through  the  cell  from  cathode  through  diaphragm 
to  anode.  By  suitably  adjusting  the  rate  of  flow,  a  continuous  regenera- 
tion takes  place,  undisturbed  by  H2S04  concentration  changes. 

Le  Blanc  later  introduced  a  technical  apparatus  similar  to  the  bell- 
jar  alkali-chlorine  cell,1  thus  dispensing  with  the  diaphragm.  The  spent 
liquors  flow  downwards  through  the  cathode  compartment  (the  inside 
of  the  be  11  jar)  and  into  the  outer  vessel,  containing  the  anodes.  A 
suitable  rate  of  flow  leads  to  the  formation  of  an  unalterable  middle 
layer  in  the  belljar.  The  current  efficiency  is  85  per  cent.,  87'5  per 
cent,  of  the  total  dissolved  chromium  leaving  the  anode  chamber  as 
H2Cr207.  The  anodic  current  density  is  high,  and  the  process  cheap 
and  effective. 

Miiller  and  Seller2  have  investigated  the  behaviour  of  the  Pb02 
electrode,  and  shown  it  to  act  catalytically,  as  in  the  oxidation  of 
HI03  to  HI04.3  There  is  no  question  of  an  overvoltage  effect,  as  the 
potential  of  a  platinum  anode  under  the  same  conditions  (when  no 
oxidation  takes  place)  is  more  positive  than  that  of  the  Pb02 
electrode.  But  whilst  the  potentials  of  the  Pt  and  Pb02  electrodes 
in  H2S04  alone  are  very  similar,  that  of  the  PbO,  electrode  is 
lowered  on  the  addition  of  Cr'"  ions  owing  to  their  depolarising 
action,  and  that  of  the  Pt  electrode  is  raised.  The  following  table 

TABLE  LX 


Current 
density 

Platinum  anode 

Pb02  anode 

«•"• 

N'g^+o 

N  .  HsSO., 

N.H2S04  + 

1    litre  Cr2°3 

Amp./  cm.2 
0-0046 
0-023 

Volts 
+  1-997 
2-027 

Volts 
+  2-003 
2-104 

Volts 
+  1-985 
2-064 

Volts 
+  1-758 
1-974 

0-046                     2-065 

2-208 

2-129 

2-052 

contains  some  of  their  figures  (6  =  20°).  This  behaviour  is  closely 
connected  with  the  powerful  chemical  oxidising  action  of  Pb02  in 
the  same  case.  The  influences  of  current  density,  temperature,  and 
concentration  were  also  investigated. 


P.  371. 


"  Zeilsch.  Elektrochem.  11,  863  (1905). 


s  P.  146. 


398    PKINCIPLES  OP  APPLIED  ELECTROCHEMISTKY    [CHAP. 

Schmiedt l  has  also  studied  the  subject,  particularly  investigating ; 
the  effect  of  various  addition  agents  on  the  oxidation,  several  of  which 
are  used  technically.     He  found,  for  example  : 

TABLE  LXI 

Current  efficiency 

Addition                               At  start  At  finish 

Per  cent.  Per  cent. 

82-2  71-3 

1  per  cent.  KF                           93'4  86'2 

O'l  per  cent.  KF                          92'8  73'9 

1  per  cent.  Na2HPO4                      98'2  82'8 

Small  quantities  of  boric  acid,  KCN,  and  sodium  vanadate  and 
molybdate  also  had  marked  effects.  The  mechanism  of  their  action 
is  not  understood.2 

Potassium  Ferricyanide. — This  salt,  previously  chemically  prepared 
by  passing  chlorine  into  K4FeCy6,  the  reaction  being  2K4FeCy6  -f  Clr 
— >2  K3FeCy6  +  2KC1,  is  now  almost  exclusively  made  by  electrolytic 
oxidation.  The  net  result  of  the  anodic  reaction  is  simply 

FeCy6""-- >FeCy6'"  +  0. 

Whether  it  actually  proceeds  thus  or  by  the  intermediate  action  of 
discharged  oxygen  is  uncertain.  At  the  cathode  alkali  and  hydrogen 
are  produced.  The  total  reaction  is  therefore 

2K4FeCy6  +  2H20 >  2K3FeCy6  +  2KOH  +  H2. 

The  electrolysis  has  been  studied  by  v.  Hayek.3  Working  at  room 
temperature  with  a  well-agitated,  saturated,  slightly  alkaline  K4FeCy6 
solution,  and  a  current  density  0'3  amp. /dm.2,  he  could  reduce  the 
K4FeCye  concentration  from  20  per  cent,  to  1/7  per  cent,  with  a  100 
per  cent,  current  efficiency.  The  last  fractions  were  also  readily 
oxidised,  and  the  K3FeCy6  separated  by  crystallisation. 

Potassium  Permanganate. — This  important  salt  is  prepared  from 
potassium  manganate,  K2Mn04,  made  in  its  turn  by  fusing  together 
Mn02  and  potash  in  presence  of  air,  the  reaction  being 

Mn02  +  2KOH  +  J02 *  K2Mn04  +  H20. 

The  chemical  method  formerly  used  for  the  further  transformation 
into  permanganate  consisted  in  leading  in  C02  into  the  dissolved  melt. 
K2C03  and  KMn04  resulted,  and  Mn02  was  precipitated 

3K2Mn04  +  2C02 >  2K2C03  +  2KMn04  +  Mn02 

We  see  that  one-third  of  the  Mn02  originally  used  had  to  be  worked  up 
again,  and,  more  important  still,  that  two-thirds  of  the  alkali  originally 
used  were  converted  into  carbonate. 

These  losses  are  now  avoided  by  electrolytically  oxidising  between 

1  Dissertation  (Charlottenbur^,   1HOH).  '2  Cf.  p.  146. 

3  Zeitsch.  Anorg.  Chem.  39,  240  (1904). 


xxii.]  POTASSIUM  PERMANGANATE  399 

iron  or  nickel  electrodes.  As  catholyte,  alkali  is  used,  which  becomes 
stronger  during  the  electrolysis.  The  anolyte  consists  of  the  alkaline 
solution  obtained  by  lixiviating  the  original  melt.  Its  alkali  content 
also  increases,  owing  to  OH'  migration  from  the  catholyte,  and  if  the 
original  manganate  concentration  was  sufficiently  high,  the  perman-. 
ganate  finally  crystallises  from  the  strongly  alkaline  liquors.  The 
equation  for  the  electrolysis  is 

2K2Mn04  +  2H20  — *  2KMn04  +  2KOH  +  H2. 

There  is  no  trouble  with  Mn02,  and  caustic  alkali  is  formed,  which  is 
evaporated  down,  and  used  for  the  next  fusion. 

Two  types  of  processes  are  used.  In  one  there  is  no  diaphragm, 
and  the  light  catholyte  rests  on  top  of  the  heavy  anolyte.  As  K*  ions 
are  the  only  ones  which  migrate  from  anolyte  to  catholyte,  there  is  no 
danger  of  mixing  from  that  reason,  whilst  the  diffusion  of  the  manganese 
salts  upwards  is  more  than  neutralised  by  Mn04"  and  MnO/  migration 
downwards.  There  is,  however,  some  loss  owing  to  oxygen  liberated 
at  the  anode  carrying  permanganate  with  it  up  to  the  cathode,  where 
reduction  ensues.  Also  the  catholyte  must  continually  be  diluted  to 
maintain  the  density  difference. 

This  process  has  been  studied  by  Askenasy  and  Klonowski.1  Their 
electrolyser  was  a  cylinder,  the  electrodes  iron.  Using  an  electrolyte 

containing  about  88  ??    -  K2Mn04,  and  working  at  60°  with  2'8-2'9 
litre 

volts,  they  obtained  with  anodic  current  densities  of  8'5-13  amps./dm.2 
the  following  relations  between  current  efficiency  and  percentage  of 
manganate  oxidised. 

Percentage  oxidised  Current  efficiency 

Per  cent.  Per  cent. 

70-75  (maximum)  24 

50  50 

48  64 

27  67 

For  technical  work  they  recommend  a  current  density  of 
8'5  amps./dm.2  at  the  anode  (a  containing  vessel  of  iron),  that  at  the 
cathode  being  ten  times  as  great.  Using  2'8  volts,  about  O7  K.W.H. 
would  be  required  per  kilo.  KMn04.  This  corresponds  to  a  67  per  cent, 
current  efficiency,  which  in  turn  denotes  an  oxidation  of  only  about 
one-third  of  the  manganate. 

The  second  type  of  process  employs  a  diaphragm  of  cement  or 
some  other  suitable  alkali-resistive  material.  It  has  been  studied 
by  Askenasy  and  Klonowski,2  also  by  Brand  and  Ramsbottom.3  The 
former  investigators  used,  as  before,  sheet-iron  electrodes,  the  latter 

1  Zeitsch.  Elektrochem.  16,   170  (1910).  -  Loc.  cit. 

3  Jour.  Prakt.  Chem.  82,  336  (1910). 


400    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 


electrodes  of   iron  and   nickel  gauze.     The   following   Table    (LXII) 
contains  some  of  the  results  given  by  iron  electrodes. 

TABLE   LXII 


Composi- 

tion of 
anolyte 
grams 

e 

Voltage 

Anodic 
current 
density 

Per  cent, 
oxidised 

Current 
efficiency 

Obser- 
vers 

Remarks 

litre 

76-45 
K.,Mn04 
50KOH 

|    60° 

7  volts  \ 

0-152 
amp./cm." 

(  61-5 

57 

A.K.  j 

Anolyte 
not 
stirred 

Do.             do. 

6-5 

0-083 

68-4 

60 

do. 

do. 

Do.              do. 

do. 

do. 

81-5 

36 

do. 

do. 

Do. 

do. 

5     { 

0-11- 

0-0275 

j  70-75 

55 

do.   | 

Anolyte 
stirred 

58K2Mn04 
55KOH, 

J18°-20° 

/4-02- 
(4-62 

0-02 

100 

36-8 

B.R. 

do. 

(  3'23— 

Do. 

do. 

1  3-48 

0-0125 

100 

57-7 

do. 

do. 

Do. 

do. 

/2'47- 
\2-52 

0-005 

100 

76-3 

do. 

do. 

Askenasy  and  Klonowski  also  worked  with  lower  manganate  con- 
centrations, which  gave  worse  results,  and  at  25°,  which  made  little 
difference1. 

It  will  be  noticed  that  the  current  efficiencies  obtained  by  Brand  and 
Ramsbottom  were  much  better,  and  their  voltages  lower,  in  spite  of  the 
lower  temperature.  The  reasons  were  undoubtedly  the  lower  current 
densities  employed,  and  the  fact  that  the  gauze  anode  permits  far  better 
circulation,  and  consequently  lessens  polarising  effects.  Table  LXIII 
contains  their  figures  for  nickel  anodes,  which  are  seen  to  be  even  better. 

TABLE   LXIII 


Anolyte 
grams 
per  litre 

$ 

Anodic 
current 
density 

Voltage 

IVr  cent, 
oxidised 

Current 
efficiency 

Remarks 

Per  cent. 

40KjMnO4\ 
140KOH  / 

18°-20° 

0-0083- 
0-0208 
amp./  cm.2 

; 

100 

48-53 

Electrolyte 
not  stirred 

Do. 

do. 

do. 

— 

80              100 

Electrolyte 
stirred 

Do. 

do. 

do. 

— 

100 

86-6 

do. 

55KOH    } 

do. 

0-005         2-06-2-11 

100 

83-4 

do. 

Do.              do. 

0-0125       2-77-2-87 

100             72-6 

do. 

Do. 

do. 

0-02           .'J-60-4-66 

100 

55-2 

do. 

xxii.]  PERCHLORATES  401 

Better  current  efficiencies  are  obtained  and  lower  voltages  required 
than  with  iron.  Stirring  obviously  has  a  great  influence.  Without  it 
oxygen  evolution  commences  at  the  beginning  of  the  electrolysis. 
Brand  and  Ramsbottom  believe  that  the  oxidation  does  not  simply 
proceed  as  follows  : 

Mn04"  -  —  *  MnO/  +  0, 

but  that  oxygen  plays  a  part.  The  special  influence  of  nickel  is  ascribed 
to  the  formation  of  a  nickel  superoxide  on  the  electrode,  which  acts  as 
an  oxygen  carrier,  as  Pb02  perhaps  does  in  the  H2Cr207  regeneration.1 

They  also  followed  the  changes  in  the  anode  potential  during  the 
electrolysis.  At  nickel  anodes  the  initial  potential  difference  was  about 
-f-  0*65  volt.2  It  rose  slowly  to  -f-  O7  volt,  then  rapidly,  then  slowly 
again,  finally  reaching  +  Tl  volts.  With  iron,  an  initial  figure  of 
-|-  0'88,  rising  to  +  0*93  volt,  was  observed.  The  first  slow  rise  with  the 
nickel  is  ascribed  to  concentration  polarisation  ;  the  rapid  rise  accom- 
panies the  initial  oxygen  evolution,  and  the  final  slow  rise  corresponds 
to  the  usual  overvoltage  increase.  With  iron  the  first  two  stages  do 
not  appear.  Oxygen  is  evolved  from  the  start,  as  no  oxide  is  formed 
which  catalyses  the  oxidation  of  the  manganate.  The  figures,  in  fact, 
correspond  well  with  parallel  experiments  made  with  KOH  solu- 
tions (no  manganate),  and  with  similar  experiments  of  Foerster  and 
Piguet.3 

Brand  and  Ramsbottom  finally  showed  that  Na2Mn04  oxidises  just 
as  easily  as  K2Mn04.  With  a  strongly-agitated  solution,  and  a  current 
density  of  0*0125  amp.  /cm.2  at  a  nickel  anode,  the  current  efficiency 
was  90  per  cent. 

Perchlorates.4  —  Of  these  salts,  potassium  and  ammonium  perchlorates 
are  important,  being  used  in  the  firework  and  explosives  industries.  The 
sodium  and  calcium  salts  are  also  prepared,  but  only  as  intermediate 
stages  in  the  production  of  the  first  named. 

The  oxidation  of  the  C103'  ion  to  the  CIO/  ion  occurs  readily  and 
with  high  current  efficiency  if  the  anode  potential  is  sufficiently  high. 
It  has  been  shown  to  take  place  in  all  probability  as  follows 

(a)  2C103'  —  >  2C103  +  20; 

(6)  2C103  +  H20  —  >  CIO/  +  C102'  +  2H'-f  J02  ; 

(c) 


CKY  ions  are  first  discharged  and  decompose  water,  giving  a  mixture 
of  HC104  and  HC102  and  liberating   oxygen.     (Concentrated  HC103 

1  Cf.  pp.  133-134,  146,  152  (footnote),  397. 

2  Foerster  has  found  the  potential  of  the  NiOo  electrode  in  2'8  n.  KOH  to  be 
-}-  0-60  volt. 

a  Zeitsch.  Elektrochem.  10,  714  (1904). 

4  Winteler,  Zeitsch.  Elektrochem.  5,  50,   217   (189S)  ;   7,  635  (1901)  ;  Oechsli, 
Zeitsch.  Elektrochem.  9,  807  (1903)  ;  Couleru,  Chem.  Zeit.  30,  213  (1906). 

2  D 


402    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAI'.J 

breaks  up  similarly  into  HC104  and  HC102.)  The  HC102  is  then  oxidised 
by  the  liberated  oxygen  to  HC103,  the  total  result  being 

CIO/  +  H20 — >  C104'  +  2H*  +  2  0. 

Any  direct  OH'  discharge  means  a  loss  of  current,  the  CIO/  ions  being] 
unaffected  by  oxygen.  The  oxidation  is  therefore  carried  out  in  aci3 
solution,  in  which  OH'  discharge  is  difficult,  and  an  electrode  is  usecH 
with  a  high  oxygen  overvoltage,  such  as  smooth  platinum.  If  this  be] 
platinised,  or  if  the  electrolyte  contain  a  little  free  alkali,  the  current! 
efficiency  at  once  falls.  Other  circumstances  favouring  high  over! 
voltage,  and  therefore  good  results,  are  high  current  density  and  IOMJ 
temperature.  With  these  conditions  fulfilled,  the  CIO/  concentration 
in  the  electrolyte  is  of  minor  importance. 

In  practice  a  concentrated  (perhaps  60-70  per  cent.)  slightly  acid 
solution  of  the  soluble  NaC103  is  electrolysed  between  smooth] 
platinum  anodes  and  iron  cathodes.  The  temperature  is  kept  below] 
10°  by  cooling  coils.  The  electrodes  may  also  be  cooled.  An  anodioj 
current  density  of  about  8  amps./dm.2  is  employed.  Under  thesJ 
circumstances,  with  6*5-7  volts  on  the  bath,  the  total  chlorate  present! 
can  be  oxidised  at  a  mean  current  efficiency  of  85  per  cent.  At  the 
commencement  it  is  100  per  cent.,  but  falls  throughout.  Good 
circulation  is  necessary.  The  high  voltage  is  due  partly  to  the  grealj 
resistance  of  the  electrolyte  at  such  a  low  temperature,  and  partly  tq 
the  high  anode  potential.  The  oxygen  evolved  in  the  later  stageaj 
contains  much  ozone.  The  production  of  one  kilo,  of  NaC104  (fromj 
chlorate)  needs 

1000     100     96540  X  2       6-8     _     r  „ 

122   '    85   '       3600       '  1000  ~ 

As  the  sodium  salt  is  deliquescent  it  is  not  worked  up  as  such,  buft 
KC104  is  precipitated  by  adding  KC1.  In  this  connection  it  is  im-j 
portant  for  all  the  chlorate  to  have  been  oxidised  ;  otherwise  the  pre-j 
cipitated  KC104  will  contain  some  KC103  dissolved  in  it  as  a  solicH 
solution,  which  cannot  be  removed  by  washing.  NH4C104  is  prepared] 
similarly  by  adding  NH4C1.  According  to  Angeli  it  is  also  conve- 
niently made  by  oxidising  a  strong  CaCl2  solution  anodically  until 
all  is  converted  into  perchlorate,  adding  NH4C1  and  subsequently] 
evaporating. 

Persulphates  and  Hydrogen  Peroxide. — Persulphuric  acid  was 
disco vcrt-d  by  Berthelot,  and  the  solid  salts  first  prepared  by  Marshall.1 
The  formukc  given  below  show  the  close  relationship  between  H2S208, 
H,02,  and  the  interesting  acid  H2S06  discovered  by  Caro.2 

"ins.  Chem.  Soc.  59,  771  (/«//). 
-'  Zeitsch.  Angew.  Chem.  11,  845  (1898). 


xxii.]  PERSULPHATES  403 

OH  0  .  SO,  .  OH  0  .  S02  .  OH 

I  I  I 

OH  OH  0  .  S02  .  OH 

Hydrogen  peroxide  Caro's  acid  Persulphuric  acid 

Carols  acid  and  H2S208  are  respectively  monosulphonated  and 
disulphonated  hydrogen  peroxide,  while  if  H2S208  be  hydrolysed  the 
first  product  is  H2S05  and  the  second  H202.  The  salts  of  H2S208  are 
exclusively  prepared  by  electrolysis,  and  extensively  used  as  oxidising 
agents  and  in  photography.  H202,  now  prepared  to  an  ever-increasing 
extent  from  H2S208  or  its  salts,  finds  great  application  on  account  of  its 
disinfectant,  germicidal,  oxidising,  and  bleaching  qualities. 

As  in  the  oxidation  of  chlorates,  the  anodic  process  consists  in 
the  discharge  of  the  anions  —  here  S04"  ions. 


To  bring  about  this  reaction,  the  same  conditions  are  necessary  as  iu 
preparing  perchlorates  —  viz.  low  temperature,  high  current  density  at 
a  smooth  platinum  anode,  and  an  acid  solution,  or  at  least  one  free 
from  alkali.  All  these  circumstances  render  OH'  discharge  difficult. 
The  necessary  anode  potential  is,  however,  higher  than  is  the  case 
with  perchlorates.  Oxygen  is  evolved  from  the  start,  and,  of  course, 
contains  ozone.  A  low  temperature  is  also  necessary,  on  account  of  the 
tendency  of  persulphate  solutions  to  decompose  as  follows  : 

R2S208  +  H20—  >  R2S04  +  H2S04  +  J0r 

The  production  of  persulphates  has  been  much  studied.1  Two 
methods  can  be  employed,  both  designed  to  avoid  cathodic  reduction. 
A  diaphragm  can  be  used.  In_the  anode  compartment  is,  perhaps,  a 
concentrated  (NH4)2S04  solution,  in  the  cathode  compartment  H2S04  of 
medium  strength.  The  electrolyte  is  well  cooled  by  means  of  coils,  as 
the  temperature  is  best  kept  at  about  15°.  The  anode  is  smooth  platinum 
and  the  anodic  current  density  high  ;  the^athode  can  be  lead  and  of 
a  much  larger  surface.  Crystalline  (NH4)2S208,  containing  a  few  per 
cents,  of  (NH4)2S04  continually  precipitates,  whilst  fresh  (NH4)2S04 
is  continually  added.  The  current  efficiency  exceeds  70  per  cent.  In 
the  catholyte,  hydrogen  is  evolved,  and  the  total  reaction  is 

(NH4)2S04  +  H2S04  —  >  (NH4)2S208  +  H2. 

It_is  simpler  to  use  no  diaphragm  (Mailer  and  Friedberger)  and  to 
^addjnstead  O2  per  cent.  KoCr04  as  in  the  chlorate  process.    _Cathodic- 
reduction    is    thereby    sufficiently  avoided.     With  smooth  platinum 
electrodes  and  an  anodic  current  density  of  50^  amps.  /dm.2,  the  electro- 
lyte being  concentrated  (XH4)2S04,  the  current  efficiency  is  70-80  per 

1  Particularly  see  Elbs  and  Schonherr,  Zeitsch.  Elektrochem.  2,  245  (1896); 
Miiller  and  Friedberger,  Zeitsch.  Elektrochem.  8,  230  (1902). 

2  D  2 


404    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY^  [CHAP. 

cent,  and  the  voltage  about  7  volts.  Working  thus,  the  solution 
becomes  alkaline  at  the  cathode,  and  if  the  OH'  ions  reach  the  anode 
oxygen  evolution,  meaning  increased  current  losses,  will  be  facilitated. 
Hence  acid  must  be  continually  added.^-This  can  be  avoided  by  using, 
instead  of  (NH4)2S04,  the  soluble  KHS04.  K2S2Os  is  precipitated,  and 
at  the  cathode  hydrogen  evolved  without  OH'  production.  The  total 
equation  is 

2KHS04  —  >K2S208  +H_. 

The  yield  is  not  as  good  as  when  making  (NH4)2S208.      One  kilo,  of  the 


^^ 

It  should  be  finally  mentioned  that,  exactly  as  in  the  electrolytic 
regeneration  of  H2Cr207,  'certain   ions  exert  a    markedly    favourable 
catalytic  influence  on  the  oxidation.      Particularly  active  are  the  Clr 
and  F^ions.1 

Hydrogen  Peroxide  is  technically  prepared  either  from  H2S208 
solutions  or  from  solid  K2S208.  The  former  method  is  used  by  the 
Consortium  fur  elektrochemische  Industrie,2  the  latter  method  is  due 
to  Pietzsch  and  Adolph.3  In  the  former  process,  H2S04  solutions  (S.G. 
about  1*5),  cooled  by  suitable  coils  and  with  the  addition  of  some 
HC1  or  HF,  are  electrolysed  without  a  diaphragm,  using  platinum 
electrodes  at  high  current  densities.  An  H2S208  solution  results  at  about 
50  per  cent,  current  efficiency,4  containing  up  to  40  per  cent,  of  the  acid 
together  with  unoxidised  H2S04.  This  is  distilled  under  reduced 
pressure.  The  H2S208  is  hydrolysed  to  H202,  which  distils  off,  and 
the  residue,  after  cooling  and  diluting,  is  re-electrolysed.  The  losses 
as  oxygen  during  the  distillation  are  small,  provided  that  no  traces 
of  platinum  resulting  from  anodic  attack  during  the  electrolysis  are 
present.  Lately  hollow  cooled  anodes  have  been  used,  instead  of  having 
the  cooling  coils  in  the  electrolyte.  The  current  efficiency  is  thereby 
considerably  raised. 

Pietzsch  and  Adolph  gently  heat  or  distil  K2S208  with  1'4  S.G. 
HaS04,  KHS04  and  H202  resulting.  The  latter,  after  cooling,  can  be 
sucked  off  from  the  precipitated  KHS04  (which  is  reoxidised  electro- 
lytically)  or  else  distilled  off  at  80°  ;  20-30  per  cent,  solutions  result 
directly,  and  the  distillation  apparatus  is  small  for  its  output.  The 
loss  as  oxygen  is  very  low,  whilst  K2S208  can  be  prepared  with  a  better 
current  efficiency  than  H2S208. 

Hyposulphites.  —  Up  to  now  these  very  valuable  reducing  agents, 
of  which  Na2S204  is  the  most  used,  have  been  prepared  by  the  chemical 
reduction  of  NaHS03  solutions  by  metallic  zinc  (4NaHS03  -(-  Zn  —  > 
Na2S204  +  Na2S03  +  ZnS03  +  2H20).  Electrolytic  methods  have 
achieved  little  success,6  owing  to  incomplete  understanding  of  the 

1  See  p.  146.  2  D.R.P.  217538,  217539  (1905);    E.P.  23548  (1910). 

a  E.P.  23158,  23660  (1HW).  4  See  also  Zeitoch.  Elektrochem.  1,  417  (1895). 

6  Zeitsch.  Elektrochem.  10,  361,  450  (1904). 


XXIL]  HYPOSULPHITES  405 

processes  and  factors  involved.  The  subject  has  been  lately  thoroughly 
studied  by  Jellinek.1  His  work  has  put  the.  matter  on  a  different 
footing,  and  it  is  highly  probable  that  a  technical  electrolytic  method 
will  be  shortly  introduced. 

Consider  a  solution  containing  bisulphite  and  hyposulphite  ions. 
The  ions  HS03'  are  in  equilibrium  with  small  quantities  of  the  ion 
S205", 

2HS03'  ^T  S205"  +  H20 


and  SaO/  formation  perhaps  actually  takes  place  through  the  inter- 
mediation of  these  ions. 

S205"  +  2H*  —  >  S204"  +  H20  +  2  ©. 
But  the  relations  are  rendered  clearer  if  we  use  the  equation 

2HSCV  +  2H'  —  >  S204"  +  2H20  +  2  ©. 

The  potential  of  an  indifferent  electrode  in  a  solution  containing  these 
ions  is  given  by  the  equation 


and  Jellinek  has  shown  E.P.  to  be  —  0*009  volt.  A  solution  in  which 
[HS(V]  =  [S20/]  =  1  will  have  [H']  =  circ.  v/5~.  10~3,  and  we  calculate 
the  electrode  potential  under  these  circumstances  to  be  —  0*163  volt. 
As  the  reversible  potential  of  a  hydrogen  electrode  under  these 
conditions  is  about  —  0*157  volt,  such  a  solution  would  decompose 
spontaneously  according  to  the  equation 

Na2S204  +  2H20  -  >  2NaHS03  +  H2 

if  the  hydrogen  were  given  an  opportunity  of  being  evolved.  Further, 
the  reverse  process  —  i.e.  cathodic  HS03'  reduction  at  a  platinised 
platinum  cathode  —  would  be  impossible,  as  the  reaction  H"  --  >  JH2-f-0 
would  preferably  set  in.  If,  however,  an  electrode  be  used  with  a 
considerable  hydrogen  overvoltage  the  reduction  can  take  place. 

In  the  chemical  reduction  process,  the  zinc  used  is  such  a  metal, 
and  the  reaction  can  be  regarded  as  essentially  electrochemical.  The 
anodic  process  is  Zn  —  >  Zn"  -f-  2©.  Owing  to  the  high  hydrogen 
overvoltage  at  zinc,  reduction  sets  in,  instead  of  H'  discharge.  The 
total  reaction  becomes 

Zn  +  2H'  -f  2HS03'  —  >Zn"  +  S204"  +  2H20, 
or 

Zn  +  4HS03'  -  *  S204"  +  2S03"  +  Zn"  +  2H20, 

which  is  the  equation  on  p.  404,  written  ionically.  As  E.P.  for  the 
HS03/-S204//-H"  electrode  is  -0*009  volt,  and  E.P.  Zn...^2n-0'7Q  volt, 
it  is  obvious  that  much  chemical  energy  is  wasted  in  the  reduction  by 

1  Zeitsch.  Ekktrochem.  17,  157, 


406      PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY 

zinc.     Iron,  for  which  E.P.  Fe.. ^  Fe  is —0*44  volt,  would  be   more 

suitable,  but  it  unfortunately  is  made  passive  by  the  HS03'  ion,  and 
does  not  reduce.  The  electrochemical  method,  which  allows  of  the 
use  of  the  necessary  cathodic  polarisation  and  no  more,  is  the  most 
economical. 

Previous  workers  had  noticed  that,  after  a  certain  S204"  concen- 
tration, varying  under  different  conditions,  had  been  reached,  no  more 
was  apparently  formed,  but  S203"  ions  instead.  This  was  attributed  to 
further  reduction  as  follows, 

S204"  +  2H'  — >  S203"  +  H20  +  2  ©. 

Jellinek  showed  the  formation  of  thiosulphate  to  be  due  rather  to  a 
chemical  decomposition  of  the  hyposulphite 

2Na2S204  +  H20-  ->  Na2S203  +  2NaHS03. 

Practically  none  is  produced  by  reduction.  The  discovery  of  this  fact 
led  him  to  recognise  that  the  essential  condition  for  the  production  of  a 
high  hyposulphite  concentration  is  a  high  current  concentration l — i.e.  a 
current  which  is  very  large  compared  with  the  volume  of  the  catholyte. 
The  rate  of  electrochemical  formation  of  the  hyposulphite  is  thus  made 
great  in  comparison  with  its  rate  of  chemical  decomposition,  and  its 
concentration  rises.  To  keep  a  hyposulphite  solution  at  a  given 
strength  a  definite  current  concentration  is  needed.  If  exceeded,  the 
solution  becomes  stronger  ;  if  not  reached,  weaker. 

For  example,  working  on  this  principle,  Jellinek,  starting  with 
5  n.  NaHS03,  could  prepare  a  10  per  cent.  Na2S204  solution  with  an  80 
per  cent,  current  efficiency,  using  5  amperes  for  every  100  c.c.  of  catho- 
lyte. Of  this,  1'06  amperes  was  the  critical  current  necessary  to  neutra- 
lise the  chemical  decomposition.  Besides  low  current  concentrations.2  a 
high  temperature  and  H'  migration  from  the  anode  (where  the  reaction 
is  HS03'  +  OH'  -  -*  HSO/  +  H'  +  2  0)  must  also  be  avoided. 
Current  density  is  of  minor  importance.  The  electrode  processes  are 
almost  reversible.  To  neutralise  the  heat  resulting  from  the  high 
current  concentration,  efficient  artificial  cooling  must  be  employed. 
To  prevent  entrance  of  H'  ions,  Jellinek  employed  an  apparatus  divided 
by  two  diaphragms  into  three  compartments,  an  Na2S04  solution 
flowing  through  the  middle  neutral  chamber. 


Literature 

Engelhardt.     Die  Elektrolyse  des  Wassers. 

Schlotter.     Ueber  die  elektrolytische  Gewinnung  von  Brom  und  lod. 

Haber-Moser.     Die  elektrolytischen  Prozesse  der  organischen  Chemie. 

»  See  p.  30. 

2  It  should  be  noticed  that  the  successful  results  of  the  chemical  reduction  by 
zinc  are  largely  due  to  the  fact  that  the  conditions  are  those  of  an  electrolysis  at  a 
very  high  current  concentration. 


CHAPTER  XXIII 

METALS  FROM  FUSED  ELECTROLYTES— CAUSTIC  SODA  AXD 
CHLORINE  FROM  FUSED  SALT 

General. — Strongly  electropositive  metals  (i.e.  those  having  a  great 
affinity  for  oxygen)  are  not  easily  prepared  by  chemical  methods.  For 
the  liberation  of  the  metal  from  its  very  stable  compounds — generally 
oxides  or  halides — demands  a  still  more  electropositive  metal  or  other 
powerful  reducing  agent,  and  often  very  high  temperatures.  Such 
operations  are  therefore  costly,  destructive  to  apparatus,  difficult 
to  carry  out,  and  furnish  poor  yields.  The  chemical  processes  for 
sodium  and  aluminium  manufacture  are  examples. 

Turning  to  electrochemical  methods,  we  are  often  debarred  from 
depositing  from  aqueous  solutions,  as  strongly  electropositive  metals 
decompose  water.  On  the  other  hand,  the  electrolysis  of  anhydrous 
fused  salts  can  be  employed.  Chemically  it  is  simple  and  direct.  It 
generally  requires  lower  temperatures  than  the  corresponding  chemical 
methods.  And  there  are  other  advantages.  Thus  we  know  that  the 
satisfactory  electrodeposition  of  zinc  from  aqueous  solution  is  not  easy.1 
From  fused  ZnCl2  the  metal  is  obtained  molten.  Again,  current 
densities  can  be  employed  much  higher  than  with  aqueous  solutions. 
For  with  a  fused  product  there  is  no  trouble  owing  to  spongy  or  powdery 
deposits,  and  no  concentration  polarisation  to  be  feared.  On  the  other 
hand,  more  care  and  attention  are  necessary  for  successful  working. 
The  heat  required  to  warm  up  the  starting  materials  and  to  neutralise 
radiation  losses  must  be  considered.  Also  wear  and  tear  of  plant  due 
to  the  high  temperatures  and  the  fused  electrolytes  are  often  increased. 
Further,  there  are  electrochemical  defects  peculiar  to  the  electrolysis  of 
fused  salts.2 

Processes  of  this  kind  are  the  only  ones  which  have  ever  been  used 
for  the  production  of  magnesium  and  calcium,  have  displaced  older 
chemical  methods  in  the  manufacture  of  sodium  and  aluminium,  and 
have  been  applied  on  a  large  experimental  scale  to  the  production  of 
zinc. 

1  Pp.  282-283.  Chap.  XII. 

407 


408    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 


1.  Sodium 

It  is  unfortunate  that  the  melting-point  of  NaCl  is  so  high  as  800°, 
for  the  direct  production  of  sodium  from  it  is  thereby  made  very  difficult. 
The  conditions  otherwise  are  ideal — a  cheap  and  pure  raw  material 
containing  40  per  cent,  metal  and  nothing  else  but  chlorine.  Other  salts 
have  been  suggested  (notably  NaN03  by  Darling),  but  the  only  one; 
used  technically  to  any  extent  is  NaOH.  It  is  cheap  (only  dear  com- 
pared with  NaCl),  and  its  low  melting-point  allows  easy  working  and 
handling  of  the  inflammable  sodium.  The  most  important  processes 
using  NaOH  are  the  Castner  process  and  the  Rathenau-Suter  '  contact 
electrode  '  process. 

Gastner  Process. — The  molten  NaOH  is  contained  in  a  cast-iron 
pot  A  (Fig.  100).  This  is  provided  underneath  with  an  extension,  B, 
through' which  passes  the  cathode  C,  sealed  into  B  by  NaOH  which  has 
solidified.  C  may  be  of  iron,  nickel  or  copper ; 
preferably  of  iron.  Resting  on  the  flange  of 
the  pot  and  insulated  from  it  is  hung  the  ring- 
shaped  nickel1  anode  D,  sometimes  per- 
forated with  holes  to  allow  free  circulation 
of  the  electrolyte.  Between  the  electrodes  is 
hung  a  short  cylindrical  screen,  E,  which  dips 
beneath  the  liquid  surface,  forming  a  cathode 
chamber,  and  is  continued  below  by  an  ex- 
tension of  fine  wire  gauze.  The  whole  is  of 
nickel  or  iron,  and  is  carefully  insulated.  The 
gauze  prevents  globules  of  sodium  reaching 
the  anode,  they  being  unable  to  pass  through 
the  meshes.  The  top  of  the  vessel  is  suitably 
covered  to  minimise  heat  losses.  The  covers 
can  be  readily  detached,  and  are  provided 
with  holes  permitting  the  escape  of  evolved 

gases.  The  metal  liberated  at  the  cathode  is  ladled  out  by  means 
of  perforated  spoons,  which  retain  the  sodium,  allowing  the  NaOH  to 
flow  through.  The  whole  apparatus  is  bricked  in,  and  in  order  to 
protect  the  cell  walls  the  bath  is  designed  to  work  at  the  right 
temperature  with  a  layer  of  solidified  electrolyte  coating  the  sides. 

The  temperature  must  be  kept  as  low  as  possible.  This  was  fully 
recognised  by  Castner,  who  specified  that  it  should  not  rise  more  than 
20°  above  the  melting-point.  This  is  327° 2  for  pure  NaOH,  but  the 
impure  material  can  melt  as  low  as  300°,  and  electrolysis  is  most  con- 
veniently carried  out  at  315°-320°.  Above  325°  the  yield  practically 
falls  to  zero.  The  temperature  must  also  be  kept  constant,  as 

1  Preferable  to  iron,  as  it  is  more  passive  in  NaOH  containing  any  NaCl. 
*  Zeitsch,  Elektrochem.  15,  539  (1MH), 


Fio.  100. — Castner  Sodium 
Cell. 


xxiii.]  SODIUM  409 

convection  currents  disturb  the  electrolysis.  Whilst  working,  a  number 
of  slight  explosions  occur.  They  are  generally  harmless,  but  account 
for  the  use  of  small  units. 

According  to  J.  W.  Richards l  the  cells  at  Niagara  are  about  18"  in 
diameter  and  2'  deep.  The  cathode 2  is  provided  with  an  enlargement 
at  the  top,  4"  in  diameter,  which  constitutes  the  active  cathodic  surface. 
A  unit  holds  about  250  Ibs.  of  molten  NaOH,  takes  1,200  amperes  at 
about  5  volts,  and  works  with  45  per  cent,  current  efficiency.3  Assum- 
ing the  cathode  enlargement  to  be  6"  long,  its  active  surface  will  be 
about  4'8  dm.2  and  the  cathodic  current  density  250  amps./dm.2.  The 
units  used  in  England  are  of  smaller  size  (500  amperes),  work  at 
slightly  lower  current  densities,  and  take  about  4'5  volts. 

Theory. — The  mechanism  of  this  electrolysis  was  first  recognised 
by  Janeczek,4  who  showed  that  when  anhydrous  NaOH  is  electrolysed 
hydrogen  is  at  first  not  produced  at  the  cathode,  but  results  on  con- 
tinuing the  electrolysis.  He  considered  the  successive  reactions  to  be 

(a)  2NaOH — »  2Na'  -f  20H' 

I  I 

Metal  at  cathode.     Water  +  oxygen  at  anode. 

(6)  2Na  +  2H20  — >  2NaOH  +  H2. 

Thus  the  hydrogen  is  the  product  of  a  second  chemical  reaction,  and  we 
see  at  once  one  of  the  causes  of  the  poor  yield  of  sodium.  The  ques- 
tion was  later  treated  more  fully  by  Le  Blanc  and  Erode.5  They  found 
that  if  fresh  commercial  NaOH  be  taken  and  electrolysed  hydrogen 
and  oxygen  are  evolved.  As  the  electrolysis  is  continued,  hydrogen  is 
produced  with  more  and  more  difficulty,  until,  at  a  certain  decomposi- 
tion voltage,  sodium  is  liberated.  If  the  melt  has  been  dehydrated 
previous  to  the  electrolysis  by  heating  or  by  treatment  with  metallic 
sodium,  no  hydrogen  is  evolved.  They  therefore  regard  the  cathodic 
production  of  hydrogen  as  rising  from  the  water  contained  in  the 
molten  NaOH,  which  they  showed  to  be  very  hygroscopic,  and  to^part 
with  its  water  with  difficulty.  Investigating  the  anodic  process,  they 
conclusively  proved  the  formation  of  water. 

Their  final  formulation  of  the  processes  occurring  is 

(1)  NaOH >Na'  +  OH' 

I  I 

Sodium         Water  +  oxygen  at  anode, 
at  cathode. 

(2)  H,0 — >H-  +  OH' 

i        ; 

Hydrogen     Water  +  oxygen  at  anode, 
at  cathode. 

This  is  the  chief  source  of  loss  if  the  temperature  be  kept  low. 

1  Electrochem  Ind.  1,  14(1902).     -  Said  (obviously  erroneously)  to  be  of  carbon. 

3  Richards'  calculation  of  the  daily  yield  is  incorrect.     He  assumes  90  per  cent, 
current  efficiency,  whereas  about  half  is  obtained  in  practice. 

4  Ber,  8,  1018  (1875).  5  faitsch.  Elektrochem.  8,  717  (1902). 


410    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

(3)  Na      +     H20     —  >  NaOH  +  JH8 

(dissolved  (formed  at 

fronTcathode)  anode) 

This  can  occur  in  both  anode  and  cathode  compartments.  It 
takes  place  to  a  greater  extent  at  the  anode  at  higher  temperatures, 
when  the  rate  of  diffusion  of  the  dissolved  sodium  is  greater.  When 
this  happens,  both  hydrogen  and  oxygen  are  liberated  at  the  anode, 
and  explosions  may  result. 

(4)  2Na        +  02  —  >Na202 

(from  cathode) 

This  occurs  at  the  anode. 

(5)  Na202  +  2Na  -  >  2Na20 

I  2H20 
4NaOH 


(6) 

This  ionic  process  can  occur  at  the  anode  if  the  melt  is  saturated 
with  sodium  at  a  high  temperature.  No  gas  is  liberated,  and  the 
current  efficiency  is  zero.1 

Equations  (2)-(6)  all  represent  sources  of  loss  in  the  Castner  process. 
When  it  is  well  worked,  (2)  is  the  most  important,  but  at  higher  tempera- 
tures the  others  come  into  play.  As  Le  Blanc  and  Erode  point  out, 
the  Castner  process  as  worked  can  never  give  higher  than  50  per  cent. 
current  efficiencies,  but  better  figures  would  result  if  the  water  could 
be  kept  away  from  the  cathode  and  the  sodium  from  the  anode.  A 
cathode  diaphragm  is  needed,  uncorroded  by  molten  NaOH  ;  and 
a  current  of  dry  air  passed  through  the  anode  compartment  to  remove 
the  water  would  be  advantageous.  Ewan  2  has  patented  both  devices. 
He  suggests  a  diaphragm  of  A1203  or  sodium  aluminate,  but  its  efficiency 
is  very  doubtful. 

The  reasons  for  the  frequent  explosions  are  therefore  as  follows  :  — 

(1)  In  the  cathode  compartment.     Hydrogen  and  air  which  enters 
from  above. 

(2)  In  the  anode  compartment.     Sodium  diffusing  through   and 
liberating  hydrogen  from  the  anodic  water. 

(3)  The    screen    and   gauze  acting  as  a  bipolar  electrode.     This 
would  cause  explosions  in  both  compartments,   but  is  unlikely  to 
occur  except  in  a  badly  designed  cell  or  with  a  gauze  of  too  fine  a 
mesh. 

The  immediate  cause  of  explosions  would  be  the  action  of  anodic 
oxygen  or  air  on  tiny  globules  of  molten  sodium,  small  enougli  in 
case  (2)  to  pass  through  the  gauze  screen. 

It  can  now  be  clearly  seen  why  the  temperature  must  be  kept  low 
and  constant.  Convection  and  diffusion  are  minimised  (the  elec- 

1  See  p.  4ir..  '  E.P.  14,739  (1902). 


XXIIL]  SODIUM  411 

trolyte  becomes  far  more  viscous  near  its  melting-point),  and  losses 
are  consequently  lessened.  One  might  imagine  that  the  rapid  fall  of 
the  current  efficiency  above  330°  is  due  to  increased  solubility  of  the 
metal  in  the  melt.  The  work  of  v.  Hevesy  *  shows  this  to  be  incorrect. 
He  determined  the  solubilities  of  sodium  and  potassium  in  their 
anhydrous  molten  hydroxides,  conclusively  showing  them  to  be  cases 
of  real  solubility,  not  of  colloidal  suspension  or  formation  of  sub-oxide. 
For  sodium  he  obtained — 

Grams  Na  per 
100  grams  NaOH 
480°  25-3 

600°  10-1 

800°  6-9 

The  solubility  thus  decreases  rapidly  with  rising  temperature. 

With  potassium  the  solubilities  are  much  smaller.  Le  Blanc 
and  Erode  had  attributed  the  impossibility  of  preparing  this  metal 
by  the  Castner  process  to  its  enormous  affinity  for  oxygen,  v.  Hevesy 
showed  the  correctness  of  this  view  by  parallel  experiments  on  the 
production  of  the  two  metals,  using  cathodes  capped  in  magnesite 
crucibles  and  thus  protected  from  the  air.  Much  higher  yields  of 
potassium  than  sodium  resulted  (at  the  same  temperatures),  corre- 
sponding to  the  greater  solubility  of  sodium  in  its  fused  hydroxide. 
Thus,  between  320°-340°,  a  27  per  cent,  yield  of  Na  and  a  55  per  cent, 
yield  of  K  resulted.  Finally,  to  confirm  the  idea  that  the  rapid  decrease 
in  the  yield  of  sodium  with  rising  temperature  is  due  essentially  to 
its  increased  rate  of  diffusion,  diffusion  experiments  in  their  fused 
hydroxides  were  carried  out  with  both  sodium  and  potassium.  The 
rate  of  diffusion  of  K  was  found  to  be  very  low  and  practically  constant 
between  300°  and  550°,  whereas  that  of  sodium  rose  perceptibly  at 
330°  and  very  rapidly  at  340°. 

The  Helmholtz-Thomson  rule,  applied  to  the  decomposition  volt- 
age of  molten  NaOH,  gives  the  very  uncertain  figure  of  3*1  volts. 
Fortunately  we  have  experimental  determinations  by  Sacher 2 
and  by  Le  Blanc  and  Brode.3  The  decomposition  voltage  of  the 
water  present  in  moist  NaOH  was  found  to  be  1*3  volts  at  about  330°, 
the  voltage  for  sodium  formation  2'2  volts.  Assuming  a  current 
efficiency  of  45  per  cent,  and  an  average  voltage  of  4'5  volts,  we  have 
for  the  energy  efficiency  of  the  Castner  process — 

2-2 

X  45  =  22  per  cent. 
4"o 

1  Zeitsch.  Elektrochem.  15,  539  (1909). 
-  Zeitsch.  Anorg.  Chem.  28,  385  (1901). 
:!  Zeitsch.  Elektrochem.  8,  697  (1902). 


412    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 


Also  that  one  ton  of  sodium  requires 

4'5  X  100  X  96540  X  1000  x  1000 


=  11700K.W.H. 


45  X  23  X  3600  x  1000 

Or  1  H.P.  year  gives  0*56  ton  sodium. 

Becker  Process.1 — In  this  process,  said  to  be  worked  in  France, 
a  mixture  of  NaOH  and  Na2C03  is  electrolysed  in  a  cell  similar  to 
the  Castner,  but  with  one  or  two  essential  differences.  Firstly,  no 
wire-gauze  curtain  is  used.  Secondly,  above  the  cathode  (in  shape  a 
truncated  cone  sloping  in  upwards)  is  put  a  '  collector  '  dipping  into 
the  melt,  under  which  the  sodium  aggregates  together.  This  collector, 
provided  with  a  deep  vertical  flange  to  prevent  electrolyte  getting  on 
its  upper  surface,  has  a  discharge  pipe  through  which  the  sodium  is 
drawn  off.  It  can,  if  necessary,  be  cooled  from  above.  This  collector 
is  connected  electrically  with  the  cathode,  Becker  stating  that  the 
re-solution  of  the  sodium — thus  made '  negative  ' — is  thereby  prevented. 
The  working  temperature  near  the  electrodes  is  550°,  and  solidified 
electrolyte  coats  the  walls.  5000-ampere  units  can  be  constructed. 
It  is  claimed  that  explosions  are  avoided,  and  that  Na2C03  and  not 
NaOH  is  decomposed.  The  inventor  states  that  fourteen  5000-ampere 
units  running  for  24  hours  would  produce  500  kilos,  of  metal.  The 
current  efficiency  would  then  be — 

100  X  50  X  1000  X  26'8 

"5000  X  14  X  24  X  23      =  35  pe*  Cent' 

He  further  says  that  not  less  than  one  kilo,  of  sodium  is  produced 
per  18  K.W.H.  We  have  seen  that  the  production  of  one  kilo,  by  the 
Castner  process  requires  11  "7  K.W.H. 

The  only  essential  advantage  which  this  process  would  appear 
to  have,  i.e.  the  cheapness  of  the  raw  material,  a  Na2C03  —  NaOH 
mixture,  with  Na2C03  subsequently  fed  in,  has  been  shown  by  Le  Blanc 
and  Carrier 2  to  be  quite  illusory.  They  worked  with  varying  propor- 
tions of  Na2C03  and  NaOH,  and,  as  one  would  expect,  never  obtained 
any  C02  in  the  anode  gas,  which  was,  on  the  contrary,  nearly  pure 
oxygen.  With  a  55  per  cent.  Na2C03  melt,  sodium  was  obtained 
above  480°,  and  with  a  60  per  cent,  melt  at  550°,  but  in  very  small 
amounts.  On  raising  the  temperature,  the  yields  diminished.  Varia- 
tions of  electrolyte  composition  and  current  density  gave  no  better 
results.  They  conclude  that  the  Becker  process  is  merely  the  Castner 
process,  slightly  modified  and  by  no  means  improved. 

'  Contact  Electrode  '  Process.— The  other  important  method  for 
sodium  production  from  NaOH  is  the  '  contact  electrode  '  process, 
used  by  the  Griesheim  Elektron  Co.  Its  principle  is  Bunsen's  idea 

'  E.P.  11,673  (1899).     Also  Becker,  Kl>ltr&meiallurgi<  <l>r  AlhilimetaUe. 
•  Zeitsch.  Elektrochem.  10,  568  (1904). 


XXIII.] 


SODIUM 


413 


of  using  a  cathode  which  does  not  dip  into  the  electrolyte,  but  is  as  far 
as  possible  merely  in  contact  with  it.1  The  liberated  metal  is  thus 
much  less  exposed  to  the  electrolyte,  and  larger  yields  should  be  possible. 
The  apparatus  employed  is  very  simple.  A  large  iron  anode  in  the 
middle  of  a  shallow  NaOH  bath  is  surrounded  at  a  suitable  distance 
by  a  ring  of  metal  cathodes,  which  make  contact  with  the  electrolyte 
by  the  surface  film  only.  The  distance  apart  of  the  electrodes  is  so 
regulated  that  no  anodic  gases  come  into  contact  with  the  sodium. 
Screens,  such  as  in  the  Castner  process,  or  as  were  used  in  a  former 
'  contact  electrode '  process,  are  unnecessary.  Cathodic  current 
densities  up  to  the  high  figure  of  1000  amps./dm.2  can  be  used. 
Above  that  value,  overheating  at  the  contact  film  occurs.  The  current 
efficiency  is  stated  to  be  35  per  cent.  The  voltage  is  not  known,  but 
wrill  doubtless  be  very  high  with  such  enormous  current  densities.2  To 
this  disadvantage  we  must  add  the  further  probable  one  that  the  Na 
has  to  be  collected  in  much  smaller  quantities  at  a  time  than  in  the 
Castner  process. 

Sodium  from  Fused  Salt. — The  production  of  sodium  by  electro- 
lysing fused  NaCl  has  been  the  subject  of  much  investigation.  The 
difficulties  are  enormous,  and  no  satisfactory  solution  has  been  yet 
reached.  Pure  NaCl  melts  at  803° — impure  material  down  to  775°. 
The  metal  boils  at  877°,  and  at  800°  its  vapour  pressure  is  already 
very  high.  It  is  difficult  to  thoroughly  separate  the  anodic  and 
cathodic  products.  And,  lastly,  the  wear  and  tear  of  the  apparatus  is 
very  great.  Grabau  and  Hulin  came  nearest  to  solving  the  problem, 
but  their  cells  proved  unsuitable  when  tried  on  the  large  scale.  At 


Cathode 


Water   |?| 


-Water 


-Anode 


Solidified 
NaCl 

-Water 


SocLkuun, 
FIG.  101. — Seward-v.  Kiigelgen  Sodium  Cell. 

present  it  is  believed  that  small  quantities  of  metal  are  being  prepared 

directly  from  fused  salt  at  Holcomb  Rock,  Virginia,  by  a  method 

due  to  Seward  and  v.  Kiigelgen,  and  at  Basel,  using  a  process  devised 

1  Cf.  p.  419.  2  Compare  pp.  419,  420. 


414    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

by  Stockem.  The  former  inventors  surround  their  cathode  with  a 
metal  water-cooled  hood.  Under  this  the  sodium  collects,  eventually 
running  off  through  the  hollow  electrode  into  a  vessel  below.  The 
graphite  anode  forms  the  side  lining  of  the  cell,  the  bottom  of  which 
is  also  water-cooled  (Fig.  101). 

Ashcroft  Process. — A  few  years  back  a  method  for  the  production 
of  sodium  from  molten  salt  was  patented  by  Ashcroft.1  Though  it, 
too,  has  been  thoroughly  tested,  and  is  at  present  abandoned,  a  brief 
description  of  it  may  find  a  place  here,  as  the  problem  itself  is  an 
attractive  one  and  the  proposed  solution  ingenious.  The  electrolysis 
was  carried  out  in  a  double  cell.  In  the  first  compartment  fused  salt 
was  electrolysed  between  a  carbon  anode  and  a  molten  lead  cathode. 
The  molten  lead-sodium  alloy  formed  was  transported  to  the  second 
chamber,  where  it  became  anode  in  a  bath  of  molten  NaOH,  metallic 
sodium  being  deposited  at  the  cathode. 

The  essential  parts  of  the  cell  are  shown  in  Fig.  102.  Salt  is  charged 
in  at  A,  at  B  chlorine  is  led  off,  and  C  is  the  anode.  The  lead-sodium 


FIG.  102.— Ashcroft  Sodium  Cell. 

alloy  leaves  the  vessel  at  D,  and  passes  along  a  pipe  connection  (in 
which  it  is  cooled)  to  the  NaOH  cell,  which  is  considerably  larger  and 
works  at  a  lower  current  density.  The  sodium  produced  at  the  cooled 
spherical  cathode  collects  in  the  cone  (similar  to  that  used  in  the 
Becker  cell)  and  is  run  off.  The  depleted  lead  alloy  circulates  back 
to  the  salt  compartment  through  a  second  pipe  connection,  being 
heated  up  by  the  hot  sodium-rich  alloy  travelling  in  the  opposite 
direction.  The  temperature  of  the  salt  chamber  is  kept  at  about  770°, 
as  low  as  is  possible  without  solidification.  The  NaOH  compartment 
is  at  330°.  Pure  lead  melts  at  326°,  but  its  sodium  content  makes  it 
quite  easily  fluid  at  that  temperature. 

The  current  density  at  the  cathode  in  the  salt  chamber  is  about 
200  amps./dm.8    At  the  anode  it  is  probably  more,  though  Carrier 2 

1  Trans.  Amer.  Electrochem.  Soc.  9,  123  (1906). 

2  Electrochem.  Ind.  4,  477  (1906). 


XXIIL]  SODIUM  415 

states  that  at  300  amps/dm.2  a  graphite  anode  becomes  rapidly 
corroded.  At  the  lead-sodium  alloy  in  the  caustic  chamber  it  is  far 
lower.  The  salt  compartment  takes  about  7,  the  NaOH  compart- 
ment 2  volts.  Ashcroft  claimed  a  current  efficiency  of  90  per  cent. 
when  working  smoothly.  One  ton  of  sodium  would  thus  require 

1000  x  100  X  96540  x  9  X  1000 


or  1  H.P.  year  gives  0'56  ton  of  sodium,  exactly  as  in  the  Castner 
process.  The  surprisingly  high  current  efficiency  is  due  to  the  very 
small  solubility  of  both  sodium  and  chlorine  in  the  molten  salt,  to 
the  small  sodium  vapour  pressure  of  the  lead-sodium  alloy,  and  to  the 
absence  in  the  NaOH  cell  of  any  complications  due  to  water  and 
oxygen. 

This  NaOH  compartment  must,  however,  be  kept  at  a  low  tempera- 
ture. For  example,  Carrier  *  worked  on  a  similar  process  in  which 
the  NaOH  and  NaCl  cells  were  not  spatially  separated,  but  in  very 
close  contact.  The  NaOH  cell  was  at  700°-800°,  and  yielded  no  trace 
of  sodium  or  any  other  product  even  with  large  currents  (800  amperes) 
passing.  Of  course  at  this  high  temperature  the  sodium  would  at 
first  rapidly  dissolve,  but  after  saturation  of  the  melt  one  wrould  expect 
it  to  deposit.  This  interesting  phenomenon  may  result  from  one 
of  two  causes.  The  metal  dissolved  in  the  electrolyte  may  ionise 
(Na  +  ©  -  >  Na')  at  the  anode  more  easily  than  the  metal  dissolved 
in  the  lead.  Then  anode  and  cathode  reactions  would  be  the  converse 
of  one  another,  and  the  electrolyte  would  remain  unchanged,  Na" 
ions  being  discharged  at  the  cathode,  the  metal  diffusing  to  the  anode, 
and  there  becoming  re-ionised.  Or  the  melt,  in  virtue  of  its  large 
content  of  dissolved  metal,  may  perhaps  simply  behave  as  an  electronic 
or  metallic  conductor,  as  do  solutions  of  sodium  in  liquid  ammonia. 
Which  view  is  correct  we  cannot  yet  decide. 

There  are  no  experimental  data  on  the  decomposition  voltage  of 
molten  NaCl.  But  we  know  that  at  800°  calcium  can  displace  sodium 
to  a  great  extent  from  Nal,  and  that  at  lower  temperatures  the  reaction 
is  reversed.2  The  same  statement  probably  holds  for  the  chlorides. 
We  also  know  that  the  decomposition  voltage  of  CaCl2  at  800°  is 
3-24  volts.3  That  of  NaCl  is  consequently  a  little  lower.  We  will 
suppose  it  to  be  3'0  volts.  The  energy  efficiency  of  the  complete 
Ashcroft  process,  of  which  the  final  result  is  the  splitting  up  of  NaCl 
into  sodium  and  chlorine,  is  thus 

o 

90  X      =  30  per  cent.  (Castner  process  22  per  cent.). 

«7 

1  Metall.  Chem.  Engin.  9,  253  (1911).  Cf.  also  Trans.  Amer.  Electrochem.  Soc. 
9,  362  (1906). 

-  Danneel  and  Stockem,  Zeitsch.  Elektrochem.  11,  209  (1905).          3  See  p.  419. 


416    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

The  decomposition  voltage  in  the  Ashcroft  NaCl  compartment 
is  of  course  less  than  3  volts,  as  not  sodium,  but  an  unsaturated 
lead-sodium  alloy,  is  the  product.  The  high  voltage  is  due  to  the 
large  current  density.  The  conductivity  of  molten  NaCl  is  very 
high.  According  to  Poincare,1  K  is  4*09  at  780° ;  according  to 
Arndt,2  whose  figures  are  probably  more  correct,  it  is  rather  lower, 
3-34  at  800.° 

The  practical  difficulties  preventing  the  smooth  running  of  the 
process  are  enormous.  Crude  salt  is  apt  to  deposit  its  impurities 
as  a  crust  on  the  cathode,  sending  the  voltage  up.  And  the  widely 
differing  temperatures  needed  in  the  separate  parts  of  the  apparatus 
cause  considerable  strains  and  wear  and  tear.  Reasons  such  as  those 
have  led  to  an  abandonment  which  may  be  permanent.  By  using, 
not  crude  rock  salt,  but  a  suitable  low  melting  mixture  of  chlorides, 
and  continuously  feeding  in  a  purer  salt,  different  results  might 
perhaps  be  obtained. 

2.  Magnesium 

Magnesium  was  first  electrochemically  prepared  by  Bunsen  by 
the  electrolysis  of  fused  anhydrous  MgCl2.  It  is  now  made  technically 
by  the  electrolysis  of  fused  anhydrous  carnallite,  preferred  to  MgCl2 
because  it  occurs  naturally,  is  less  volatile,  and  can  be  dehydrated 
far  more  easily.  When  a  MgCl2  solution  is  evaporated  to  dryness  and 
fused,  the  salt  is  hydrolysed,  much  HC1  is  lost,  and  the  resulting 
residue  contains  MgO.  The  magnesium  in  a  carnallite  solution  is 
largely  present  as  the  complex  MgCl3'  anions,  not  as  Mg"  ions,  and 
the  above  reaction  is  much  less  to  be  feared. 

The  electrolysis  is  said  to  take  place  in  an  iron  pot  which  serves 
as  cathode,  and  using  a  carbon  anode.  No  descriptions  of  exact 
technical  apparatus  are  available,  and  it  seems  more  probable  that 
the  electrolyte  is  allowed  to  solidify  round  the  walls  of  the  pot,  and 
that  the  iron  cathode,  like  the  anode,  is  dipped  into  the  melt.  Some 
kind  of  anode  screen  or  diaphragm  will  be  necessary,  as  fused  magnesium 
is  lighter  than  fused  carnallite,  and  will  swim  about  on  the  surface  of 
the  electrolyte.  Magnesium  melts  at  633°.  MgCl2  melts  at  708°,  and 
anhydrous  carnallite  therefore  lower.  The  best  working  temperature 
would  appear  to  be  about  650°,  and  A.  Oettel 3  has  shown  that  under 
these  conditions  in  fact  the  current  efficiency  is  highest.  It  is,  however, 
difficult  to  keep  the  temperature  of  the  melt  sufficiently  constant  to 
avoid  solidification  of  the  magnesium,  and  the  process  can  easily  be 
disturbed  on  that  account.  Oettel  recommends  700°-750°.  Borchers 4 
also  worked  at  700°. 

No  data  exist  on  the  decomposition  voltage  of  carnallite  melts. 

1  Ann.  Chim.  Phya.  [6],  21,  289  (1890).        *  Zeitach.  Elektrochem.  12,  337  (!!><><;). 
3  Dissertation  (Dresden,  1908).  4  Zeitach.  Elektrochem.  1,  361  (1895). 


xxm.]  MAGNESIUM  417 

The  value  will  probably  be  higher  than  that  for  pure  MgCl2,  particularly 
as  the  magnesium  may,  even  in  the  fused  state,  be  largely  present 
as  complex  anions.  The  decomposition  voltage  of  fused  MgCl2  is 
also  unknown.  But  we  know  that  sodium  displaces  magnesium  from 
its  fused  salts.  Hence  the  decomposition  voltage  of  fused  MgCl2  must 
be  less  than  that  of  fused  NaCl  at  the  same  temperature.  The 
latter  is  approximately  3*0  volts  at  8000.1  At  700°  it  will  be  greater, 
perhaps  3*2  volts  (for  super-cooled  fused  NaCl).  We  can  therefore 
say  that  at  700°  the  decomposition  voltage  of  MgCla  is  less  than  3'2 
volts,  and  will  assume  the  value  for  carnallite  to  be  equal  to  this  figure. 
Borchers  used  5-8  volts  in  the  electrolysis,  A.  Oettel  4-8  volts,  depending 
on  current  density  and  temperature.  We  will  assume  for  calculation 
6  volts,  and  a  current  efficiency  of  75  per  cent.2  The  energy  efficiency 
is  therefore 

*V2 

75  X  -  r  =  40  per  cent. 
6 

One  kilo,  of  metal  requires 

1000  X  2  X  96540  X  100  X  6  = 
24'3  X  75  X  3600  X  1000 

F.  Oettel3  has  elucidated  certain  points  in  connection  with  this 
process.  Under  ordinary  conditions,  when  magnesium  globules  are 
formed  at  the  cathode,  they  do  not  easily  coalesce  ;  the  yield  is  conse- 
quently collected  with  difficulty,  and  the  small  particles  readily  catch 
fire.  Oettel  found  that  this  disinclination  to  aggregate  together  could 
be  removed  by  the  addition  of  a  little  CaP2  (first  recommended  by 
Deville  and  Caron).  This  acts  very  powerfully,  undoubtedly  through 
a  surface  tension  effect,  though,  according  to  Oettel,  it  also  dissolves 
traces  of  oxide  from  the  surface  of  the  globules.  A  source  of  loss  in 
the  use  of  impure  carnallite  is,  for  a  reason  already  discussed,4  the 
presence  of  a  small  quantity  of  FeCl3.  If  too  high  a  voltage  is  used, 
or  if  the  MgCl2  content  of  the  bath  becomes  too  low,  the  magnesium 
may  contain  potassium.  In  that  case  it  is  liable  to  catch  fire  during 
the  electrolysis. 

The  Aluminium  und  Magnesium  Gesellschaft,  Hemelingen,  uses 
a  rather  different  electrolyte,5  consisting  of  a  molten  mixture  of  equi- 
molecular  proportions  of  MgCl2,  KC1,  and  NaCl.  It  is  prepared 
from  carnallite  by  the  addition  of  the  necessary  amount  of  NaCl, 
none  of  the  materials  needing  a  special  purification.  During  the 
electrolysis  (a  continuous  one),  anhydrous  MgCl2  is  added  to  keep 

1  See  p.  415. 

~  90-95  per  cent,  in  practice,  according  to  F.  Oettel.     A;  Oettel  obtained  up 
to  75  per  cent. 

'  Zeitsch.  Elektrochem.  2,  394  (1895).  4  P.  162. 

•"'  Zeitsch.  Elektrochem.  7,  408  (1901). 


418    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

the  bath  composition  constant.  The  electrolyte  is  further  always 
kept  basic  by  a  suitable  quantity  of  alkali,  and  CaF2  is  added  as 
usual.  This  process  has  been  studied  on  a  small  scale  by  Holder.1 
The  best  working  temperature  was  found  to  be  750°-800°,  when  a 
70  per  cent,  current  efficiency  was  obtained.  A  cathodic  current 
density  of  27-30  amps./dm.2  was  used.  (Borchers  employed  10 
amps./dm.2,  A.  Oettel  30-40  amps./dm.2) 

Finally,  according  to  Lorenz,2  one  must  believe,  judging  from  the 
appearance  of  some  of  the  magnesium  sticks  now  on  the  market,  that 
the  '  contact  electrode '  method 3  has  been  applied  to  the  production  - 
of  this  metal  also.  A.  Oettel 4  has  shown  this,  by  laboratory  experi- 
ments, to  be  quite  possible.  At  the  same  time,  it  is  inconvenient, 
as  the  metal  rod  formed  is  somewhat  brittle.  Further,  in  consequence 
of  the  high  current  densities,  the  potassium  content  is  likely  to  rise 
high. 

3.  Calcium. 

Calcium  is  now  technically  prepared  by  the  electrolysis  of  the  fused 
chloride.  The  scale  of  production  must  be  small,  as  the  only  applica- 
tion of  the  metal  is  to  make  calcium  hydride,  sometimes  employed  to 
generate  hydrogen. 

The  electrolysis  of  fused  CaCl2  is  subject  to  a  disturbance  also 
encountered  with  MgCl2  and  ZnCl2.5  It  is  difficult  to  prepare  the  pure' 
anhydrous  salt  from  the  hydrated  chloride,  owing  to  hydrolysis  andj 

OTT 
the  gradual  formation  of  some  hydroxy- chloride  such  as  Ca^™   .\ 

In  consequence  the  liberated  metal  is  slowly  attacked  by  the  meltJ 
displaces  the  hydrogen,  and  forms  an  insoluble  oxy-chloride.  The 
bath  thickens,  the  conductivity  decreases,  and  the  yield  of  calcium 
falls.  As  much  as  17  per  cent,  of  metallic  calcium  can  be  taken  up 
by  the  melt,  together  with  several  per  cents,  of  iron  (assuming  an 
iron  containing  vessel),  and  the  condition  of  the  electrolyte  grows 
worse  with  each  cooling  and  re-heating.  Fresh  CaCl2  should  be  used 
each  time  for  small-scale  work,  and  when  operating  continuously  the 
electrolyte  must  be  occasionally  completely  changed.  Regeneration 
by  HC1  is  hopelessly  slow. 

Bunsen  and  Matthiessen 6  were  the  first  to  investigate  the  elec- 
trolysis of  the  molten  alkaline  earth  chlorides,  including  CaCl2.  They 
could  not  get  satisfactory  yields  of  metal,  which,  liberated  at  a  high 
temperature  and  in  a  finely  divided  condition,  readily  caught  fire,j 
making  the  melt  basic.  Pure  CaCl2  melts  at  780°,  and  pure  calcium 

1  Dissertation  (Zurich,   1904) 

-  Elektrochemie  geschmolzener  Sake,  p.  72.     Zeit*c1i.  Elektrochem.  7,  252  ( 1901).    \ 
3  See  pp.  412,  419.  '   Loe.  dt.  5  Pp.  416,  421. 

6  Lieb.  Ann.  93,  277  (1555). 


XXTTI.I  CALCIUM  419 

higher — 800°.  Hence,  allowing  for  impurities,  750°  will  give  a  lower 
limit  to  the  working  temperature,  and,  as  the  finely  divided  metal 
burns  in  air  not  far  above  800°,  and  as  the  molten  metal  very  easily 
forms  '  metal-fog/  there  is  only  a  small  range  of  safe  working  tempera- 
ture. They  made  several  attempts  to  overcome  this  difficulty,  using 
low-melting  mixtures  of  alkaline  earth  chlorides  and  a  very  high 
cathodic  current  density,  but  only  succeeded  in  preparing  small 
quantities  of  very  finely  divided  calcium-rich  alloy. 

Many  years  later,  Borchers  &  Stockem x  and  Ruff  &  Plato a 
worked  on  the  same  subject,  but  on  a  larger  scale,  and  eventually 
produced  small  quantities  of  fairly  pure  compact  metal.  The  latter 
authors,  after  many  preliminary  experiments,  used  a  mixture  of  100 
parts  CaCla  and  16'5  parts  CaF2,  melting  at  660°.  Keeping  the  mass 
of  the  electrolyte  at  about  760°,  the  temperature  in  the  immediate 
neighbourhood  of  the  cathode  was  raised  above  800°  by  the  use  of  a 
very  high  cathodic  current  density  (3--5  amps./mm.2).  The  calcium 
was  consequently  produced  molten,  and,  on  moving  away,  quickly 
chilled  and  solidified.  About  30  volts  were  required,  the  high  value 
being  essentially  due  to  the  enormous  current  density.-  (The  decom- 
position voltage  of  molten  CaCl2  at  800°  has  been  found  by  Arndt 
and  Willner3  to  be  3'24  volts.)  Ruff  and  Plato's  work  forms 
a  comprehensive  study  of  one  method  of  electrolytically  winning 
metallic  calcium,  but  it  is  unlikely  that  a  similar  process  will  ever  be 
technically  used. 

The  process  now  employed 4  is  based  on  the  contact  electrode 
principle  used  for  the  production  of  sodium.5  But  the  relative  positions 
of  the  melting-points  of  metal  and  electrolyte  permit  an  important 
modification.  Not  only  does  the  iron  cathode  barely  dip  under  the 
surface  of  the  melt,  but  it  can  be  steadily  raised  by  means  of  a  gearing. 
A  solid  rod  of  the  precipitated  metal  is  thus  continually  removed  from 
the  electrolyte  and  itself  forms  the  cathode,  the  iron  merely  acting  as 
a  lead.  The  advantages  are  obvious.  The  metal  is  at  once  obtained 
in  massive  coherent  form,  free  from  any  impurity  except  an  adhering 
skin  of  CaCl2 ;  and  a  much  better  current  efficiency  results  than 
with  the  older  method,  any  tendency  of  metal  to  dissolve  or  to  form 
'  metal-fog '  being  minimised.  The  published  technical  details  are 
extremely  meagre.  The  electrolyte  contains  CaCl2  only>  no  CaF2 
being  added.  The  temperature  must  consequently  lie  between  780°- 
800°.  The  current  density  is  very  high,  100  amps./cm.2 

Several  laboratory  investigations  of  the  process  have  been  published. 
Wohler6  used  a  cast-iron  vessel  externally  heated,  a  carbon  anode, 

1  Zeitsch.  Elektrochem.  8,  757  (1902).  -  Ber.  35,  3612  (1902). 

3  Zeitsch.  Elektrochem.  14,  216  (1908). 

4  Rathenau,  Zeitsch.  Elektrochem.  10,  508  (1904).  5  P.  412. 
6  Zeitsch.  Elektrochem.  11,  612  (1905). 


420    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

and  an  8  mm.  iron  rod  as  cathode.  A  water-cooled  cathode  presented 
no  particular  advantage.  His  electrolyte  consisted  of  100  parts 
CaCl2 :  17  parts  CaF2  (M.P.  6600).1  The  bath  was  kept  at  665°-6SO°, 
the  latter  temperature  being  reached  near  the  cathode  where  the 
current  density  was  high.  The  electrolyte  deteriorated  with  time, 
as  has  already  been  described.  With  a  fresh  bath  38  volts  were  used. 
The  cathodic  current  density  was  varied  between  50-250  amps./dm.2, 
but  had  little  effect  on  the  current  efficiency.  On  the  other  hand, 
it  is  of  prime  importance  that  the  cathode  be  regularly  and  rapidly 
raised  from  the  electrolyte,  as  the  chief  source  of  loss  is  the  metal-fog 
formation.  The  current  efficiency  obtained  was  82  per  cent.  With 
too  high  an  anodic  current  density  the  anode  effect  was  observed 
(at  5'6  amps./cm.2  with  carbon  or  8  amps./cm.2  with  graphite),  and 
the  bath  voltage  rose  to  about  80  volts. 

Goodwin  2  worked  under  rather  different  conditions.  He  employed 
a  CaCl2  bath  and  kept  it  at  just  above  800°.  Fused  'calcium  was 
consequently  produced.  It,  however,  solidified  immediately  above  the 
surface  of  the  melt,  and  by  continually  raising  the  cathode  massive 
metal  resulted  in  the  form  of  a  rod  of  very  irregular  cross-section.  An 
absolutely  regular  rate  of  raising  the  cathode  is  essential.  An  average 
run  gave  a  26' 6  per  cent,  current  efficiency,3  using  17' 7  volts  and 
163  amperes.  The  best  run  was  with  160  amperes  at  19  volts  and  gave 
a  41 '2  per  cent,  yield.  The  cathodic  current  density  varied  between 
3-2-20  amps./dm.2,  much  lower  than  was  the  case  with  Wohler.  This 
is  one  cause  of  the  far  lower  voltage  used.  Another  reason  would 
be  the  higher  working  temperature.  The  lower  current  density  was 
doubtless  also  partly  responsible  for  the  low  current  efficiency.  Other 
causes  would  be  the  greater  tendency  to  '  metal-fog '  formation  at 
the  higher  temperature,  and  the  loss  of  liquid  globules  of  metal. 

Frary,  Bicknell  and  Tronson 4  used  a  containing  vessel  of  Acheson 
graphite  (water-cooled  at  the  bottom)  which  acted  as  anode,  and  a 
water-cooled  iron  cathode  1"  in  diameter.  They  found  CaCl2  without 
any  addition  of  CaF2  to  be  the  most  suitable  electrolyte .  Two  important 
conditions  were  the  regular  and  continuous  raising  of  the  cathode  rod 
and  the  careful  regulation  of  the  cathode  temperature.  If  the  latter 
were  allowed  to  rise  too  high,  liquid  globules  of  metal  floated  away ; 
when  it  fell  too  low,  spongy  masses  of  calcium  resulted.  The  best 
current  density  was  about  9'3  amps./dm.2  The  voltage  varied  between 
18  and  31  volts,  usually  near  the  latter  figure  ;  and  the  current  efficiency 
between  45  per  cent.-lOO  per  cent.,  usually  exceeding  80  per  cent. 

1  See  Ruff  and  Plato,  p.  419. 
-  Jour.  Amer.  Chem.  Soc.  27,  1403  (1905). 

'•'•  Using  a  similar  apparatus,  Tucker  and  Whitney  obtained  60  per  cent.     Jour. 
Amer.  Chem.  Soc.  28,  85  (1W6). 

4  Trans.  Amer.  Electrochem.  Soc.  18,  117  (1910). 


XXIIL]  ZINC  421 

In  consequence  of  the  high  voltage,  the  methods  proposed  are  very 
imperfect  as  far  as  energy  efficiency  goes.  In  the  actual  technical 
process  the  voltage  will  be  less  than  that  observed  by  Wohler,  owing 
to  the  higher  temperature,  and  greater  than  that  observed  by  Goodwin, 
owing  to  the  heavier  current  density.  We  will  assume  25  volts,  and 
with  an  80  per  cent,  current  efficiency  we  have 

3*24 

Energy  efficiency  =  80  X  — —  =  10  per  cent. 

25 

The  production  of  one  kilo,  of  metallic  calcium  requires 

1000X2X96540X100X25  = 
40x80x3600x1000 

All  investigators  agree  as  to  the  enormous  voltage  losses,  and  they 
can  only  be  ascribed  to  the  use  of  the  contact  electrode  and  very  high 
current  densities.  The  specific  conductivity  (K)  of  fused  CaCl2, 
according  to  Arndt  and  Gessler,1  is  1 '9  at  800°.  According  to  Poincare  2 
it  is  1*22  at  760°,  but  this  lower  figure  probably  holds  for  impure  basic 
material. 

4.  Zinc 

The  electrolytic  production  of  zinc  from  fused  ZnCl2  has  been  fully 
investigated,  both  in  the  laboratory  and  technically.  It  has  been 
proposed  to  prepare  ZnCl2  from  crude  ZnO  and  HC1,  to  evaporate, 
fuse,  and  electrolyse,  the  metal  being  thus  obtained  in  a  compact 
molten  state.  Again,  fused  ZnCl2  is  a  principal  product  of  the 
Swinburne-Ashcroft  and  Baker  processes  of  chlorine  smelting  of 
complex  sulphide  ores. 

The  electrolysis  of  fused  ZnCl2,  or  rather  its  preparation  previous  to 
electrolysis,  presents  peculiar  difficulties  which  have  been  investigated 
by  Lorenz  and  his  pupils.3  If  ZnCl2  be  melted  in  an  open  crucible  in 
air  and  electrolysed  between  carbon  electrodes,  large  quantities  of 
gas  are  evolved  at  both  electrodes,  carrying  away  ZnCl2  vapours,  the 
melt  becomes  badly  conducting  and  turbid,  and  no  zinc  is  produced. 
After  stopping  the  current,  the  electrolyte  is  found  to  be  strongly 
basic.  If,  before  melting,  some  JSTaCl  be  added,  the  electrolysis  proceeds 
a  little  better,  and  some  zinc,  though  not  much,  results.  The  reasons 
for  this  behaviour  are  that  ZnCl2  can  never  be  procured  in  the  pure 
state,  but  always  contains  water,  whilst  water  vapour  is  further 
produced  by  the  flame  used  for  heating.  If  the  ZnCl2  be  fused  in  a 
vessel  to  which  the  flame  gases  have  no  access,  and,  before  electrolysing, 

1  Zeitsch.  Elektrochem.  14,  662  (1908).     Also  p.  159. 
-  Ann.  Chim.  Phys.  |6]  21,  289  (1890). 

:!  Zeitsch.  An&rg.  Chem.  10,  78  (1895),  20,  323  (1899) t  23,  281  (1900)t  38,  389 
(1904). 


422    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

heated  until  fuming  has  ceased,  on  passing  the  current,  hydrogen  and 
oxygen  are  still  at  first  evolved.  But  gradually  the  conductivity 
increases,  the  stream  of  hydrogen  becomes  less,  chlorine  appears 
instead  of  oxygen,  and  zinc  is  produced.  The  fused  salt  is  now  a 
perfectly  clear  highly  refracting  liquid  which  solidifies  to  a  porcelain- 
like  hygroscopic  white  mass.  The  water  can  therefore  to  a  large  extent 
be  removed  by  heat  and  the  last  portions  electrolysed  out.  Lorenz 
recommends  for  the  same  purpose  the  addition  of  strong  HC1  or  the 
bubbling  through  of  HC1  gas  during  the  last  stages  of  evaporation, 
thus  neutralising  the  hydroxy-compounds  and  producing  water  which 
is  blown  off. 

Pure  ZnCl2  melts  at  3650,1  a  considerably  higher  temperature  than 
that  previously  given  for  impure  material.  But  traces  of  water  lower 
its  M.P.  very  considerably.  The  most  reliable  figures  for  the  conduc- 
tivity of  the  pure  fused  material  are  those  of  Lorenz  and  H.  Schultze,2 
viz. : — 

K  =  0-026  at* 400° 

0-057  at  450° 

0-104  at  500°. 

These  figures  are  exceptionally  low  for  a  fused  salt.  When  electrolysed 
it  furnishes  zinc  and  chlorine  readily  and  with  good  current  efficiency. 
Thus  Lorenz  obtained  98  per  cent,  at  500°  in  a  V-tube  apparatus. 
Within  certain  limits  the  results  are  better  the  higher  the  current 
density.  The  deficiency  is  due  to  vaporisation  and  metal-fog  forma- 
tion. Griinauer3  found  that  this  second  source  of  loss  is  much 
diminished  if  a  mixture  of  ZnCl2  and  an  alkaline  chloride  is  used.4 
Thus,  working  at  600°,  and  with  lower  current  densities  than  in  the 
case  quoted  above,  he  obtained  the  following  figures : 

TABLE  LX1V 

,„  Current  efficiency 

Electrolyte  per  ^ 

ZnCl2  73-9-75-9 

[ZnCy-HKCl)  '.CM-94-7 

[ZnClo]  +  [NaCl]  83'9-89'9 

[ZnClo]  +  1-2  [NaClJ  89'6-91'2 

Suchy  *  has  determined  the  E.M.F.  of  the  cell  Zn  |  fused  ZnCl2  |  C18 
at  various  temperatures,  a  figure  which  is  identical  with  the  decom- 
position voltage  of  the  fused  salt.  Amongst  other  figures  he 
obtained — 

1  Zeitsch.  Anorg.  Chem.  39,  434  (1MI). 
-  Zeitsch.  Anorg.  Chem.  20,  323  (/W). 
:<  Zeitsch.  Anorg.  Chem.  39,  389  (/W-/). 
4  Cf.  p.  162. 
'  Zeitsch.  Anorg.  Chem.  27,  1^2 


xxni.]  ZINC  423 

TABLE  LXV 

6  Volts 

450°  C.  1-643 

500°  1-611 

550°  1-576 

600°  1-535 

650°  1-494 

Lorenz  x  has  made  a  few  direct  determinations  of  decomposition  voltage 
and  obtained  1'49  volts  at  500°-600°. 

Technical. — Swinburne  2  lias  briefly  described  the  electrolysis  which 
constitutes  the  last  stage  of  the  Swinburne-Ashcroft  process.  In 
preparing  his  ZnCl2,  he  boiled  down  and  removed  the  residue  of  oxygen 
by  a  preliminary  electrolysis,  using  cheap  carbon  anodes  which  were 
oxidised  by  the  evolved  gas.  The  main  electrolysis  took  place  in 
vessels  internally  heated  by  the  current,  between  molten  zinc  as 
cathode  and  carbon  anodes.  These  vats  were  built  of  firebrick  cased 
with  iron,  and  took  about  3,000  amperes.  This  was  considered  a 
small  load,  and  10,000-ampere  units  were  projected,  but,  if  tested, 
there  are  no  available  accounts  of  their  behaviour.  Four  volts  were 
used,  the  current  efficiency  being  nearly  theoretical. 

The  same  process  has  been  described  elsewhere 3  more  fully.  The 
final  fusion  of  the  ZnCl2  took  place  in  enamelled  iron  pans.  '  With 
jproper  working,  the  amount  of  basic  material  formed  is  not  great/ 
The  preliminary  electrolysis  was  carried  out  in  a  brick-lined  iron  vessel, 
the  cathode  being  of  molten  zinc  and  the  anode  of  cheap  carbon  blocks, 
and  was  continued  until  all  the  water  and  basic  compounds  were 
decomposed,  a  very  pure  product  resulting.  The  energy  thus  expended 
was  about  10  per  cent,  of  that  employed  in  the  final  electrolysis. 
This  took  place  in  a  sheet-iron  vessel  lined  with  firebrick.  The  roof  was 
of  cast-iron,  gas-tight,  and  provided  with  a  charging  port  for  ZnCl2. 
Into  it  the  carbon  anodes  were  bolted,  reaching  almost  to  the  bottom 
of  the  cell.  The  chlorine  passed  out  through  a  suitably  placed  flue. 
The  molten  zinc  cathode  made  connection  with  a  small  tank  outside 
by  means  of  a  channel  through  the  wall.  A  slight  vacuum  was  main- 
tained in  the  cell  to  prevent  chlorine  losses,  and,  working  with  pure 
material,  the  electrolysis  is  said  to  have  proceeded  very  smoothly. 

One  interesting  point  is  that  the  rule  was  to  electrolyse  a  mixture 
of  ZnCl2  and  NaCl,  not  ZnCl2  only,  the  reason  being  that  the  resistance 
of  the  melt  and  also  fuming  were  thereby  diminished.  This  addition 
of  an  alkaline  chloride  has  also,  however,  a  favourable  effect  on  the 
current  efficiency.  In  the  technical  process  NaCl  was  added  until  the 
melt  contained  28  per  cent,  metallic  zinc,  whilst  Lorenz  and  Griinauer  4 

1  Zeitsch.  Anorg.  Chem.  12,  272  (1896). 
-  Electrochem.  and  Metall.  3,  68  (1903). 
3  Electrochem.  Ind.  3,  63  (1905).  4  P.  422. 


424    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP,  j 

obtained  with  a  melt  of  about  the  same  composition  a  15  per  cent.  1 
better  current  efficiency  than  when  using  the  pure  ZnCl2.     In  the  I 
present  case,  almost  theoretical  current  efficiencies  resulted  when  using  j 
a  well  dehydrated  melt.    The  current  density  was  about  43  amps. /dm.2  j 
at  the  cathode,  and  the  cell  took  about  4*5  volts.     (Swinburne  gave  I 
four  volts.)     The  working  temperature  was  450°,  under  which  circum- 
stances the  decomposition  voltage  is  1*64  volts.     The  energy  efficiency 
was  therefore — 

98  X  -    '-  =  36  per  cent. 
4*5 

One  ton  of  zinc  would  require 

1000  X  1000  X  96540  X  2  X  100  x_4-5  =  „ 

65-4  X  98  X  3600  X  iOOO~~ 
One  H.P.  year  would  yield  1*75  tons  of  zinc. 

Vogel l  has  described  work  on  the  electrolysis  of  fused  ZnCl2  in 
externally  heated  cells,  the  ZnCl2  being  previously  prepared  by  the 
action  of  commercial  HC1  on  crude  ZnO.  The  chief  point  of  interest 
here  was  the  method  employed  for  the  preparation  of  the  anhydrous ; 
salt — viz.  evaporation  in  vacuo.  By  this  means  the  water  is  removed 
more  rapidly,  more  effectually,  and  at  a  lower  temperature  than  when 
working  under  atmospheric  pressure.  The  lower  temperature  is 
probably  the  most  important  factor.  As  in  the  case  of  CaCl2,  so  also 
here  the  basic  salt  will  lose  its  water  far  more  readily  if  it  is  not  heated 
too  high  in  the  process.  The  vacuum  used  was  26-27  in.  of  mercury, 
and  allowed  of  satisfactory  dehydration. 

The  electrolysis  was  carried  out  in  an  unglazed  fireclay  cell. 
After  preliminary  failures,  a  run  of  250  hours  was  made,  using  600 
amperes  and  producing  over  3  cwt.  of  zinc  at  9T5  per  cent,  current 
efficiency.  The  temperature  was  kept  at  450°-500°,  and  the  voltage 
fluctuated  between  4  and  5,  of  which  over  1  volt  was  lost  at  the 
electrode  contacts.  The  energy  efficiency  was  consequently 

i .  fff> 

91-5  x  -—  =  33  per  cent. 
4*o 

The  electrolysis  voltage  (allowing  for  the  contact  losses)  was  consider- 
ably less  than  that  required  by  the  internally  heated  cells  previously 
described,  and  that  in  spite  of  the  electrolyte  consisting  of  the  badly 
conducting  ZnCl2  with  no  admixture  of  salt.  This  was  due  to  the  small 
current  density,  apparently  only  about  16  amps./dm.2  (very  low  for  a 
fused  salt  electrolysis),  which  in  its  turn  was  rendered  possible  by  the 
external  heating.  Low  current  density,  external  heating,  and  absence 
of  salt  all  combined  to  produce  the  rather  low  current  efficiency. 
The  electrochemical  conditions  being  favourable,  it  is  not  easy  to 

1  Trans.  Farad.  Soc.  2,  56  (1906). 


XXIIL]  ALUMINIUM  425 

see  why  the  above  processes  are  not  worked  technically.  The  answer 
is  that  the  problem  of  the  complete  dehydration  of  ZnCl2  has  not  yet 
been  fully  solved  on  a  large  scale.  The  addition  of  HC1  to  the  frothing 
mass  just  before  the  final  fusion  (as  suggested  by  Lorenz)  would  be 
an  exceedingly  difficult  and  troublesome  operation.  The  preliminary 
electrolysis  is  too  expensive,  both  as  regards  power  and  also  con- 
sumption of  anodes ;  and  the  difficulties  of  satisfactory  vacuum 
dehydration  increase  enormously  with  the  size  of  the  plant,  owing 
to  the  multiplication  of  leaks.  To  really  serve,  the  vacuum  must 
be  a  very  good  one.  Another  disturbance  encountered  technically  is 
due  to  the  difficulty  of  preparing  iron-free  ZnCl2  melts.  Any  iron 
present  in  such  an  electrolyte  is  deposited  before  the  zinc.1  Not  only 
is  the  current  efficiency  thereby  lowered  and  the  purity  of  the  product 
impaired,  but  the  metal  becomes  viscous,  which  necessitates  a  rise  of 
temperature  and  dislocates  the  working  conditions. 


5.  Aluminium 

The  entire  world's  production  of  aluminium  is  now  obtained 
electrochemically  by  the  electrolysis  of  a  solution  of  A1203  in  fused 
cryolite  (Na3AlF6).  The  two  almost  identical  processes  employed 
both  date  from  the  years  1886-1889.  The  Hall  process  is  used  in 
America,  the  Heroult  process  on  the  Continent  and  in  Great  Britain. 

The  Hall  cell  consists  of  a  cast-iron  box,  thickly  lined  with  carbon, 
of  external  dimensions  6'  X  3'  X  3'.2  The  internal  dimensions  are 
length  4J'  and  breadth  2J',  and  the  actual  depth  of  the  electrolyte  is  6". 
The  carbon  lining  acts  as  a  cathode  until  sufficient  molten  aluminium 
has  collected,  whilst  the  anode  system  consists  of  a  series  of  carbon 
rods,  3"  in  diameter,  and  15"-1S"  long  when  new.  Each  bath  contains 
40-50  of  these,  arranged  in  four  rows.  They  dip  right  down  into  the 
electrolyte,  ending  1"  or  so  above  the  layer  of  aluminium  at  the  bottom.3 
A  tapping-hole  is  provided.  The  electrolyte  consists  simply  of  a 
solution  of  alumina  (15-20  per  cent.)  in  cryolite  (J.  W.  Richards).4 
A1F3  is  present  in  excess,  and  CaF2  is  often  added  (Haber).  During 
the  electrolysis  a  layer  of  charcoal  is  kept  on  the  surface  of  the  melt. 
This  tends  to  prevent  loss  of  heat  by  radiation  and  loss  of  cryolite  by 
vaporisation,  and  also  minimises  the  burning  away  of  the  electrodes 
by  the  air  at  the  points  where  they  emerge  from  the  bath.  On  the 
top  of  this  charcoal  is  thrown  fresh  alumina,  which  is  dried  by 
the  hot  gases  and  stirred  in  when  the  bath  needs  replenishing.  The 

1  Zeitsch.  Anarg.  Chem.  39,  461  (1904). 
-  Haber,  Zeitsch.  Elektrochem.  9,  360  (1S03). 

<!  Later  forms  of  the  Hall  cell  are  said  to  employ  fewer  electrodes  of  a  much 
larger  cross-section. 

4  Electrochem.  Ind.  1,  158  (1903). 


426    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

A1F3  Haber  mentions  is  probably  added  to  replace  the  slight  vaporisation 
losses  which  occur. 

The  cryolite  used  needs  no  particular  purification.  It  will  contain 
small  quantities  of  iron  and  siHca,  but  these  will  soon  be  removed  at 
the  commencement  of  the  electrolysis,  and  any  subsequent  additions 
need  not  be  taken  into  account.  But  a  pure  alumina  is  essential. 
And  this  is  generally  prepared  by  the  Bayer  process  from  bauxite, 
a  hydrated  oxide  of  iron  and  aluminium,  containing  some  silica  and 
titanic  acid.  The  crude  material,  after  careful  roasting  to  ensure  that 
all  the  iron  is  in  the  ferric  state,  is  digested  with  caustic  soda  under 
pressure,  the  alumina  being  thereby  dissolved.  After  dilution  and 
filtration  to  remove  ferric  hydroxide,  the  liquors  are  treated  with 
some  precipitated  aluminium  hydroxide  made  in  a  previous  operation, 
when  about  70  per  cent,  of  the  A1203  present  in  the  super-saturated 
solution  is  precipitated.  After  washing  and  drying,  it  is  ready  for 
use,  while  the  NaOH,  after  concentration,  is  utilised  for  treating  a 
fresh  quantity  of  bauxite. 

A  unit  such  as  the  one  described  takes  about  10,000  amperes, 
200-250  for  each  anode  carbon,  and  absorbs  5'5  volts.  The  current 
density  at  the  cathode,  assuming  the  whole  bottom  of  the  bath  but 
no  part  of  its  sides  to  be  active,  works  out  at  nearly  100  amps. /dm.2 
At  the  anode  it  is  perhaps  500  amps./dm.2  x  The  joule  heat  keeps 
the  electrolyte  liquid,  but  a  solidified  layer  is  allowed  to  form  on  the 
inside  walls  of  the  cell  in  order  to  protect  them  from  corrosion.  In 
the  early  days  of  the  aluminium  industry,  when  smaller  units  were 
employed,  larger  current  densities  were  required  to  keep  the  bath 
molten — up  to  250  amps./dm.2  at  the  cathode.  A  higher  bath  voltage 
was  consequently  necessary,  7-8  volts.  This  is  now  avoided. 

Cryolite,  according  to  J.  W.  Richards,  melts  at  800°  ;  according 
to  Pyne  2  at  1000°,  which  is  probably  more  correct.  The  former  value 
may  have  been  determined  for  cryolite  with  a  considerable  excess 
of  A1F8.  The  addition  of  A1203  lowers  its  melting-point — a  mixture 
containing  5  per  cent.  A1203  melts  at  915°  (Pyne).  Further  addition 
of  A1203  appears  to  raise  the  M.P.,  and  a  mixture  with  20  per  cent. 
A1203,  roughly  a  saturated  solution,  melts  at  1015°,  higher  than  the 
temperature  for  pure  cryolite.  If,  then,  pure  cryolite  and  A1203  alone 
are  used,  the  electrolysis  temperature  cannot  be  much  below  1000°.  As 
a  matter  of  fact  it  is  kept  as  low  as  possible,  and,  according  to  Haber, 
fluctuates  between  900°-1000°,  and  never  exceeds  1065°.  Richards 
puts  it  as  900°-950°.  If  we  suppose  the  percentage  of  dissolved 
A1203  to  be  high,  which  it  must  be,  or  else  the  voltage  increases,  then 
these  figures  certainly  indicate  the  presence  in  the  melt  of  considerable 
quantities  of  substances  other  than  cryolite  and  A1203— e.g.  the  A1F3  and 

1  In  newer  cells,  lower. 

-  Trans.  Amer.  Electrochem.  tioc.  10,  03  (1906). 


XXIII.] 


ALUMINIUM 


427 


CaF2  mentioned  by  Haber.     The  specific  resistance  (  —  J  of  the  melt 

Richards  states  to  be  3  ohms/cm.3 

Neumann1  has  also  described  the  aluminium  bath  as  at  present 
used,  here  presumably  the  Heroult  type.  It  is  round  in  section,  not 
rectangular,  and  is  made  of  wrought  iron.  The  bottom  is  covered 
with  carbon  plates  and  serves  as  cathode  at  the  beginning  of  the 
electrolysis.  The  anodes  are  distributed  regularly  over  the  whole 
surface  of  the  melt.  They  are  40  cm.  long  and  of  large  cross-section, 
35  X  35  cm.,  the  result  being  that  the  anodic  current  density  is  only 
80-100  amps./dm.2  The  baths  are  built  in,  but  space  is  left  for  the 
passage  of  a  regulated  current  of  air  for  the  purposes  of  cooling  their 
sides  and  of  producing  a  solidified  lining  of  electrolyte  (Fig.  103). 


m    W/ 

^   -M -Electrolyte 


Carbon, 


FIG.  103. — Heroult  Aluminium  Cell. 


The  greater  part  of  the  surface  of  the  bath  is  also  covered  with  a  crust 
during  working,  and  this  must  be  broken  through  when  necessary 
to  add  alumina.  Of  this  the  electrolyte  contains  10-20  per  cent.,  and 
some  NaCl  is  sometimes  added  at  the  start  to  facilitate  the  fusion 
of  the  electrolyte.  The  working  voltage  is  about  6  volts,  but  varies 
considerably  with  the  distance  apart  of  anode  and  cathode.  The 
temperature  is  below  1000°.  From  statements  from  other  sources 
we  gather  that  an  average  unit  carries  7000  amps,  and  absorbs 
7  volts.2 

The  mechanism  of  the  process  is  simple.  The  dissolved  A1203 
furnishes  Al'"  and  0"  ions.  The  AT"  ions  are  cathodically  discharged 
on  the  bottom  of  the  bath,  and  the  metal  drawn  off  at  suitable  intervals. 
Calculating  from  the  specific  gravities  of  aluminium,  cryolite,  and 
alumina  at  room  temperature,  one  would  not  expect  the  metal  to 

1  Zeitsch.  Elektrochem.  16,  230  (1910). 

-  At  Dolgarrog,  the  Aluminium  Corporation  has  baths  taking  63'5  h.p.  and 
containing  ten  anodes  (baked  at  1300°-1400°).  The  metal  is  tapped  twice 
weekly,  a  cell  yielding  7-7 £  cwts.  per  week  (corresponding  to  0'29  metric  ton 
per  h.p.  year).  2  kilos,  (almost)  A1203,  O'l  kilo,  cryolite,  and  0'67  kilo,  of 
anodes  are  consumed  per  kilo,  of  metal.  The  whole  cell  is  lined  with  carbon, 
which  is  renewed  every  6-8  months  (Sept.  1912). 


428    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP.: 

collect  on  the  bottom  of  the  cell,  but  rather  to  float  on  the  surface.] 
J.  W.  Richards1  has,  however,  shown  that  fused  aluminium  is 
denser  than  fused  cryolite,  or  the  solution  of  A1203  in  cryolite.  Thus  :; 


Substance  C 

Molten      Solid 

Commercial  aluminium  2'54  2'66 

Commercial  Greenland  cryolite  2  '08  2  '92 

Do.  saturated  with  A1203.  2'35  2*90 

Cryolite  +  aluminium  fluoride  (Al2F6.6NaF  -f  2A12F6)  T97  2'96 

Do.  saturated  with  AL>0;5  2'  14  2'98 

The  anodic  oxygen  does  not  leave  the  bath  as  such,  but  attacks  the 
carbons,  forming  CO,  which  subsequently  burns  to  C02  at  the  surface 
of  the  melt. 

Anodes.  —  This  loss  of  anodes  is  a  heavy  item  in  the  cost  of  alumin- 
ium production,  as  they  must  be  of  good  quality,  containing  a  minimum 
percentage  of  ash.  Otherwise  the  aluminium  would  become  con- 
taminated  to  too  great  an  extent  with  iron  and  silicon.  And  they 
should  not  disintegrate  during  use.  Clacher  2  has  described  the  manu- 
facture of  the  anodes  used  by  the  British  Aluminium  Co.  Petroleum 
coke  is  first  calcined  up  to  temperatures  of  2000°  (?)  to  remove  volatile 
matter.  It  is  then  pulverised,  mixed  with  a  suitable  binding  medium, 
moulded,  and  kilned  in  a  furnace  for  five  days  at  1400°,  after  which 
the  product  is  slowly  cooled.  The  finished  blocks  have  a  section  of 
lO"  square.  For  every  pound  of  metal  produced,  0'5-0'7  Ib.  of  anode  is 
consumed,  approximately  equivalent  to  the  quantity  of  metal  formed 
according  to  the  equation  A1203  +  3C  --  >  2A1  +  SCO.  The  anodes 
are  also  themselves  directly  attacked  by  atmospheric  oxygen.  This 
burning  away  of  the  anodes  necessitates  their  continually  being 
lowered  deeper  into  the  cell.  It  is  as  far  as  possible  minimised  by 
coating  the  surface  of  the  electrode  with  some  suitable  material—  e.g. 
whitewash. 

The  current  efficiency  obtained  is  about  75-80  per  cent.,  though  it 
can  vary  considerably.  Haber  quotes  the  following  data  :— 

Voltage  5'5  volts.  Current  7520  amps. 

Yield  of  metal)    . 

24  ho        I  kilos.  Current  efficiency  71  per  cent. 

Energy  consumption)  1  H.P.  year  \ 

per  ton  metal       /  ^'W  yi.-lds      / 

The  losses  are  chiefly  due  to  the  metal-fog  phenomenon.  Anode 
and  cathode  are  only  3  cm.  apart,  and  the  aluminium  is  far  above  its 
melting-point  (665°).  There  is  therefore  a  continual  regeneration  of 
alumina  at  the  anodes.  Another  loss  that  must  not  be  overlooked 
(Haber)  is  that  due  to  short  circuits  between  anode  and  cathode. 

1  Zeitsch.  Eleklrochem.  1,  367  (1895).        •  Metall.  Ckem.  Engin.  9,  137  (lull). 


XXIIL]  ALUMINIUM  429 

This  is  to  some  extent  inevitable  with  constant  individual  adjustment 
of  the  anodes.  Our  knowledge  of  the  decomposition  voltage  of  the 
cryolite  solution  is  very  scanty.  No  reliable  experimental  data  exist. 
If  we  calculate  according  to  the  Helmholtz-Thomson  Rule,  as  Richards 
lias  done,  we  get  the  following  values  : — 

NaF  4-7  volts 

A1F3  4-0  volts 

A1203  2-8  volts. 

These  values,  though  far  from  being  absolutely  correct,  will  stand 
in  the  right  order,  and  we  thus  see  that  the  A1203  will  most  easily 
undergo  decomposition.  When  a  very  high  current  density  is  used  at 
the  cathode,  the  liberation  of  sodium  has  been  observed.  Fluorine  can 
be  formed  anodically  if  the  A1203  content  of  the  electrolyte  has  become 
very  low,  or  if  the  current  density  used  is  too  high.  In  that  case  there 
is  a  marked  '  anode  effect/  and  the  voltage  rises  considerably.  The 
correct  decomposition  voltage  of  A1203  will  be  less  than  the  above 
figure.  The  value  of  2'2  volts  suggested  by  Richards  is  probably  near 
the  correct  one,  and  we  shall  use  it  in  the  following  calculation. 
(Richards  further  says  that  it  can  be  regarded  as  2*8  volts,  the  decom- 
position voltage  of  A1203  calculated  as  above,  minus  O6  volt,  which 
corresponds  to  the  heat  of  formation  of  CO  from  electrode  and  evolved 
oxygen.  The  coincidence  is  of  course  really  very  largely  a  chance  one.) 
The  energy  efficiency  is  therefore,  assuming  a  voltage  of  six  volts — 

2'2 

75  x  -  -  =  27'5  per  cent. 
6 

The  study  of  the  production  of  aluminium  by  this  method  has  been 
the  object  of  several  laboratory  investigations.1  Haber  and  Geipert 
carried  out  three  experiments  with  currents  of  300-400  amperes, 
their  cell  consisting  of  a  thick-walled  carbon  crucible  hollowed  from 
a  carbon  block.  A  carbon  anode  was  used,  its  distance  from  the 
bottom  of  the  crucible  (0*5-1  cm.)  being  capable  of  exact  regulation. 
The  first  experiment  was  carried  out  with  1000  grams  artificial  cryolite 
containing  an  excess  of  A1F3  and  200  grams  A1203.  During  the  run 
of  five  hours.  885  grams  more  of  cryolite  and  927  grams  of  A1203  were 
gradually  added.  The  average  current  was  310  amperes.  The 
voltage  started  at  7'5  and  gradually  rose  during  the  experiment  to 
10  volts.  This  increase  was  due  to  the  progressive  consumption  of 
the  anode,  causing  a  rise  in  current  density.  The  cathodic  current 
density  was  about  3  amps. /cm.2,  the  anodic  about  10  amps. /cm.2  at 
the  start,  and  rising  continuously  during  the  run.  A  current  efficiency 
of  54  per  cent,  was  obtained.  The  current  density  being  considerably 

1  Haber  and  Geipert,  Zeitsch.  Elektrochem.  8,  1,  26  (1902).  Thompson,  Electro- 
chem.  2nd.  7,  19  (1909).  Neumann  and  Olsen,  Zeitsch.  Elektrochem.  16,  230 
(1910).  Richardson,  Trans.  Amer.  Electrochem,  Soc.  19,  159  (1911). 


430    PKINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

higher  than  that  used  technically,  the  voltage  was  accordingly  alsoj 
greater.  The  electrolyte  was,  further,  too  rich  in  A1203,  and  therefore] 
too  dense,  and  small  particles  of  aluminium  were  seen  to  rise  to  the] 
surface  and  re-oxidise.  The  second  experiment  was  very  similar.! 
The  cathodic  current  density  and  mean  voltage  were  still  higher — 
3'36  amps./cm.2  and  9*3  volts.  The  yield  was  45  per  cent.  The 
third  run  was  carried  out  with  natural  cryolite,  containing  a  smaller 
percentage  of  A1FS  and  therefore  melting  less  easily,  but  also  being 
less  volatile.  The  electrolysis  proceeded  much  as  before,  the  current! 
efficiency  being  43*5  per  cent.  The  chief  difficulty  lay  in  bringing 
the  anode  sufficiently  near  to  the  bottom  of  the  cell  without  short- 
circuiting. 

Discussing  the  above  results  in  a  later  paper,1  Haber  recommends 
5  amps./cm.2  as  the  most  suitable  anodic  current  density,  and  1  part 
alumina  :  5  parts  cryolite  as  the  best  electrolyte.  To  ensure  a  low 
voltage,  anode  and  cathode  should  be  as  close  together  as  possible. 
This,  however,  has  the  disadvantage  of  lowering  the  current  efficiency. ' 
The  alumina  used  must  be  easily  soluble,  otherwise  a  pasty  mass 
settles  down  over  the  cathode,  causing  non-uniform  current  densities, 
raising  the  voltage,  and  probably  assisting  in  the  formation  of  short 
circuits.  The  consumption  of  the  anode  carbons  was  found  to  be 
almost  equal  to  the  weight  of  aluminium  produced.  As  we  have  seen, 
it  is  far  less  in  technical  practice.  The  carbon  anodes  are  the  chief 
source  of  the  small  quantity  of  silicon  in  the  product.  Those  used 
by  Haber  and  Geipert  contained  O183  per  cent.  Si02. 

Thompson  used  cell  and  anode  of  graphite  and  a  current  of 
900  amperes.  He  could  not  satisfactorily  reproduce  the  results  of 
Haber  and  Geipert,  chiefly  because  his  current  density  was  too  high, 
10  amps./cm.2  at  the  anode,  his  normal  working  voltage  being  about 
10  volts.  The  alumina  in  the  bath  became  rapidly  exhausted,  and 
a  marked  anode  effect  followed,  due  to  the  discharge  of  F'  ions  at  a 
high  current  density.  The  voltage  rapidly  rose  and  the  electro- 
lysis was  disturbed.  The  best  current  efficiency  obtained  was  49 
per  cent. 

Neumann  and  Olsen  used  vessels  of  iron  and  hard  carbon  anodes, 
which  do  not  disintegrate  as  Acheson  graphite  does.  The  anodic 
current  density  was  lower,  about  2  amps./cm.2,  and  under  these  circum- 
stances the  electrolysis  proceeded  quietly  and  regularly.  An  '  anode 
effect '  was  rarely  noticed  unless  the  current  density  exceeded 
4  amps./cm.2  The  height  of  the  anodes  was  regulated  every  half-hour, 
and  the  electrodes  were  kept  6  cm.  or  less  apart.  The  average  voltage 
was  9  volts.  Current  efficiencies  up  to  70  per  cent,  were  obtained,  but 
more  often  60  per  cent.  Electrolysis  with  an  anodic  current  density 

1  Zeitach.  Elektrochem.  8, 


XXIIL]  THE  ACKER  PROCESS  431 

of  1  amp. /cm.2  or  less  (as  in  the  Heroult  process)  required  external 
heating.  This  is,  of  course,  due  to  the  fact  that  heat  losses  by  radiation 
are  far  greater  proportionally  with  a  laboratory  unit  than  with  a 
technical  one. 

Richardson  employed  a  cell  of  wrought-iron  plates.  The  bottom 
(cathode)  was  graphite,  the  sides  being  of  sand  and  asbestos  and 
ultimately  of  solidified  electrolyte.  The  anodic  current  density  was 
3  amps./cm.2 ;  the  voltage  varied  very  considerably,  the  mean  value 
being  10-12  volts.  He  obtained  average  current  efficiencies  of 
70-75  per  cent.,  but  they  also  showed  great  variations  (46-93  per 
cent.).  The  working  temperature  and  the  composition  of  the  electrolyte 
corresponded  closely  to  those  used  in  practice. 

The  results  of  these  investigations  point  to  the  advisability  of  using 
low  anodic  current  densities.  Not  only  are  '  anode  effects '  thereby 
excluded,  but  both  voltage  and  anode  consumption  are  less  in  the 
regular  course  of  work.  The  excessive  anode  losses  observed  by 
Haber  and  Geipert  are  directly  attributable  to  their  very  high  current 
densities.  There  can  be  little  doubt  that  the  '  anode  effect '  which 
occasionally  appears  is  not  due  to  a  layer  of  oxygen  or  oxides  of  carbon, 
but  to  one  of  fluorine  gas,  which  is  formed  owing  to  the  bath  having 
become  depleted  of  alumina.  It  thus  falls  into  line  with  all  other  anode 
effects  due  to  evolved  halogens,1  particularly  with  that  noticed  by 
Muthmann,  Hofer  and  Weiss 2  in  the  electrolysis  of  pure  molten  cryolite, 
which  was  very  marked  at  a  current  density  of  4-5  amps./cm.2 

6.   The  Acker  Process  for  Chlorine  and  Caustic  Soda 

This  process,3  the  best  conceived  and  most  successful  of  those 
involving  the  electrolysis  of  fused  NaCl,  was  worked  for  several  years 
at  Niagara  on  a  considerable  scale,  but  has  not  been  re-started  since 
the  works  were  burnt  down  some  years  back.  The  difficulties  met 
with  were  undoubtedly  enormous. 

The  fused  NaCl  was  electrolysed  in  a  cell  of  cast-iron  lined  with 
magnesia  bricks.  The  cathode  covering  the  bottom  of  the  cell  was  of 
fused  lead.  On  this  rested  a  6*  layer  of  molten  salt,  into  which  dipped 
four  graphitised  carbon  anodes,  attached  to  carbon  rods  passing 
through  the  roof  of  the  cell.  These  rods  were  coated  with  fireclay 
and  cement  to  protect  them  from  the  hot  gases.  The  lower  (active) 
surface  of  an  anode  was  6*5  dm.2,  and  at  a  current  density  of  about 
300  amps./dm.2  each  anode  took  2000  and  each  unit  8000  amperes. 
The  anodes  were  not  attacked  under  these  conditions  as  might  have 
been  expected.4  The  chlorine  was  drawn  off,  together  with  about  nine 

1  P.  164.  -  Lieb.  Ann.  320,  237  (1901). 

3  Trans.  Amer.  Electrochem.  Soc.  1,  165  (1902).  Electrochem.  Ind.  1,  54  (1902). 
Zeitsch.  Elektrochem.  9,  364  (1903).  '  Electrochem.  Ind.  4,  477  (1906). 


432    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

times  its  volume  of  air  (entering  the  cell  through  gaps  in  the  walls), 
and   converted   into    bleach.     The    cathodic    lead-sodium    alloy    left 

Steam 


FIG.  104.— Acker  Cell.     Front  Elevation. 

the  electrolysis  cell  and  entered  a  smaller  chamber  where  it  was  acted 
on  by  a  jet  of  steam  at  40  Ib.  pressure.  This  oxidised  the  sodium  to 
NaOH  and  blew  the  mixture  of  lead  and  fused  alkali  over  a  partition 


FIG.  105.— Acker  Cell.     Side  Elevation. 

into  a  third  chamber.  Here  separation  into  a  heavy  layer  of  lead 
and  a  lighter  layer  of  caustic  took  place.  The  former  returned  to  the 
electrolysis  cell  by  a  suitable  channel,  and  the  latter  continually  flowed 


XXIIL]  THE  ACKER  PROCESS  433 

off.     The  hydrogen  burnt  with  a  large  flame  at  the  mouth  of  this 
chamber. 

It  will  be  seen  that  the  steam  jet,  besides  separating  the  sodium 
from  the  lead,  also  circulated  the  latter.  By  means  of  a  horizontal 
partition  in  the  electrolysis  cell  it  was  arranged  that  the  metal  was 
always  efficiently  charged  with  sodium  between  successive  treatments 
by  the  steam.  Fig.  104  is  a  sectional  front  elevation  of  the  cell,  showing 
the  electrolysis  and  steam  chambers,  whilst  the  side  elevation  (Fig.  105) 
shows  the  steam  jet  and  the  separating  chamber. 

The  working  temperature  in  the  cell  was  about  850°,  the  melting- 
point  of  the  salt  used  being  775°  (impure).  Each  cell  absorbed 
between  6-7  volts.  The  current  efficiency  averaged  93-94  per  cent., 
rarely  fell  below  90  per  cent.,  and  often  touched  100  per  cent.  Before 
being  fed  in  through  charging  hoppers,  the  crude  salt  used  was  dried,  but 
not  otherwise  purified.  Calcium  and  magnesium  salts  present  conse- 
quently appeared  in  the  product  as  hydroxides.  At  the  temperatures 
worked  at  the  NaOH  was  always  commercially  anhydrous,  even  when 
using  excess  of  steam.  It  was  98  per  cent,  pure  after  standing  and 
packing,  the  2  per  cent,  being  mainly  ]STa2C03  and  CaO. 

Many  difficulties  were  encountered  in  working  out  this  process. 
At  one  time  bleach  could  not  be  made  satisfactorily  owing  to  a  little 
HC1  in  the  chlorine,  probably  derived  from  aqueous  vapour  entering 
the  furnace.  Large  quantities  of  chlorine  used  to  escape  into  the  air. 
The  costs  for  repairs  were  very  high.  Crusts  of  insoluble  salts  forming 
over  the  surface  of  the  lead  cathode  were  a  great  source  of  trouble. 
At  the  beginning,  too  low  current  densities  were  used  and  gave 
bad  results.  What  the  exact  reasons  were  that  decided  against  the 
restarting  of  the  process  is  not  generally  known. 

The  advantages  of  the  process  were  the  comparatively  cheap  first 
cost  of  plant  and  the  avoidance  of  all  evaporation  charges.  Against 
these  points  we  must  set  the  high  power  expenditure  and  cost  of 
repairs.  Assuming  6'5  volts  and  a  93  per  cent,  current  efficiency,  we 
calculate  that 

1  ton  NaOH  required 

1000  X  96540  X  100  X  6-5  _ 

93X40X3600 
1  H.P.  year  produced  1'39  tons  NaOH. 


Literature 

Lorenz.     Die  Elektrolyse  geschmolzener  Salze,  vol.  i. 


2   F 


CHAPTER  XXIV 
ELECTROTHERMICS    IN  THE   IRON  AND  STEEL   INDUSTRY 

General. — The  introduction  of  electrochemical  methods  into  iron  and 
steel  production  has  come  comparatively  late,  and  the  recent  great 
advances  consequently  appear  the  more  striking.  There  is  little 
doubt  that  in  the  near  future  the  electrometallurgy  of  iron  and  steel 
will  rank  with  electrolytic  copper  refining,  the  fixation  of  atmospheric 
nitrogen  by  the  electric  arc,  and  the  electrolytic  manufacture  of  potash 
and  soda  as  a  chemical  industry  of  the  first  rank. 

We  can  distinguish  three  separate  fields  in  which  electrothermic 
methods  are  employed.  They  are 

(a)  The  production  of  pig-iron  from  ore  ; 

(6)  The  production  of  refined  steel  from  pig-iron,  from  steel  of  a 
poorer  quality,  or  from  scrap ; 

(c)  The  manufacture  of  ferro-alloys,  for  use  in  steel  refining  or  in 
the  production  of  alloy  steels. 

The  historical  order  of  development  is  the  reverse  of  the  above. 
It  is  a  commonplace  how  the  rise  of  the  ferro-alloy  industry  about 
twelve  years  back  resulted  essentially  from  a  crisis  in  the  carbide 
industry,  the  works  being  compelled  to  use  their  plant  in  some 
other  way  or  else  to  shut  down.  About  the  same  time  certain 
investigators  were  experimenting  on  electric  steel  production,  their 
guiding  idea  being  the  fact  that  electric  heat  permits  of  far  better 
utilisation  than  heat  from  fuel.  The  economic  success  of  the  electro- 
thermic  ferro-alloy  industry  undoubtedly  acted  as  a  stimulus,  the 
result  being  the  rapid  development  of  electric  steel  refining  which  has 
taken  place  in  the  last  decade.  A  circumstance  which  has  already 
favoured,  and  which  will  favour  still  more  in  the  future,  the  utilisation 
of  these  methods  is  the  increasing  demands  made  by  modern  conditions 
on  all  kinds  of  steel,  whether  for  machine  or  tool  work  or  for  con- 
structional purposes.  Far  greater  strains  and  more  severe  usage 
generally  must  be  borne  than  has  hitherto  been  the  case,  and  suitable 
high  quality  materials,  whether  carbon  or  alloy  steels,  are  best  prepared 
in  the  electric  furnace.  The  subject  of  the  electrical  production  of 

434 


ELECTROTHERMAL  PIG-IRON  435 

pig-iron  has  recently  attracted  serious  attention,  but  only  in  certain 
countries  where  ore  and  water-power  are  plentiful  and  suitable  fuel  dear, 
and  it  is  unlikely  that  the  industry  will  find  a  footing  elsewhere. 

1.  Pig-iron  Production 

The  Blast  Furnace  Process. — The  essential  processes  occurring  in 
the  ordinary  blast  furnace  are  as  follows  :  The  charge  consists  of 
ore  (usually  a  hydrated  Fe203  or  magnetite  with  impurities),  slag- 
forming  materials  (exact  nature  depending  on  the  character  of  the 
ore),  and  fuel  (generally  coke).  This  fuel  serves  a  double  purpose. 
It  is  burnt  to  CO  at  the  bottom  of  the  furnace  by  air  blown  in  through 
the  tuyeres.  The  liberated  heat  liquefies  the  reduced  iron  and  the 
slag,  and  preheats  the  descending  charge,  whilst  the  CO  formed  effects 
the  reduction  of  the  ore.  In  doing  this,  it  is  partly  oxidised  to  C02, 
and  the  mixed  gases,  essentially  nitrogen,  CO,  and  C02,  leave  the  top 

CO 

of  the  furnace.     The  ratio  n  -  in  this  mixture  is  usually  about  2:1. 

C02 

The  heat  carried  by  the  gases  is  used  for  steam  raising.  Often  the 
heat  of  combustion  of  the  CO  content  is  similarly  utilised,  the  gases 
being  burnt  under  steam  boilers.  In  other  cases,  after  filtering  out 
the  dust,  they  are  burnt  in  gas  engines. 

The  Electric  Furnace  Process. — In  an  electric  pig-iron  furnace 
the  conditions  are  rather  different.  The  charge — as  before  a  mixture 
of  ore,  flux,  and  fuel — is  fed  down  over  an  electric  arc.  No  air  is 
blown  in.  The  reduction  of  the  iron  oxide  takes  place  through  the 
solid  fuel,  and  the  necessary  energy  required  to  effect  this,  together 
with  the  heat  absorbed  by  metal,  slag  and  gases,  is  provided  by  the 
current,  not  by  the  combustion  of  further  quantities  of  fuel.  As  in 
the  blast  furnace,  the  fuel  will  partially  give  CO,  partially  C02.  The 
obvious  essential  difference  between  the  two  methods  consists  therefore 
in  the  fact  that  the  energy  absorbed  ly  the  reaction 

iron  oxide  -f-  carbon  — >  iron  -f-  oxides  of  carbon 
is  supplied  in  the  one  case  by  the  combustion  of  more  fuel  and  in  the 
other  by  electrical  energy. 

Thermochemical  Comparison.— An  exact  comparison  of  the  two 
processes  is  only  possible  if  certain  specific  conditions  are  assumed. 
Such  calculations  have  been  made  by  several  authors.1  We  will  here 
treat  the  problem  on  the  broadest  lines  only.  Suppose  first  the  ore 
to  be  pure  Fe203,  and  to  be  reduced  by  carbon  with  the  formation  of 
a  gaseous  mixture  of  two  volumes  CO  :  one  volume  C02.  The  equation 
will  be  451  Q3  +  9C >  8Fe  +  6CO  +  3 


1  J.  W.  Richards,  Electrochem.  Ind.  5,  165  (1907),  and  Trans.  Amer.  Electrochem. 
Soc.  15,  53  (1909).  Harden,  Electrochem.  Ind.  7,  16  (1909) ;  Catani  and  Neumann, 
Electrochem.  Ind.  7,  153  (1909) ;  Yngstrom,  Metall.  Chem.  Engin.  8,  11  (1910). 

2  F  2 


436    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

The  approximate  heats  of  formation  at  room  temperature  are 

Fe203  201,000  Gals,  per  kilo.-mol. 
CO      29,200  Cals.  do. 

C02    97,200  Cals.  do.  ; 

and  we  calculate  that  the  formation  of  one  ton  of  iron  according  to 
the  above  equation  is  endothermic,  and  requires 

1000 

(804000  -  292000  -  175000)   £=  752000  Cals. 

448 

at  room  temperature. 

But  both  iron  and  gases  leave  the  furnace  at  a  high  temperature. 
It  is  generally  assumed  that  the  iron  would  require  300  Cals.  per  kilo. 
for  melting  and  heating  to  the  tapping  temperature.  A  ton  therefore 
needs  300,000  Cals.  Assuming  that  the  gases  leave  the  furnace  at 
800°,  and  that  CJ}  (mean  value)  between  0°  and  800°  for  CO  and  C02 
is  6*9  and  9'1  respectively,  we  calculate  that  123,000  Cals.  are  required 
per  ton  of  iron.  We  must  now  take  account  of  the  impurities  present, 
some  of  which  are  reduced  and  enter  the  iron,  but  most  of  which  are 
slagged  ofi.  We  can  suppose  100,000  Cals.  to  be  employed  in  this 
reduction  (chiefly  of  silica).  And  we  can  roughly  assume  that  750  kilos. 
of  slag  are  produced  per  ton  of  iron,  and  that  each  kilo,  carries  500  Cals. 
This  will  be  an  outside  figure,  and  we  can  neglect  radiation  losses,  etc. 
The  total  energy  which  must  be  supplied  is  therefore 

For  reduction  of  iron  752,000  Cals. 

For  heating  and  melting  iron  300,000  Cals. 

For  reduction  of  impurities  100,000  Cals. 

For  heating  and  melting  of  slag  375,000  Cals. 

Carried  by  gases  123,000  Cals. 


Total      1,650,000  Cals. 

1650000  X  4-19 

equivalent  to  -  -  =  1920  K.W.H. 

3600 

When,  however,  fuel  is  used,  far  more  must  be  added  than  would 
be  calculated  from  the  above  figure.  The  use  of  fuel  means  the  use 
of  air  to  burn  it,  and  this  involves  the  heating  up  of  enormous  quan- 
tities of  dilute  combustion  gases.  Assuming  that  this  fuel  also  gives 
a  mixture  of  2CO  :  1C02,  we  have 

3C  -f-  202  +  8N2 — >  2CO  +  C02  -f  8N2. 

Taking  Cp  for  nitrogen  as  equal  to  that  for  CO,  we  calculate 

1 1  ;it  liberated  by  C02  formation  97,200  Cals. 

Heat  liberated  by  CO  formation  2  X  29,200  Cals. 


Total          155,600  Cals. 
Heat  absorbed  by  gases  800  [9'1  -f  10(6'9)]=  62,500  Cals. 


xxiv.]  ELECTROTHERMAL   PIG-IRON  437 

Hence  36  kilos,  of  carbon  only  produce  (155,600  -  62,500)  Calories 
net.    To  get  1,650,000  Cals.  we  need 


Adding  the  240  kilos,  required  for  the  reduction  of  the  iron  and  60-70 
kilos,  for  carbonisation  of  the  iron  and  for  reduction  of  impurities,  we 
arrive  at  a  figure  (940-950  kilos.)  which  agrees  very  well  with  the 
1,000-1,100  kilos,  of  90  per  cent,  coke  required  in  modern  blast  furnace 
practice  per  ton  of  iron  produced. 

We  must  also  take  into  account  the  difference  between  the  calorific 
values  of  the  furnace  gases  in  the  two  cases.  If  we  calculate  from  the 
above  data  the  sum  of  the  heat  actually  carried  by  the  gases  from 
the  furnace  and  the  heat  which  can  be  produced  by  the  oxidation  of 
their  CO  content  to  C02,  we  arrive  at  a  value  (per  ton  iron)  of  about 
4,600,000  Cals.  for  a  blast  furnace  and  1,150,000  Cals.  for  an  electric 
furnace.  The  estimates  of  the  respective  proportions  of  these  amounts 
of  heat  which  are  available  for  power  production,  after  satisfying  all 
the  needs  of  the  furnace,  vary  very  widely.  If  we  assume,  however, 
that  the  surplus  is  greater  in  the  case  of  the  blast  furnace  by  about 
400  K.W.H.  per  ton  of  iron  produced,  we  shall  not  be  far  wrong. 
Broadly  speaking,  then,  the  electric  furnace  will  begin  to  compete  with 
the  blast  furnace  when  1,920  K.W.H.  (or  2,300  K.W.H.  if  the  furnace 
gases  be  taken  into  account)  are  cheaper  than  0*7  ton  coke.  This 
calculation  holds  for  the  given  conditions  of  ore  and  fuel  only,  assumes 
a  very  high  electric  furnace  efficiency,  and  takes  no  account  of  questions 
of  plant  or  metallurgical  advantages  and  disadvantages. 

Stassano's  Experiments.  —  Stassano  was  the  first  worker  in  this 
field  to  build  a  pig-iron  electric  furnace  of  technical  size.  He  experi- 
mented with  several  types  of  construction,  and  particularly  endeavoured 
to  produce  a  high-grade  material  (not  an  ordinary  cast-iron)  from 
the  ore  in  a  single  operation.  He  managed,  in  fact,  to  produce  a  ductile 
iron  containing  a  total  of  0'35  per  cent,  of  impurities.  But  this  was 
only  achieved  by  the  use  of  exceptionally  pure  materials  and  by  care- 
fully watching  the  operation  in  a  manner  impossible  under  technical 
conditions. 

The  Haanel  Reports.  —  In  Canada  there  are  enormous  deposits  of 
different  iron  ores,  many  very  considerable  undeveloped  water-powers, 
and  vast  quantities  of  wood  suitable  for  charcoal  making,  but  com- 
paratively little  coal.  These  circumstances  suggested  to  the  Canadian 
Department  of  Mines  that  there  would  be  a  great  future  for  electrical 
iron  ore  smelting  in  that  country  if  it  proved  at  all  practicable  techni- 
cally. The  immediate  outcome  has  been  the  valuable  series  of  reports 
compiled  by  Dr.  Haanel,  the  director  of  the  Department.  A  com- 
mission visited  Europe  in  1904  to  report  on  the  different  electro- 


438    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [OTTAP. 

thermal  processes  then  used  in  the  iron  and  steel  industry.  The  most 
important  experiments  witnessed  on  electrothermal  reduction  of  iron 
ores  were  carried  out  by  Keller  x  at  Livet.  Although  using  a  furnace 
really  designed  for  ferro-chrome  or  ferro-silicon  manufacture,2  Keller 
was  able,  by  suitable  variations  of  the  charge,  to  produce  at  will  white 
or  gray  cast-iron,  in  all  cases  exceedingly  low  in  sulphur.  His  furnace 
worked  perfectly  smoothly.  The  metal  was  further  so  hot  that  good 
castings  could  directly  be  made  when  tapping  the  furnace.  The  energy 
consumption  varied  rather  considerably.  The  average  was  about  2,290 
K.W.H.  per  ton  (2'8  tons  per  H.P.  year),  whilst  310  kilos,  of  coke  and 
17  kilos,  of  carbon  electrodes  were  also  necessary. 

An  important  series  of  experiments  was  next  carried  out  at  Sault 
Ste.  Marie  in  Canada  by  Heroult.3  Keller  had  shown  that  a  good  quality 
hematite  could  be  reduced.  It  now  remained  not  only  to  see  whether 
Keller's  results  could  be  improved  upon,  but  also  to  discover  whether 
electrical  methods  were  also  applicable  to  Canadian  conditions — to 
see,  that  is,  whether  magnetite  and  ores  rich  in  sulphur  but  poor  in 
manganese  could  be  smelted,  and  whether  charcoal  could  be  used  instead 
of  coke  as  a  fuel.  Preliminary  experiments  were  first  made  with 
different  kinds  of  furnaces.  In  one  case  compressed  air  was  blown  in 
near  the  top  in  order  to  burn  the  CO,  and  thus  preheat  the  charge,  but 
the  latter  became  sticky  and  tended  to  hang,  and  the  electrode  was 
rapidly  consumed.  The  final  type  of  furnace  was  exceedingly  simple. 
A  carbon  crucible  formed  one  electrode,  the  side  walls  being  of  firebrick, 
the  whole  enclosed  in  an  iron  casing  and  open  at  the  top.  The  second 
electrode  was  also  of  carbon,  suspended  vertically  in  the  middle  of 
the  furnace  and  adjustable  by  a  pulley.  Round  this  electrode  the 
charge  was  heaped  up.  The  furnace  took  about  5,000  amps,  at 
36  volts,  its  power  factor  being  0*92,  considerably  higher  than  that 
of  Keller's  furnace  in  Livet. 

The  results  were  very  encouraging.  Using  hematite,  the  energy 
consumption  was  1,700  K.W.H.  per  ton  of  product  (1  H.P.  year  =  3'8 
tons).  Magnetite,  with  which  difficulties  had  been  anticipated  on 
account  of  its  high  electrical  conductivity,  also  smelted  readily,  the 
energy  needed  per  ton  of  iron  being  about  1,900  K.W.H.  (1  H.P.  year 
=  3*4  tons).  From  sulphur-rich  ores,  magnetite  and  roasted  pyrrhotite, 
a  pig-iron  very  low  in  sulphur  was  readily  obtained.  The  roasted 
pyrrhotite  furnished  a  product  containing  4  per  cent.  Ni  at  an  energy 
consumption  of  2,570  K.W.H.  per  ton  (2- 6  tons  per  H.P.  year),  a  much 
more  favourable  result  than  was  obtained  by  Sjostedt 4  in  his  earlier 
experiments.  Finally,  an  ore  with  nearly  18  per  cent.  Ti02  gave  a 
product  with  only  1-1/3  per  cent.  Ti,  nearly  all  that  metal  entering 
tin-  slat.'.  The  experiments  also  proved  the  suitability  of  charcoal 

1  Electrochem.  Ind.  2,  280,  479  (1904). 

3  Trans.  F<i,<nl.  ,s'or.  2,  120  (HiM). 

*  Trans.  Amer.  Electrochem.  Soc.  5,  2:i!{  (I'.ni). 


xxiv.]  ELECTROTHERMAL  PIG-IRON  439 

as  a  reducing  agent.  The  material  employed  was  of  poor  quality, 
and  on  that  account,  and  also  because  large  quantities  burnt  away 
at  the  top  of  the  furnace,  the  actual  amount  consumed  during  the 
experiments  has  little  significance.  Haanel  calculates  O5  ton  per  ton 
of  pig-iron  under  normal  conditions,  and  8  kilos,  of  electrodes.  After 
the  conclusion  of  the  experiments  the  furnace  was  employed  in  the 
manufacture  of  ferro-nickel  pig  from  roasted  pyrrhotite.  The  smelting 
was  stated  to  proceed  very  smoothly,  but  the  power-consumption  was 
greater  (3,010  K.W.H.  per  ton)  than  that  given  by  Heroult  and 
Haanel,  and  the  electrode  consumption  far  higher. 

Lyon  Furnace. — The  furnaces  of  both  Keller  and  Heroult  were  of 
the  type  used  for  years  in  the  ferro-alloy  industry.  Investigations 
have  also  been  carried  out  with  furnaces  closely  resembling  the  ordinary 
blast  furnace  in  construction.  The  guiding  ideas  here  are  (a)  the 
collection  of  the  gases,  and  (6),  by  causing  them  to  stream  up  through 
a  long  column  of  charge,  the  utilisation  as  far  as  possible  of  their  heating 
and  reducing  properties  before  they  leave  the  furnace.  Such  a  shaft 
furnace,  designed  by  Lyon,  has  been  working  some  years  in  California.1 
The  ore  smelted  is  a  very  pure  magnetite  (68-70  per  cent.  Fe),  and 
the  reducing  agent  is  charcoal,  J  ton  per  ton  of  pig-iron.  In  general 
shape  of  hearth,  shaft,  and  position  of  electrodes,  the  furnace  resembles 
the  one  described  below  (Fig.  106).  As  to  its  exact  behaviour  we 
have  only  scanty  data.  It  is  fed  with  three-phase  current,  each  phase 
(they  are  apparently  not  linked)  having  two  electrodes,  and  taking 
10,000-21,400  amps,  at  35-75  volts,  thus  750  K.W.  The  total 
power  in  the  furnace  is  therefore  2,250  K.W.  As  the  output  per 
day  is  25  tons  (tapped  5  tons  at  a  time),  the  energy  consumption  is 

OOKA   ^    04. 

-~  -  =  2160   K.W.H.  per  ton  of  pig  (3  tons   per  H.P.  year). 

aO 

This  is  a  maximum  expenditure,  as  we  assume  here  cos  0=1.     More 
furnaces  are  being  built. 

The  Domnarfvet  Experiments. — The  question  was  simultaneously 
studied  in  Sweden,  and  the  furnace  described  below  devised  by  Gron- 
wall,  Lindblad  and  Stalhane.2  The  figures  given  refer  to  a  600  K.W. 
unit  at  Ludwiga.  The  furnace  body  itself  consisted  of  two  parts, 
crucible  and  shaft.  The  latter  varied  in  internal  diameter  at  different 
heights  (Fig.  106)  very  much  as  does  an  ordinary  blast  furnace.  The 
reason  of  this  form  of  construction  is  that  the  descending  charge 
was  then  found  not  to  fill  up  the  crucible,  but  to  lie  in  it  at  a  definite 
angle — 50°-55°  to  the  vertical — and,  by  means  of  arching  the  crucible 
roof,  a  contact  between  wall,  electrode  and  charge  was  avoided,  there 
being  always  a  ring-shaped  layer  of  gas  between  the  charge  and 
the  places  where  the  electrodes  passed  through  the  roof  ;  otherwise  the 

1  Trans.  Amer.  Electrochem.  Soc.  15,  39  (1909). 

-  Trans.  Farad.  Soc.  5,  306  (1909)  ;  Slahl  und  Eisen,  29,  1801  (1909) ;  MetalL 
Engin.  8,  11  (1910). 


440    PEINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 


temperature  became  high  enough  to  fuse  the  lining  at  those  points. 
This  lining  was  of  magnesite,  as  were  also  the  sides  of  the  crucible. 
The  shaft  was  lined  with  suitable  firebricks,  and  its  weight  supported 
on  cast-iron  pillars  (not  shown).  The  three  electrodes  consisted  each 
of  two  carbons  63"  X  11"  X  11",  the  cross-section  of  each  electrode  being 
thus  242  inch2.  They  were  clamped  in  steel  holders,  passed  through 
guides,  and  entered  the  furnace  through  water-cooled  gas-tight  stuffing 
boxes.  Regulation  was  effected  by  a  cable  and  pulley. 

The  furnace  gases  were  collected  and  utilised.     For  this  purpose  the 
furnace  was  closed  and  provided  with  special  charging  arrangements, 

Gases 


Hot  Gases  to 
Dustcatcher 
and,  Fan,. 


iCoLd  Gases 
I  fromDnstcatfher 
Y    and  Fan. 


FIG.  106. — Domnarfvet  Pig-iron  Furnace. 

designed  to  avoid  any  possibility  of  an  explosion  on  charging.  Finally, 
an  important  feature  was  the  means  used  to  cool  the  roof  of  the  crucible. 
A  portion  of  the  gas  leaving  the  furnace  throat  was  sucked  off  into  a 
dust-catcher  by  means  of  a  fan,  and  thence  forced  down  into  the 
crucible  by  three  pipes  or  tuyeres  (A).  It  entered  at  about  200°, 
abstracted  much  heat  from  the  roof  and  the  upper  surface  of  the  ore, 
gave  it  up  to  the  descending  charge,  and  was  very  effective  in  pro- 
longing the  life  of  the  roof.  The  other  exit  for  the  furnace  gases  worked 
automatically  when  the  pressure  reached  a  certain  figure.  Three- 
phase  current  was  used.  Cos  6  at  25  cycles  was  0*8-0*9,  at  60  cycles 
0*7,  and  the  furnace  voltage  was  about  40  volts.  During  the  tests 
described  by  Haanel,  the  current  was  very  low,  the  furnace  being 
worked  at  far  below  its  full  load. 


XX IV.  I 


ELECTROTHERMAL  PIG-IRON 


441 


Using  a  magnetite  containing  60-63  per  cent,  iron,  the  energy 
consumption  with  this  furnace  averaged  3180  K.W.H.  per  ton  of 
product  (2'1  tons  per  H.P.  year).  The  carbon  consumption  was  low, 
the  equivalent  in  coke  or  charcoal  of  about  280  kilos,  of  pure  carbon 
per  ton  of  iron.  About  8  kilos,  of  electrodes  were  burnt  per  ton  of 
iron,  the  total  consumption  amounting  to  30  kilos,  per  ton.  The 
furnace  ran  exceedingly  quietly,  and  the  electrodes  needed  no  adjust- 
ment over  long  periods.  The  pig-iron  obtained  had  occasionally  1  per 
cent,  or  less  of  carbon,  but  generally  2-3  per  cent.  The  sulphur  was 
exceedingly  low  ;  all  the  phosphorus  entered  the  iron.  The  results  of 
these  trials  were  regarded  as  satisfactory,  and  a  2000  K.W.  furnace  of 
essentially  the  same  construction  was  installed  at  Trollhattan.  This 
furnace  has  run  for  six  months  uninterruptedly,  yielding  much  im- 
proved results.1  It  takes  two-phase  current,  the  phase  voltage  varying 
between  50-90  volts,  and  is  provided  with  four  electrodes,  each  2  metres 
long  and  66  X  66  cm.  in!  cross-section.  These  have  an  average  life 
of  750  working  hours,  and  the  total  electrode  consumption  per  ton  of 
iron  (burnt  and  scrapped)  amounts  to  only  10  kilos.  The  proportion  of 
charcoal  required  is  greater  than  with  the  smaller  Ludwiga  furnace, 
but  the  average  energy  expenditure  per  ton  of  iron  has  fallen  to 
2390  K.W.H.  (2-7  tons  per  H.P.  year). 

The  following  table  contains  a  summary  of  the  results  obtained  in 
this  field.  The  great  differences  apparent  are  largely  to  be  ascribed 
to  differences  in  composition  of  charge,  etc.,  etc.  The  superiority  of 
the  Heroult-Haanel  results  (short  runs)  over  all  others  is  nevertheless 
remarkable. 

TABLE  LXVI 


Furnace 

Load  of 
same 

Ore  used 

K.W.H. 
per  ton 

Tons 
per 
H.P.  year 

Keller 

400-700  K.W. 

Hematite,  —  69    per 

cent.  Fe 

2290 

2-8 

Heroult 

170  K.W. 

Hematite,  —  62  per 

cent.  Fe 

1700 

3-8 

Do. 

Do. 

Magnetite,  55-59  per 

cent.  Fe 

1900 

3-4 

Do. 

Do. 

Pyrrhotite,    46    per 

cent.   Fe  -f  2  per 

cent.  Ni                        2560-3010 

2-2-2-6 

Lyon 

2250  K.W. 

Magnetite,  68-70  per  ! 

cent.  Fe                            2160                3'0 

Ludwiga 

600  K.W. 

Magnetite,  60-63  per 

cent.  Fe                             3180                 2'1 

Trollhattan 

2000  K.W. 

Magnetite,  50-67  per 

cent.  Fe 

2390 

2-7 

Zeitsch.  Elektrochem.17,  649  (1911). 


442    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

Questions  of  fuel  and  energy  apart,  electrothermal  methods  of 
pig-iron  production  possess  certain  advantages  dependent  on  the 
high  temperature  of  reduction.  Firstly,  refractory  ores  can  be  treated 
which  will  not  satisfactorily  yield  to  ordinary  metallurgical  methods. 
The  possibility  of  thus  producing  a  good  pig-iron  from  titaniferous 
ores  has  been  repeatedly  shown.1  Secondly,  a  metal  very  low  in 
sulphur  is  made  as  the  result  of  a  simple  operation.  It  can  be  run 
directly  into  an  electric  steel  furnace  and  refined  under  exceptionally 
favourable  conditions.  In  spite  of  these  facts,  however,  it  is  improbable 
that  this  industry  will  gain  a  footing  in  many  countries.  Blast-furnace 
reduction  will  be  cheaper. 

2.  Electric  Steel.     General 

The  first  workers  on  electric  steel  were  chiefly  concerned  with 
making  a  product  to  compete  with  crucible  steel.  Because  of  the 
high  costs  of  fuel  and  labour  in  this  latter  process,  and  because  of  the 
short  life  of  the  small  units  necessarily  used,  the  field  was  essentially 
promising,  and  the  electric  furnace  has  achieved  a  very  large  measure 
of  success  therein.  It  has,  in  the  last  three  or  four  years,  commenced 
to  invade  other  fields,  and  is  likely  to  be  largely  used  in  the  future  for  the 
production  of  a  metal  suitable  for  rail  steel,  etc. 

The  various  methods  of  working  which  have  been  or  are  more  or 
less  used  can  be  classified  as  follows  : — 

(a)  Starting    material   pig-iron.     Charged    cold    or   in   the    fused 
condition  fr6m  blast  furnace  or  electric  furnace.     Then  treated  with 
fluxes  and  either  with  iron  oxide  (ore),  by  means  of  which  the  carbon 
is  oxidised,  or  with  scrap  low-carbon  steel  or  low-carbon  iron,  or  with 
a  mixture  of  the  same.     Product  similar  to  open-hearth  steel. 

(b)  Starting  material  steel  from  an  ordinary  Bessemer  or  acid 
converter  and  fluxes.      Refined  to   steel  of  open-hearth  quality,  or 
further  to  crucible -steel  quality. 

(c)  Starting    material     open-hearth     (Siemens-Martin,    Wellman) 
steel  and  fluxes.     Refined  to  a  crucible  steel. 

(d)  Starting  material    scrap  steel  or  low-carbon  iron  and  fluxes. 
Refined  to  a  crucible  steel. 

A  '  crucible  steel '  can,  of  course,  be  either  a  carbon  or  an  alloy 
steel.  So  far  (c)  and  (d)  have  been  most  frequently  used  ;  (a)  and  (b)  not 
to  such  an  extent.  The  United  States  Steel  Corporation  has  refined 
acid  converter  steel  in  the  Heroult  furnace,  the  Kjellin  furnace  has 
been  employed  to  produce  high-grade  steel  from  pig-iron  and  scrap 
or  pig-iron  and  briquetted  ore,  and  it  is  intended  to  use  electric  refining 

1  Sault  Ste.   Marie  experiments.     Also   Klectr.   63,   934   ( W09)  ;  Gin,   Trans. 

Avn.tr.  Electrochem.  Soc.  11,  291  (UW7);    Greene  and  Macgregor,   Tmn*.  Amer. 

-,.  Soc.  12,  r,r> 


xxiv.]  ELECTRIC  STEEL  443 

in  combination  with  the  new  electric  iron  furnaces  to  be  erected  in 
Sweden. 

Thermochemical  Relations. — The  theoretical  minimum  of  energy 
needed  for  the  formation  of  a  ton  of  steel  under  various  given  conditions 
has  been  calculated  by  Neumann,1  using  recently  revised  values  for 
the  specific  and  latent  heats  of  iron.  With  a  cold  charge  it  is  the  sum 
"of  the  heat  required  to  heat  the  iron  to  its  melting-point,  to  melt  it, 
to  heat  the  liquid  metal  to  the  tapping  temperature  and  to  reduce  any 
iron  oxide  present.  From  this  is  subtracted  the  heat  gained  by  the 
combustion  of  the  carbon,  silicon,  etc.,  etc.,  which  are  removed.  With 
iron  or  steel  charged  molten,  the  energy  required  is  correspondingly 
less.  The  values  given  take  no  account  of  heat  lost  during  the  process 
by  radiation  and  other  causes.  Neumann  assumes  that  a  pig-iron 
containing  3*6  per  cent.  C,  1'68  per  cent.  Si,  I'l  per  cent.  Mn,  0*62 
per  cent.  P  is  refined  to  a  product  with  O96  per  cent.  C  and  0*28 
per  cent.  Si,  and  calculates — 

500  K.W.H.  per  ton  of  steel  using  cold  pig-iron  and  iron  ore. 

190  K.W.H.  per  ton  of  steel  using  liquid  pig-iron  and  iron  ore. 

460  K.W.H.  per  ton  of  steel  using  670  kilos,  pig-iron  ;    210  kilos,  ore  ;    45  kilos. 

lime  ;   285  kilos,  scrap — all  charged  cold. 

230  K.W.H.  per  ton  of  steel  using  the  same  charge,  the  pig-iron  being  molten. 
280  K.W.H.  per  ton  of  steel  using  675  kilos,  pig-iron  ;    350  kilos,   scrap — both 

charged  cold. 

53  K.W.H.  per  ton  of  steel  using  the  same  charge,  the  pig-iron  being  molten. 
330  K.W.H.  per  ton  of  steel  using  365  kilos,  pig ;   650  kilos,  scrap — charged  cold. 
210  K.W.H.  per  ton  of  steel  using  the  same  charge,  the  pig  being  molten. 

For  the  further  refining  of  open-hearth  steel,  he  gives  the  theoretical  energy 
consumption  as  77  K.W.H.  per  ton. 

Mechanism  of  Refining  Processes.2 — The  actual  mechanism  of 
electrical  steel  refining  is  interesting.  The  raw  material  contains 
excess  of  carbon,  sulphur,  and  phosphorus,  which  must  all  be  eliminated. 
The  metal  is  first  submitted  to  the  action  of  an  oxidising  and  basic 
slag,  either  in  an  electric  furnace  or  in  an  open-hearth  furnace,  before 
charging  into  the  former.  This  slag  generally  contains  Fe203,  which 
oxidises  the  carbon  to  CO  and  the  phosphorus  to  P205,  the  latter 
entering  the  slag.  The  metal,  now  low  in  carbon  and  phosphorus,  still 
contains  most  of  its  sulphur  and  also  dissolved  oxides  (chiefly  iron). 
This  sulphur  is  present  as  sulphides  of  manganese  and  iron,  in  equili- 
brium with  the  same  sulphides  dissolved  in  the  slag.  The  ratio 

S  in  metal 

depends  essentially  on  the  temperature  and  basicity  of  the  slag,  becoming 
greater  as  these  two  factors  are  increased.     The  sulphur  can  only  be 

1  Article  in  Askenasy's  Einftihrung  in  die  technische  Ehktrochemie,  vol.  i. 
(1910). 

-  Geilenkirchen  and  Osann,  Electrochem.  Ind.  6,  405  (1908).  Amberg, 
Electrochem.  Ind.  7,  115  (1909). 


444    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

completely  removed  from  the  steel  by  converting  it  into  a  sulphide 
insoluble  in  the  latter.     CaS  is  the  only  one  we  need  consider.     But  as] 
long  as  oxides  of  iron  or  other  oxidising  agents  are  present,  formation 
of  CaS  is  impossible.     Either  by  direct  interaction  with  the  iron  oxides, 
or  else  by  oxidation  to  CaS04,  and  subsequent  reduction  by  metallic 
iron,  the  final  result  is  that  CaO  and  FeS  are  produced,  the  latter; 
dissolving  in  the  steel. 

All  oxides  of  iron  and  manganese  present,  both  in  slag  and  in 
metal,  must  therefore  first  be  reduced.  The  slag  used  for  the  purpose  is 
made  as  basic  as  possible,  and  contains  as  reducing  agent  carbon 
and  often  ferro-silicon.  When  these  oxides  are  removed,  the  metal 
becoming  at  the  same  time  partly  recarburised,  the  formation  of  CaS 
commences,  e.g.  thus  : — 

2CaO  +  2FeS  +  Fea.Si  — >  Si02  +  2CaS  -f  (2  -f  z)Fe. 

The  product  is  a  steel  containing^ery  low  percentages  of  phosphorus 
and  sulphur  and  merely  traces  of  dissolved  oxides  and  gases,  the 
last  having  full  opportunity  to  escape  at  the  high  temperature  at 
which  the  metal  is '  killed/  Lastly,  the  steel  can  be  further  recarburised 
or  a  ferro-alloy  added.  The  conditions  essentially  making  for  satis- 
factory desulphurisation  in  the  electric  furnace  are  the  neutral  or 
reducing  atmosphere  and  the  high  temperature,  which  allows  of  very 
basic  slags  and  rapid  working.  The  frequent  occurrence  of  CaC2  in 
the  slags  of  electric  steel  furnaces  (particularly  the  Heroult  type) 
shows  that  both  these  conditions  are  fulfilled.  It  is  a  mistake  to 
assume,  as  has  been  done,  that  CaC2  actually  effects  the  deoxidation. 
When  added,  it  has  no  marked  action.  Its  formation  simply  witnesses 
to  the  reducing  atmosphere  and  high  temperature. 

The  technical  furnaces  at  present  used  in  steel  refining  are  designed 
on  one  of  two  distinct  principles.  They  are  arc  furnaces  or  induction 
furnaces.  The  latter  can  also  be  regarded  as  resistance  furnaces, 
as  the  heating  current  is  produced  in  and  flows  through  the  steel 
bath  itself.  Attempts  made  to  construct  resistance  furnaces  on  any 
other  principle  have  proved  futile  (e.g.  those  of  the  French  olcotro- 
metallurgist  Gin). 

3.  Arc  Furnaces 

In  furnaces  of  this  type  the  heating  of  the  charge  is  effected  by 
radiation  from  an  arc  or  arcs  playing  immediately  above  the  bath. 
These  arcs  can  be  arranged  in  several  ways.  We  need  here  only 
consider  in  detail  three  arc  furnaces,  those  of  Stassano,  Heroult,  and 
Girod  (Fig.  107).  In  the  Stassano  furnace  (the  simplest  form)  the 
arc  passes  between  two  almost  horizontal  electrodes.  The  Heroult 
furnace  has  two  vertical  electrodes  and  two  arcs,  passing  between 
the  electrodes  and  the  surface  of  tin-  }>;tih.  Later  types  have  more 


XXIV.] 


ELECTRIC    STEEL 


445 


arcs,  but  these  are  arranged  on  the  same  principle.  In  the  Girod  furnace 
the  arc  (or  arcs)  plays  between  one  or  more  vertical  electrodes  and 
the  surface  of  the  slag,  the  current  then  passing  through  the  steel  bath 
and  out  through  the  bottom  of  the  furnace. 


Stcussano. 


Girod. 


FIG.  107. 


Stassano  Furnace.1 — This  furnace  has  assumed  several  forms.  The 
fixed  type,  as  existing  in  the  Stassano  Steel  Works  at  Turin,  consists  of 
a  thick-walled  rectangular  chamber  with  a  slightly  arched  roof.  Three 
pairs  of  electrodes  are  introduced  through  the  two  longer  opposite 


FIG.  108.— Stassano  Steel  Furnace. 

sides.  The  raw  material  is  charged  in  at  the  two  ends.  There  are 
suitable  tapping-holes  for  slag  and  steel,  and  the  gas  outlet  is  water- 
sealed. 

The  most  usual  type  (Fig.  108)  is,  however,  different.     It  is  circular 

1  Elcdrochem.  Ind.  6,  315  (1908).     Stahl  und  Eiscn2S,   657  (1908).     Trans. 
Amer.  Eleclrochem.  Soc.  15,  63  (1909). 


446    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

in  plan,  and  is  not  fixed,  but  slowly  rotates  about  an  axis  inclined] 
7°  from  the  vertical.  The  melting  chamber  is  lined  with  dolomite,  the] 
domed  roof  with  magnesite  bricks.  These  are  surrounded  by  a  thick] 
layer  of  material  of  low  conductivity,  the  whole  being  enclosed  in  a] 
steel  shell.  The  electrodes,  which  are  of  small  cross-section  and 
symmetrically  arranged  round  the  furnace,  usually  number  three, 
are  fed  with  three-phase  alternating  current,  and  connected  ^-wise. 
Each  is  fastened  to  a  metal  rod  which  works  through  the  head  of  a] 
water-cooled  cylinder,  and  is  hand-regulated  by  a  special  hydraulic] 
arrangement  (not  shown  in  figure).  No  part  of  the  electrode  is  exposed  i 
to  the  air.  As  the  gas  outlet l  in  the  roof  is  water-sealed,  it  follows 
that  the  atmosphere  in  the  furnace  is  strongly  reducing.  Under  these! 
circumstances  the  electrode  consumption  is  small,  and  far  higher  current! 
densities  can  be  used  than  would  otherwise  be  possible.  Stassano  can! 
work  with  20  amps. /cm.2  or  more,  5-7  amps./cm.2  being  the  normal] 
figure.  Whether  so  to  work  is  really  advantageous  is  doubtful,  judging; 
from  the  considerations  on  pp.  175-179.  The  heat  losses  due  to  the 
electrodes  are  in  all  probability  very  high  in  this  furnace. 

There  is  a  closed  charging  hopper  and  a  tapping-hole,  which  serves 
to  withdraw  either  slag  or  steel,  depending  on  the  position  to  which ! 
it  has  been  brought  by  the  rotation  of  the  furnace.     The  purpose  of  j 
this  slow  rotation  is  to  effect  good  contact  between  steel  and  slag. 
Full  charges  yielding  invariably  homogeneous  products  are  thereby' 
made  possible,  and  this  possibility,  together  with  the  exclusion  of  all 
air,  is  claimed  by  Stassano  as  a  special  feature  of  his  furnace.-     The 
fixed  type  of  furnace  has  been  designed  up  to  a  capacity  of  750  K.W. 
The  rotating  furnace  has  so  far  only  been  built  for  loads  up  to  200  K.W. 
A  140-K.W.  furnace   using  three-phase    current  (^  connections)  at 
80  volts  takes  about  1100  amperes  per  phase.     A  200-K.W.  furnace  will 
be  fed  with  about  the  same  current  at  110  volts.     Stassano,  however, 
has  designed  a  600-K.W.  furnace  for  monophase  current.     There  will 
be  four  electrodes  and  two  arcs,  each  taking  2400  amps,  at  about  j 
150   volts.     These    voltages,  higher  than    those  usual    in  other  ;nr 
furnaces,  are  due  to  the  long  arcs  Stassano  employs,  and  these  arc  in 
turn  rendered  possible  by  the  quiet  manner  in  which  the  arcs  burn,  j 
with  a  minimum  of   electrode  consumption,  disturbance,  etc.     The 
power  factor  of  the  furnaces  can  rise  to  0'95,  but  is  generally  lower. 

Practically  all  the  figures  given  for  this  furnace  refer  to  the  pro- 
duction of  steel  castings,  tool  steels,  or  special  steels,  starting  with 
scrap  steel  and  soft  wrought-iron.  The  theoretical  minimum  quantity 
of  energy  necessary  is  370-400  K.W.H.  per  ton  of  steel  produced. 

1  Present  in  earlier  furnaces,  as  they  were  also  intended  for  the  reduction  of 
iron  ores ;  in  recent  furnaces  entirely  dispensed  with. 

•  It,  however,  appears  that  the  latest  furnaces  are  no  longer  capable  of 
rotation,  but  are  tiltc-1. 


XXIV.] 


ELECTRIC  STEEL 


447 


Stassano  uses  700-900  K.W.H.  in  his  750-K.W.  (5  ton)  furnaces, 
800-1000  K.W.H.  in  his  200-K.W.  (1  ton)  furnaces,  and  1000-1300 
K.W.H.  in  the  150-K.W.  (0'7  ton)  furnaces.  The  loss  of  metal  is 
very  small,  averaging  2  per  cent.  The  electrode  consumption  is  about 
10  kilos,  per  ton.  Owing  to  the  position  of  the  arcs,  roof  and  walls 
are  more  exposed  than  in  other  furnaces  to  their  action.  The  hearth 
must  be  renewed  every  3-4  weeks,  the  roof  every  4-6  weeks.  The 
furnace  itself  works  very  smoothly  and  regularly.  According  to 
Stassano,  the  various  complications  (rotation,  method  of  cooling  and 
regulating  electrodes,  method  of  attaching  cables)  are  no  serious 
disadvantage.  The  labour  required  is,  however,  disitnctly  higher 
than  with  other  furnaces.1 

Heroult  Furnace.2— This  furnace  (Fig.  109),  so  far  more  widely 
adopted  than  any  other  type,  is  of  comparatively  simple  construction. 
It  consists  essentially  of  a  shallow 
hearth,  lined  with  calcined  dolo- 
mite or  magnesite,  and  roofed 
with  silica  bricks.  This  hearth 
slopes  up  in  front  towards  a  lip, 
through  which  the  liquid  steel 
can  be  discharged  by  tilting  the 
furnace.  This  melting-chamber 
is  surrounded  by  poorly  conduct- 
ing material,  and  the  whole  en- 
closed in  a  casing  of  sheet  steel. 
The  electrodes,  two  or  three  in 
number,  enter  vertically  through 
openings  in  the  roof,  and  project 
down  to  within  one  or  two  inches 
of  the  surface  of  the  bath.  They 


FIG.  109.     Heroult  Steel  Furnace. 


are  water-cooled  at  the  points  where  they  pass  through  the  roof, 
as  well  as  at  the  cable  connections.  Regulation  is  automatically 
effected,  utilising  the  voltage  fluctuations.  Towards  the  end  of  the 
refining,  when  conditions  have  become  constant,  a  tight  joint  is  made 
between  electrodes  and  roof.  In  most  cases,  charging  hoppers  are 
provided  at  the  ends  of  the  furnaces.  The  charging  otherwise  takes 
place  through  the  tapping-opening.  The  atmosphere  in  the  furnace 
is  strongly  reducing,  as  is  shown  by  the  formation  of  CaC2  in  the  slag. 

1  At  Bonn,  where  two  1-ton  rotating  furnaces  and  a  2-ton  tilting  furnace 
are  working,  the  metal  roof  to  the  furnace  is  dispensed  with,  the  magnesite  bricks 
being  merely  covered  with  a  layer  of  sand.  A  run  of  about  two  weeks  is  usual 
before  the  hearth  needs  to  be  replaced.  At  variance  with  a  statement  made 
on  p.  446,  the  arcs  in  these  furnaces  are  not  particularly  long.  (June  1912.) 

-  Electrochem.  Ind.  5,  272  (1907).  Trans.  Amer.  Electrochem.  Soc.  15, 139  (1909). 
For  15-ton  furnaces,  Zeitech.  Elektrochem.  16,  853  (1910).  Metall.  Chem.  Engin. 
8,  179  (1910).  Trans.  Amer.  Electrochem.  Soc.  19,  205  (1911). 


448    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP 

These  furnaces  have  at  present  been  built  up  to  a  capacity  o 
15  tons ;  20-ton  units  are  being  constructed,  and  30-ton  furnaces  ar< 
projected.     In  the  smaller  furnaces,  two  electrodes  are  used,  fed  wit! 
monophase  alternating  current.     Thus  a  3-ton  furnace  takes  4000  amps 
at  100-120  volts.     The  larger  15-ton  furnaces  are  supplied  with  three 
phase  current,  and  have  three  electrodes  (section  2'  X  2')  arranged  ii 
star.     At  South  Chicago  each  arc  is  fed  by  means  of  a  750  K.W 
transformer.     The  maximum  load  it  takes  varies  between  16,000  amps 
at  45  volts  and  12,000  amps,  at  60  volts,  corresponding  in  the  latten 
case  to  a  furnace  voltage  of  100  volts.     Such  a  furnace  consumes 
2000  K.W.  as  a  maximum,  considerably  less  during  a  large  part  of 
the  heat.     Cos  0  varies,  according  to  the  stage  of  the  process,  betweed 
0'82-0'9.     Electrodes  of  both  carbon  (current  density  4-5  amps. /cm.1] 
and  graphite  (current  density  15-16  amps./cm.2)  have  been  used.     The] 
former  are  naturally  cheaper,  but  liable  to  breakages  when  made  id 
such  large  sizes. 

The  Heroult  furnace  has  been  used  for  making  steel  from  scrap,  for] 
refining  open-hearth  steel,  and  for  making  steel  of  open-hearth  quality] 
from  converter  steel.  The  following  table  contains  the  average  con| 
sumption  of  energy  per  ton  of  product  with  furnaces  of  different! 
sizes,  working  with  hot  and  cold  charges  : 

TABLE  LXVH 


Furnace 
1-ton  (250  K.W.) 
3-ton  (400   do.    ) 
5-ton  (700    do.    ) 
15-ton  (2000  do.     ) 

Molten  steel 
400  K.W.H. 
250-300      do. 
180     do. 
150     do. 

Scrap 
1000  K.W.H. 
800     do. 
700     do. 
<700     do. 

These  values  can  fluctuate  very  considerably,  according  to  the 
character  of  the  charge  and  the  exact  degree  of  refining  aimed  atJ 
The  electrode  consumption  also  varies  considerably.  If  the  charge] 
is  cold  and  the  operation  requires  a  long  time,  it  is  high  (e.g.  30  kilos. 
per  ton),  far  greater  than  in  the  Stassano  furnace.  But  with  a  molten] 
charge,  rapidly  refined,  and  with  the  furnace  atmosphere  of  a  reducinffl 
nature,  after  the  first  few  minutes  the  consumption  is  only  5-7  kilos. 
per  ton.  The  life  of  the  lining  depends  also  on  the  nature  of  the] 
refining  operation.  With  the  smaller  furnaces,  the  roof  needs  monthly] 
renewal  only,  and  the  hearth  lasts  three  months  or  longer.  But  with 
the  15-ton  furnaces,  in  which  12-16  heats  are  worked  per  day,  it  is 
necessary  to  repair  the  roof  weekly  and  to  attend  to  bad  spots  on  t  li<> 
hearth  between  each  heat.  The  loss  of  metal  is  about  6  per  cent, 
with  a  cold  and  2-3  per  cent,  with  a  molten  charge.1 

1  Most  of  the  new  furnaces  now  designed,  of  capacity  exceeding  3  tons, 
are  circular  in  shape  and  fed  with  three-phase  current.  The  largest  of  these  (in 
fact  the  largest  electric  steel  furnace  so  far  existing)  has  been  recently  installed 
in  the  Deutscher  Kaiser  Works  (Bruckhausen).  It  is  about  18'  in  diameter  and 
has  a  capacity  of  25  tons.  (June  1012.) 


xxiv.]  ELECTRIC  STEEL  449 

The  Keller  furnace  (I)  needs  no  particular  description  here,  as  it 
does  not  essentially  differ  from  the  Heroult.  An  8-ton  unit  has  been 
working  since  1905  in  France,  at  Unieux. 

Girod  Furnace.1 — This  furnace  is  shown  in  Fig.  110.  In  its  non- 
electrical parts  (lining  of  hearth  and  roof,  charging  and  discharging 
arrangements,  etc.)  it  resembles  the  Heroult  furnace.  The  electrode 
arrangement,  however,  is  essentially  different.  Through  the  roof  pass 
electrodes  of  one  polarity  (sic)  only,  and  the  current,  supposing  it 
to  have  thus  entered  the  furnace, 
passes  by  means  of  an  arc  or  arcs 
to  the  slag,  through  the  steel  bath, 
and  finally  leaves  by  a  number  of 
steel  electrodes  let  in  through  the 
hearth.  Thus,  instead  of  having 
two  arcs  in  series  as  in  the  Heroult 
furnace,  there  is  only  one,  or  (with 
more  than  one  roof  electrode)  seve- 
ral arcs  in  parallel.  The  voltage 
of  the  Girod  furnace  is  therefore 

approximately    half    that    of    the         FlG>  no._GiroTsteel  Furnace. 
Heroult  furnace,  and,  with  furnaces 

of  equal  load,  the  current  necessary  is  twice  as  great.  Two-ton 
furnaces  (300  K.W.)  have  one  carbon  and  six  steel  electrodes;  a 
12-15  ton  furnace  (1200  K.W.)  requires  four  carbon  and  sixteen 
steel  electrodes.  The  depth  of  the  steel  bath  is  about  25-30  cm. 
In  addition,  the  steel  electrodes  melt  down  to  a  depth  of  about 
5-10  cm.  When  tapping,  this  molten  part  of  course  runs  out,  but 
is  replaced  by  the  next  charge.  Both  sets  of  electrodes,  carbon  and 
steel,  are  water- jacketed,  and  the  carbon  electrodes  automatically 
regulated. 

Girod  furnaces  have  been  built  up  to  capacities  of  fifteen  tons. 
Monophase  alternating  current  is  employed.  The  voltage  varies 
between  55-75  volts.  A  300-K.W.  unit  will  take  5000-5500  amps,  at 
65-75  volts.  Its  power  factor  is  0-6-0-9.  A  1200-K.W.  unit  works 
with  20,000  amps,  at  70-75  volts.  The  current  density  in  the  carbon 
electrodes  is  about  5  amps./cm.2  These  furnaces  have  been  chiefly 
worked  with  cold  mixed  charges  of  various  kinds  of  scrap.  Girod 
claims  that  for  such  charges  his  furnace  is  undoubtedly  the  best,  on 
account  of  the  way  the  current  is  compelled  to  pass  through  the  metal 
to  be  melted.  About  800-1000  K.W.H.  are  required  with  a  2-2'5-ton 
furnace  for  1  ton  of  molten  steel,  depending  on  the  nature  of  charge 
and  finished  product.  With  a  15-ton  furnace,  700-800  K.W.H.  are 

1  Electrochem.  2nd.  6,  428  (1908).  Trans.  Amer.  Electrochem.  Soc.  15,  127 
(1909).  Metall.  6,  673  (1909). 

2  0 


450    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 


FIG.  111.— Nathusius 
Furnace. 


needed.  If  liquid  steel  is  charged  in,  the  energy  consumption  is, 
250-350  K.W.H.  per  ton,  depending  on  circumstances.  The  electrode 
consumption  is  from  13-18  kilos,  per  ton  with  a  cold  charge.  The 
metal  losses  are  6-7  per  cent.  The  roof  of  the  furnace  requires  repairs 
every  one  to  two  weeks.  The  rest  of  the  lining  is  far  more  resistive. 

Keller  Furnace  (II). — Keller  has  also  designed  a  successful  furnace 
resembling  that  of  Girod  in  that  it  has  hearth  electrodes.  They  consist 
of  a  large  number  of  V  iron  bars  attached  to  a  metal  plate  connected 
with  the  source  of  current.  The  spaces  between  the  bars  are  filled 
up  with  magnesia,  and  the  whole  conducting  hearth  is  water-cooled. 

Nathusius  Furnace. — This  furnace l  (Fig.  Ill)  should  also  be  briefly 
noticed.  Heat  is  here  generated  electrically  (from  three-phase  current) 
in  two  different  ways.  Exactly  as  in  the 
Heroult  furnace,  arcs  pass  between  the  steel 
bath  and  roof  electrodes.  But  there  are  also 
three  steel  hearth  electrodes  which  do  not, 
as  in  the  Girod  furnace,  make  contact  with 
the  steel  bath,  but  are  embedded  in  the  \ 
material  of  the  hearth.  Currents  pass  between 
these  electrodes  through  this  hearth  material, 
and  the  latter,  which  conducts  badly  at  low  i 
temperatures,  but  better  at  higher  ones,  is 
thus  heated  up.  In  this  way  the  bath  is 
heated  from  both  above  and  below.  We  shall  meet  with  this  type 
of  electrode  arrangement  again  in  the  Rochling-Rodenhauser  furnace,2 
when  it  will  be  more  fully  discussed.  A  5-ton  Nathusius  furnace  . 
apparently  consumes  550  K.W.  at  100  volts  in  its  upper  electrode 
system,  and  150  K.W.  at  a  higher  voltage  in  its  hearth  electrode  system. 
Electrodes  are  connected  in  star.  With  a  liquid  charge  of  converter 
steel,  180-300  K.W.H.  per  ton  are  required  for  a  very  soft  sled 
(0-05  per  cent.  C)  as  product.  With  a  solid  charge,  640-700  K.W.H. 
are  needed.  The  electrode  consumption  is  said  to  be  only  2  kilos,  per 
ton  for  a  liquid,  4  kilos,  per  ton  for  a  solid  charge. 

4.  Induction  Furnaces 

In  these  furnaces3  an  entirely  different  principle  of  heating  is 

dopted,  whereby  the  use  of  electrodes  is  avoided,  together  with  the 

•,ompanying  electrical  and  material  losses  and  the  possible  contamina- 

i  of  the  steel  by  pieces  of  carbon  entering  the  bath.     That  there  are 

esponding  disadvantages  we  shall  see  later.     Briefly  defined,  an 

detail.  Chem.  Engin.  9,  489  (Mil). 
se  p.  456s 

'tctrochem.  Ind.  6,  438  (1908).     Engelhardt,  Electrochem.  Ind.  6,  143   (1908). 
Electrochem.  Ind.  7,  478  (1909).     Hutton,  Engin.  82,  779 


xxiv.]  ELECTRIC  STEEL  451 

induction  furnace  is  a  step-down  transformer  with  short-circuited 
secondary.  A  transformer  is  a  machine  by  which  a  high-voltage 
alternating  current  can  be  changed  into  a  low-voltage  current  (step- 
down  transformer)  or  vice  versa  (step-up  transformer),  and  is  based 
on  the  following  principles. 

The  Transformer. — A  current,  direct  or  alternating,  passing  through 
a  coil  of  wire  (primary),  produces  a  magnetic  field  in  its  neighbourhood. 
If  a  soft  iron  core  be  pushed  into  the  coil,  the  magnetic  field  is  con- 
centrated in  this  core.  If  alternating  current  be  used,  we  have  an 
alternating  magnetic  field  in  the  core.  Suppose  now  a  second  (closed) 
coil  of  wire  be  wound  round  the  core.  The  turns  composing  this 
secondary  winding  cut  a  continually  changing  magnetic  flux.  It 
follows  that  an  E.M.F.  is  induced  in  the  coil,  and  as  the  flux  changes 
in  sign  as  well  as  in  magnitude,  this  E.M.F.  is  an  alternating  E.M.F. 
The  ratio  of  the  primary  voltage  (neglecting  the  comparatively  small 
amount  used  to  overcome  ohmic  resistance)  to  the  secondary  E.M.F. 
is  given  by  the  ratio  of  the  number  of  turns  in  the  two  windings.  If, 
for  example,  there  are  twenty  times  as  many  turns  in  the  primary  as 
in  the  secondary,  and  the  voltage  in  the  primary  is  5000,  the  voltage 
in  the  secondary  will  be  250.  The  current  in  the  secondary  is  then 
determined  by  its  resistance,  its  inductance,  and  the  E.M.F.  induced 
in  it  by  the  flux  in  the  primary.  The  formula  we  have  seen  l  to  be 
E  =  I  x/R2  +  47T2n2L2. 

Principle  of  Induction  Heating. — In  such  an  arrangement,  with 
the  exception  of  magnetic  losses  and  joule  heat  losses  in  the 
primary,  the  whole  of  the  energy  absorbed  by  the  transformer 
appears  as  the  induced  secondary  current.  If  therefore  cos  6 
is  unity,  the  product  IE  in  the  two  circuits  (neglecting  the  losses) 

must  be  identical.     And  it  follows  that  the  ratio 

current    in    primary 

turns  in  primary 

must  be  equal  to  the  ratio  .     If  cos  6  <  1,  then 

turns  in  secondary 

current  in  secondary       turns    in    primary    , 

<  .  The  current  m  the  primary 

current    in    primary        turns  in  secondary 

in  a  step-down  transformer  is  small,  and  determined  essentially 
by  the  nature  of  the  secondary  winding.  By  making  the  ratio 

turns  in  secondary 

small,  and    also   making  the    resistance    of  the 
turns    in    primary 

secondary  turns  low,  enormous  currents  can  be  induced  in  the 
latter  by  applying  a  suitable  E.M.F.  to  the  primary.  In  the 
induction  steel  furnace  there  is  only  one  turn  in  the  secondary 
winding,  and  this  consists  of  the  steel  to  be  refined,  occupying, 
in  the  simplest  form  of  furnace,  a  ring-shaped  channel,  through  the 

1  P.  181. 

2a2 


452    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 


'Primary 


centre  of  which  pass  the  primary  and  the  core  (Fig.  112).      It  is  the 
heat  produced  in  this  short-circuited  secondary  of  low  resistance  which 

effects  the  refining. 

Losses.  Power  Factor.  —  Besides  the 
heat  losses  common  to  all  furnaces,  induc- 
tion furnaces  are  subject  to  magnetic  and 
electrical  losses.  The  former  are  due  to 
hysteresis  in  the  iron  core.  The  electrical 
losses  comprise  eddy  current  losses  in 
the  core  and  joule  heat  losses  due  to  the 

0zx:--  primary  current,   and  in  the  latter  case, 

* — Care  with  a  constant  primary  voltage,  depend 

on  the  power  factor  of  the  furnace.    There 
FIG.  112.  .  ,  .  ,  ,   , 

are  two  mam  causes  which  tend  to  lower 

the  power  factor  of  induction  furnaces.  The  first  is  that,  owing  to 
insulation  and  the  necessity  of  cooling  core  and  primary,  it  is  impos- 
sible to  bring  primary  and  secondary  so  close  together  that  all  the 
lines  of  magnetic  flux  are  cut  by  both  of  them.  There  is  a  certain 
leakage  of  lines  which  are  only  cut  by  the  primary,  and  this  means 
self-induction,  instead  of  induction  in  the  secondary.  Secondly,  the  j 
low  value  of  the  resistance  of  the  secondary  compared  with  its  induct- 
ance leads  to  further  lowering  of  cos  6.  The  consequence  is  that 
pure  induction  furnaces  of  high  capacity,  in  which  the  secondary 
loop  must  either  have  a  large  cross-section,  and  therefore  a  low  resist- 
ance, or  else  must  be  of  wide  diameter,  in  which  case  its  inductance 
is  high  and  there  is  a  considerable  magnetic  leakage,  tend  to  have  very 
low  power  factors.  We  shall  see  how  in  the  Rochling-Rodenhauser 
furnace  this  tendency  has  been  largely  overcome. 

Circulation  of  Metal. — The  fact  that  the  secondary  in  the  induction 
furnace  is  molten  and  flexible  is  the  cause  of  several  interesting  pheno- 
mena observed  in  the  steel  bath.  There  is,  firstly,  a  tendency  for  the 
secondary  to  assume  a  circular  shape,  and  thus  to  enclose  as  many 
lines  of  magnetic  flux  as  possible.  The  resultant  of  gravity  and  of 
^  this  centrifugal  pressure  manifests  itself  in  an 

upward  and  outward  inclination  of  the  surface  of 
the  steel  bath  (Fig.  113),  together  with  a  perpetual 
rolling  motion  of  the  steel  round  the  longitudinal 
axis  of  the  bath  in  the  direction  indicated  by  the 
arrows.  As  a  consequence,  the  inner  side  of  the 
lining  of  the  channel  in  induction  furnaces,  which 
is  more  severely  exposed  to  the  action  of  the  slag 
than  the  outer  side,  deteriorates  more  rapidly. 
Secondly,  the  whole  molten  ring  rotates  bodily 
round  its  containing  channel  in  the  direction  of  the  longitudinal 
axis.  With  three-phase  current,  and  therefore  a  rotating  magnetic  field, 


r 

FIG.  113. 


xxiv. J  ELECTRIC  STEEL  453 

the  cause  of  this  behaviour  is  clear,  but  such  a  movement  also  appar- 
ently takes  place  to  some  extent  with  monophase  currents,  where  the 
reason  is  less  obvious.  The  combined  effects  of  these  two  movements 
ensure  excellent  mixing  of  the  metal  and  good  contact  with  the  slag. 

Pinch  Effect. — There  is  a  further  phenomenon  often  observed  in 
induction  furnaces,  more  particularly  in  those  of  small  size — the 
'  pinch  effect '  described  by  Hering.1  Two  conductors  carrying  current 
in  the  same  direction  tend  to  attract  one  another,  and  the  more  strongly 
the  greater  the  currents  flowing.  We  can  regard  a  liquid  metal  bath 
as  composed  of  a  large  number  of  flexible  parallel  conductors  in  close 
contact,  all  carrying  currents  in  the  same  direction,  and  at  once  see 
that  there  will  be  a  radial  pressure  acting  on  the  bath,  tending  to  force 
in  the  liquid  metal  towards  the  centre  and  out  at  the  ends,  and  thus 
to  decrease  the  cross-section  of  the  bath. 

Fig.  114  (a)  and  (b)  shows  the  effect  of  passing  heavy  currents 
through  a  series  of  parallel  movable  solid  conductors  and  through 
ar   liquid    conductor.      At    constant 
current   this  means    an   increase  in       °o°00000°o 
current   density    at  the    point  con-     °Q0oo0°oooo 
sidered,    and    hence    a    still    more    °l° o°000°o00 
powerful  tendency  to  shrink  together, 
an  unstable  state    of   things  which 
may  lead  to  complete  rupture  of  the 
liquid  conductor.      In  induction  fur- 
naces  this  tendency  is  partly  coun- 
teracted by  gravity.    At  the  point 
where  the  pinch  effect  sets  in  (Fig. 
114  c),  the  bath  is  in  violent  motion, 
metal  continually  running  down  the 
sloping    surfaces    and  being   forced 

back  under  them  into  the  body  of  pIG    n± pinch  Effect. 

the  melt.     But  if  the  current  density 

be  further  increased,  the  influence  of  gravity  is  overpowered,  and 
the  ring  is  broken.  The  current  at  once  stops,  and  the  metal 
joins  up  again.  If  the  current  density  be  too  high,  this  intermittent 
making  and  breaking  may  happen  many  times  in  succession,  rendering 
it  impossible  to  run  the  furnace,  and  finally  the  metal  may  become 
so  cold  that  it  no  longer  joins  up  again  but  solidifies,  necessitating 
a  complete  stoppage. 

To  avoid  this  effect,  the  first  condition  is  a  channel  of  adequate 
cross-section.  We  have  no  exact  data,  but  the  limiting  current  density 
is  probably  about  500  amps./cm.2  Secondly,  the  channel  must  be 
kept  perfectly  free  from  obstruction.  Small  pieces  of  solid  slag,  etc., 

1  Trans.  Amer.  Electrochem.  Soc.  11,  329  (1907} ;  15,  255  (1909).  Also  Harden, 
loc.  cit. 


454    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [(HAP. 


might  materially  diminish  the  cross-section  at  a  certain  point,  and 
thus  start  the  effect.  They  will  further  collect  in  any  initial  depression 
in  the  metallic  surface  which  forms,  and  increase  the  difficulty  of 
remaking  contact  when  once  broken.  If  the  pinch  effect  does  appear, 
the  power  taken  by  the  furnace  must  be  immediately  lowered.  But 
in  a  properly-designed  steel  furnace,  particularly  in  a  three-phase 
furnace,  there  should  be  no  danger  of  this.  With  smaller  furnaces, 
such  as  are  used  to  melt  copper,  brass,  aluminium,  nickel,  nickel  alloys, 
ferro-manganese,  etc.,  the  case  is  otherwise. 

The  first  induction  steel  furnace  was  designed  twenty-five  years 
ago  by  Ferranti.  Several  others  followed,  but  the  first  technically 
successful  was  that  of  Kjellin.  Since  then  the  greatest  advance  has 
been  the  introduction  of  the  Rochling-Rodenhauser  so-called '  combined 
induction  and  resistance  '  furnace. 

Kjellin  Furnace.1 — This  furnace,  of  essentially  simple  construction, 
is  shown  in  Fig.  115.  The  core  A  is  made  of  laminated  sheets  of  soft 

iron,  and,  in  order  to  minimise  the 
stray  magnetic  flux,  forms  a  closed 
circuit  just  like  an  ordinary  trans- 
former core.  The  primary  B  is  of 
copper  wire,  insulated,  and  cooled, 
together  with  the  core,  by  an  air 
blast.  Formerly  it  was  of  water- 
cooled  copper  tube.  The  ring-shaped 
channel  C  containing  the  steel  bath 
is  lined  with  calcined  magnesite,  and 
brickwork.  The  whole  is  sheathed  in 
In  a  large  unit  it  will  be 


I) 


FIG.  115.— Kjellin  Furnace. 


heat-insulated  by  suitable 
iron.  The  depth  of  the  channel  varies. 
l&"-2'.  Originally  it  was  U-shaped,  but  this  was  found  incon- 
venient, and  it  is  now  V-shaped.  It  is  covered  above  by  a  number 
of  lids,  D,  through  which  the  charging  takes  place,  and  is  discharged 
through  the  mouth  E  by  tilting.  Fixed  types  are  built  provided  with 
suitable  tapping  doors.  When  discharging,  a  certain  quantity  is 
always  left  in,  so  that  on  restarting  the  current  can  readily  pass,  ami 
the  cold  charge  is  fed  in  on  top  of  this. 

Kjellin  furnaces  have  been  built  of  all  sizes  up  to  8'5  tons  capacity. 
The  practical  economic  limit  here  reached  (and  perhaps  passed)  is 
determined  by  several  circumstances.  Firstly,  large  capacities  mean 
low  secondary  resistances  and  a  rapidly-decreasing  power  factor. 
Indeed,  to  keep  the  latter  within  workable  limits  it  is  necessary  to 
use  very  low  frequencies,  involving  expensive  generating  machinery. 

1  Elcctrochem.  2nd.  8,  294  (1905);  Trans.  Amer.  Electrochem.  Soc.  15,  173 
(1009). 


xxiv.]  ELECTRIC  STEEL  455 

For  cos  0  =  0'6,  in  itself  low,  a  2-ton  unit  must  be  worked  with 
15  periods,  a  4-ton  unit  with  8  periods,  and  the  8'5-ton  furnace 
with  5  periods  per  second.  Further,  the  shape  and  small  capacity  of 
the  channel  are  great  disadvantages  when  the  question  is  not  one 
of  making  finest  quality  crucible  steel  from  good  raw  materials,  but 
rather  of  working  up  impure  materials  by  the  use  of  several  slags 
to  good  quality  steel.  These  reasons  render  it  likely  that  the 
Kjellin  furnace  will  in  the  future  be  applied  more  particularly  to  the 
manufacture  of  comparatively  small  quantities  of  highest  quality  alloy 
and  carbon  steels,  and  to  the  melting  and  casting  of  other  metals  and 
alloys. 

A  2-ton  furnace  consumes  about  170  K.W.  It  is  fed  with  3,000 
volts  in  the  primary,  and  this  is  transformed  down  to  30,000  amps, 
in  the  secondary.  A  smaller  furnace  will  take  500  volts  in  the  primary, 
and  will  have  a  secondary  current  of  20,000  amps,  at  7  volts.  (The 
ratios  of  transformation  are  of  course  different.) 

Most  of  the  work  done  with  the  Kjellin  furnace  has  been  with  cold 
charges,  either  mixtures  of  pig-iron  and  pure  briquetted  ore  or  else 
of  pig-iron  and  scrap.  We  find  that  the  influence  of  the  furnace 
capacity  on  the  energy  required  is  particularly  great.  Using  cold  pig 
and  scrap,  a  10  K.W.  furnace  would  require  2,000  K.W.H.  per  ton  of 
steel,  a  100  K.W.  furnace  1,200  K.W.H.,  a  170  K.W.  furnace  (2-ton) 
750-850  K.W.H.,  and  a  furnace  of  500  K.W.  or  over  about  600  K.W.H. 
per  ton.  Working  with  briquettes  and  pig,  a  2-ton  furnace  requires 
1,000-1,200  K.W.H.  per  ton.  With  a  charge  of  molten  Bessemer  or 
open-hearth  steel,  the  largest  unit  needs  150-200  K.W.H.  per  ton. 
The  furnace  works  very  regularly.  The  heat  losses  are  mostly  due  to 
radiation.  The  exposed  surface  per  unit  of  volume  is  large,  and  the 
special  cooling  of  primary  and  coil  must  have  its  effect.  The  actual 
electrical  and  magnetic  losses  in  a  well- designed  furnace  hardly  exceed 
those  of  an  ordinary  transformer.  Harden  l  found  them  to  amount 
together  to  only  about  6  per  cent.,  and  that  on  a  small  60  K.W.  furnace. 
The  metal  losses  are  very  low.  The  lining  of  the  hearth  has  a  life  of 
two  to  three  months,  but  it  is  not  subjected  to  very  severe  slag  treat- 
ment. In  the  absence  of  electrodes  the  roof  lasts  a  long  time. 

Of  other  pure  induction  furnaces  we  may  mention  those  of  Colby,2 
Hiorth,3  and  Frick,4  shown  diagrammatically  in  Fig.  116.  The  Colby 
furnace  has  a  double  core  and  claims  a  higher  power  factor  than  the 
Kjellin  furnace.  In  the  Hiorth  furnace,  which  has  two  circular 
secondary  channels,  there  are  also  two  sets  of  primaries,  one  being  of 
water-cooled  tubing  embedded  in  the  magnesite  lining  below  the 

1  Electrochem.  Ind.  7,  320  (1000). 

-  Ibid.  5,  232  (1907). 

:i  Trans  Amer.  Electrochem.  Soc.  18,  191  (1910). 

4  Ibid.  19,  193  (1911). 


456    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

channels.  The  Frick  furnace  has  two  primaries  arranged  as  flat  discs 
above  and  below  the  secondary  channel.  The  power  factor  is  stated 
to  be  very  low.  It  does  not  appear  that  any  of  these  works  essentially 
better  than  the  Kjellin  furnace,  and  we  need  not  consider  them  further. 


Colby. 


Hiorth,.  Frlck. 

FIG.  116. — Induction  Furnaces. 


Rochling-Rodenhauser  Furnace.1  -  The  disadvantages  of  the 
Kjellin  furnace — small  capacity  and  low  power  factor— have  largely 
been  eliminated  in  the  Rochling-Rodenhauser  '  combined  induction 
and  resistance '  type.  Fig.  117  shows  diagrammatically  elevation 
and  plan  of  the  simplest  form  of  this  furnace,  used  with  monophase 
current.  A  is  the  soft  iron  core,  B  and  B'  are  two  primary  windings, 


FIG.  117. — Rochling-Rodenhauser  Furnace. 

and  C  is  the  steel  bath.  So  far  the  furnace  is  simply  a  double  induction 
furnace,  whose  secondary  has  the  shape  of  an  8,  the  middle  part  being 
a  comparatively  broad  hearth  allowing  of  convenient  working  and 
manipulation  of  slags.  But  to  heat  this  central  hearth  sufficiently 
further  means  .m-  necessary.  These  are  provided  by  auxiliary 

1  Trans.  Farad.  Soc.  4,  120  (1M8).      Trans.  Amer.  Electrochem.  8oc.  15,  173 
(1909).     Elektrolech.  Zeittch.  31,  903,  934  (Utto). 


ixivj  ELECTRIC  STEEL  457 

secondary  windings  D  and  D',  consisting  of  a  few  turns  of  thick  strip 
copper,  separated  from  the  primaries  B  and  B'  by  a  small  air  gap  only, 
and  connected  with  so-called  pole-plates  E  and  E'  at  the  opposite  ends 
of  the  centre  hearth. 

These  pole-plates  consist  of  plates  of  corrugated  cast  steel,  embedded 
in  the  furnace  wall 1  and  covered  with  a  thin  layer  of  calcined  dolomite, 
which  also  forms  the  lining  of  the  whole  hearth.  At  the  high  furnace 
temperature  these  oxide  layers  become  conducting  (cf.  a  Nernst  lamp 
filament),  and  an  extra  current  passes  from  D  and  D'  through  the 
steel  in  the  central  hearth.  The  heat  thus  generated  keeps  this  hearth 
at  a  high  temperature,  and  permits  of  the  use  of  refractory  slags.  The 
name  '  combined  induction  and  resistance  furnace/  it  is  obvious, 
conveys  a  false  idea  of  how  the  heating  is  effected.  All  the  heat  is 
generated  from  induction  currents,  but  in  the  pole-plate  secondaries 
a  portion  only  of  the  circuit  consists  of  fused  steel.  It  may  also 
be  noted  that,  though  the  inventors  object  to  their  pole-plates  being 
styled  electrodes,  they  are  nevertheless  as  much  electrodes  as  are 
the  steel  conductors  in  the  Girod  furnace  hearth. 

About  0-25-0-35  of  the  total  energy  taken  by  the  furnace  is  consumed 
in  these  pole-plate  circuits.  The  greater  part  of  this  is  generated  in 
the  oxide  layer  which  covers  the  pole-plates.  The  resistance  of  fused 

steel  is  low  (-    -0-00014-0-0002  at  15000,2  whilst  for  copper  at  15° 

=  O'OOOOOIT  ).  As  much  heat  will  be  generated  in  the  copper 
K 

windings  as  in  the  steel  bath  itself,  but  these  amounts  will  probably 
be  small  compared  with  the  heat  produced  in  the  pole-plate  coverings. 
Of  course  a  great  proportion  of  this  heat  will  enter  the  furnace,  but 
much  will  be  conducted  away  through  the  pole-plates  and  copper  leads, 
which  are  at  a  lower  temperature  than  the  fused  steel  bath,  and  the 
losses  in  this  circuit  must  be  considerable. 

The  effect  on  cos  6  is  twofold.  The  magnetic  leakage,  excessive 
in  the  Kjellin  furnace,  is  minimised  by  the  close  proximity  of  the 
primaries  and  the  auxiliary  copper  secondaries.  And  further,  whilst 
the  consumption  of  the  furnace  is  raised  by  30  per  cent,  or  more  through 
the  introduction  of  the  pole-plate  circuits,  this  is  achieved  without  any 
great  increase  in  self-induction,  as  resistance  is  higher  and  inductance 
lower  than  in  the  two  all-steel  circuits.3  The  result  is  that  whilst  a 
2-ton  Kjellin  furnace  wrorked  with  15  periods  gives  a  power  factor  of 
0*6  and  a  15-ton  furnace  must  be  worked  with  5  periods  to  attain 
the  same  result,  a  3-ton  monophase  Rochling-Rodenhauser  furnace 

1  Cf.  Nathusius  furnace,  p.  450. 
-  Trans.  Amer.  Electrochem.  Soc.  15,  215  (1909). 

:i  Does  the  presence  of  the  oxide  layer  introduce  a  capacity  effect  which  tends 
to  neutralise  the  effect  of  the  self-inductance  on  cos  B  ?   (Cf.  p.  180.) 


458    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

has  cos  6  =  0*7-0*8  when  worked  with  25  periods,  and  a  15-ton  tri-phase  j 
furnace,  fed  with  current  of  the  same  periodicity,  has  cos  0  =  O-G-O'T.l 
In  a  particular  case,  Rodenhauser  states  that  a  1 '5-ton  three-phase 
furnace  working  with  50  periods  had  cos  6  =  0*5  before  the  pole-plate 
circuits  came  into  play  (in  itself  a  much  better  result  than  the  Kjellin 
furnace  gives,  and  achieved  by  a  more  suitable  dimensioning  of  the 
steel  rings),  and  0'8  after  this  happened. 

A  further  great  advantage  gained  in  this  modified  induction  furnace  j 
type  is  the  possibility  of  refining  steel  in  large  quantities,  using  impure 
charges  and  several  slags.  We  have  seen  how  the  small  and  incon- 
venient dimensions  of  the  Kjellin  furnace  limit  it  in  practice  to  the 
melting  up  of  small  quantities  of  pure  raw  materials. 

Construction. — As  mother  furnaces,  there  is  an  outer  shell  of  steel 
plate  lined  with  non-conducting  brick,  and  this  again  is  lined  with  a 
stamped-in  mass  of  calcined  dolomite  which  serves  as  hearth.  The  roof 
is  of  refractory  clay.  There  is  a  working  door  at  each  end  of  the 
central  channel.  From  one  of  these  the  metal  is  discharged  by  tipping 
the  furnace.  Below  them,  under  the  surface  of  the  bath,  the  pole-plates 
are  embedded  in  the  walls.  The  core  consists  of  strips  of  soft  trans- 
former iron,  isolated  by  paper.  The  copper  secondaries  and  the 
primaries  are  so  wound  that  all  three  current  components  in  the  central 
furnace  channel  flow  in  the  same  direction  at  the  same  instant.  The 
coils  are  cooled  by  a  strong  air  blast,  filtered  to  remove  steel  particles. 
Monophase  current  furnaces  have  been  built  up  to  a  capacity  of  8 '5  tons. 
Such  a  unit  will  consume  700-750  K.W.  with  a  primary  voltage  of 
4,000-5,000  volts.  A  5-ton  furnace  takes  up  to  500  K.W.,  and  a 
3-5-ton  furnace  300-400  K.W.1 

Three-phase  Type. — Recently  furnaces  adapted  to  three-phase 
currents  have  been  built.  Such  a  furnace  has  three  soft-iron  cores, 
connected  above  and  below  by  yokes.  Round  each  core  is  a  molten 
steel  channel,  and  these  meet  to  form  a  central  hearth.  This  is  provided 
with  three  working  doors,  situated  symmetrically  between  each  pair 
of  cores,  and  below  the  doors  are  the  pole-plates.  The  copper  second- 
aries are  arranged  in  star.  The  soft-iron  core  itself  is  the  neutral 
point,  and  to  it  one  end  of  each  of  the  secondaries  is  connected,  the 
other  ends  being  attached  to  the  respective  pole-plates.  There  are 
.several  advantages  gained  by  the  use  of  three-phase  currents.  Firstly, 
a  possibility  of  a  further  increase  in  size  (15-ton  furnaces  are  contem- 
plated) ;  secondly,  an  economy  in  the  generating  m:»< -liinrry  used  ; 
lastly,  the  very  powerful  circulation  and  mixing  of  the  metal  due  to 
the  rotating  magnetic  field.  However,  so  far  1-ton  and  2-ton 
units  only  have  been  built. 

Rochling-Rodenhauser   furnaces   have   been   mostly  used   with   a 

1  A  unit  taking  14  tons  and  consuming  750  K.W.  has  recently  been  set  in 
operation  (June  l\i\'2). 


xxiv.]  ELE.CTRIC  STEEL  459 

charge  of  molten  Bessemer  steel  or  (at  Dommeldingen,  Luxemburg)  with 
molten  pig-iron.  The  energy  consumption,  as  in  the  Kjellin  furnace, 
varies  with  the  size  of  the  unit,  and  also,  of  course,  with  the  degree 
of  refining.  If  Bessemer  steel  be  refined  to  open-hearth  quality, 
about  120-160  K.W.H.  per  ton  are  needed.  To  lower  sulphur  and 
phosphorus  still  further  (O'Ol  per  cent.),  200-300  K.W.H.  are  required. 
Melting  up  cold  scrap,  as  in  the  Kjellin  furnace,  needs  900  K.W.H. 
per  ton  in  a  small  (2-ton)  furnace.  The  refining  proceeds  regularly 
and  quietly.  When  necessary,  the  hearth  is  repaired  between  the 
heats.  In  the  latest  furnaces  built,  slag  is  prevented  from  entering 
the  narrow  side  channels,  which  are  used  solely  for  heating  the  steel. 
Damage  to  the  less  accessible  parts  of  the  lining  is  thus  avoided.  The 
roof  has  a  long  life.  The  pole-plate  coverings  do  not  suffer.  Of  course 
an  excessive  current  passing  through  would  tend  to  melt  them.  The 
steel  losses  are  small. 

5.  Comparative 

We  have  already  discussed  the  nature  of  the  advantages  offered 
by  electric  heating.  In  the  present  case  the  first  important  point  is 
the  high  temperature  which  is  with  ease  attainable.  In  most  furnaces 
it  does  not  exceed  1550°-1600°  through  the  whole  mass,  but  this  is 
already  high  enough  to  permit  the  use  of  difficultly  fusible,  very  basic 
slags,  which  tend  to  quickly  desulphurise  the  metal.1  (In  arc  furnaces 
the  temperature  is  much  higher  locally.}  The  high  temperature  also 
greatly  increases  the  velocity  of  the  different  reactions,  and,  by  ren- 
dering the  charge  more  fluid,  allows  dissolved  gases  to  escape  easily. 
This  high  temperature  is  obtained  at  a  much  smaller  cost  than  if  fuel 
were  used.  The  next  point  is  the  neutral  or  reducing  atmosphere 
inside  the  furnaces.  This  is  of  the  utmost  importance,  not  so  much 
in  minimising  the  losses,  as  more  particularly  in  assisting  rapid 
desulphurisation.2 

Then  comes  the  question  of  size  of  unit  and  durability.  Compared 
with  the  crucible  steel  process .  the  advantages  are  all  on  the  side  of 
the  electric  furnace.  The  units  are  much  larger,  labour  is  less,  and  so 
are  repairs  items.  If,  on  the  contrary,  the  products  are  of  the  nature 
of  open-hearth  steel  or  a  little  better,  the  comparison  is  not  so  one- 
sided. The  units  used  in  ordinary  steel  practice  are  larger,  though 
this  difference  will  probably  disappear  largely  in  the  next  few  years. 
Secondly,  as  available  figures  show,  the  degree  of  refining  arrived  at  can 
usually  be  more  cheaply  reached  by  ordinary  metallurgical  methods. 
However  that  may  be,  it  is  significant  that  the  last  few  years  have 
seen  great  progress  in  this  direction,  more  particularly  with  the  Heroult 
and  the  Rochling-Rodenhauser  furnaces.  With  improved  electrodes 

1  P.  443.  -  Loc  cit. 


460    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP.  I 

and  refractory  materials  we  can  in  future  expect  increasing  quantities  I 
of  good  quality  steel  to  be  prepared  directly  from  Bessemer  converter 
metal  or  even  from  molten  pig-iron. 

One  feature  indeed  of  electric  steel  furnace  work  is  that  it  presents  1 
the  possibility  of  turning  impure  raw  materials  at  one  operation  into 
a  high-grade  exceedingly  uniform  product,  with  O01  per  cent,  of 
P  and  S,  and  any  required  quantity  of  carbon  or  alloyed  metal.  This 
is  possible  as  the  charge  can  be  heated  for  a  long  time  without  danger 
of  impurities  entering  from  fuel  gases. 

Confining  our  attention  now  to  the  various  electric  furnaces  used, 
the  first  obvious  comparison  is  one  between  arc  and  induction  furnaces,  j 
In  the  matter  of  power  consumption  there  appears  to  be  remarkably 
little  difference  between  the  two  types.     Working  conditions  vary 
so  much  that  an  exact  comparison  is  not  possible,  but  on  the  whole  ' 
the  induction  type  would  appear  to  give  the  best  results  with  a  solid,  ; 
the  arc  type  with  a  liquid  charge.     The  essential  differences  lie  in  ! 
other  directions.     The  first  advantage  of  the  induction  type  is  the 
elimination  of  electrodes.     The  power  losses  caused  by  electrodes  are 
partly  balanced  by  corresponding  losses  in  induction  furnaces.     But 
the  use  of  electrodes,  apart  from  the  somewhat  over-estimated  danger 
of  impurities  or  pieces  of  carbon  entering  the  bath,  also  involves  a 
constant  loss  due  to  oxidation — an  important  point  in  all  arc  furnaces. 

Further,  as  the  heat  in  the  induction  furnace  is  produced  in  the 
metal,  not  in  and  above  the  slag,  the  furnace  roof  is  much  less  severely 
attacked  by  radiation  and  by  slag  vapours.  The  absence  of  arc 
heating  also  keeps  down  the  metal  (vaporisation)  losses.  And  a 
further  advantage  is  the  fact  that  external  transformers  and  large 
cables  are  dispensed  with.  It  is  also  claimed  that  the  circulation  and 
mixing  in  induction  furnaces  are  far  superior  to  those  obtained  in  arc 
furnaces,  but  practical  results  show  the  advantage  to  be  really  illusory. 
The  difficulties  caused  by  electrode  regulation  and  load  fluctuations 
in  arc  furnaces  have  also  been  over-estimated,  though  induction 
furnaces  certainly  work  more  smoothly  in  this  respect. 

On  the  other  hand,  we  have  discussed  the  low  power  factor  of 
induction  furnaces,  and  what  it  entails.  But  the  chief  advantages  of 
arc  furnaces  lie  rather  in  the  matter  of  temperature,  and  in  their  CO 
reducing  atmosphere.  The  average  temperature  of  arc  furnaces  (e.g. 
Heroult's)  [is  probably  higher  than  that  of  induction  furnaces  (even 
Rochling-Rodenhauser).  The  essential  point,  however,  is  that,  while 
in  the  latter  the  large  mass  of  metal  is  hottest,  in  arc  furnaces  the 
thin  layer  of  slag  and  its  surface  of  contact  with  the  steel  are 
the  hottest  parts  of  the  bath,  far  hotter  than  in  induction  furnaces. 
Thus  the  refining  takes  place  more  quickly,  and  higher  melting  shijrs 
can  be  used.  The  fact  that  a  region  of  maximum  temperature  lies 
immediately  underneath  the  arc  is  a  further  advantage. 


xxiv.]  ELECTRIC  STEEL  461 

The  presence  of  carbon  electrodes,  a  drawback  for  other  reasons, 
is  responsible  for  the  valuable  reducing  atmosphere.  Arc  furnaces 
then  make  it  possible  to  remove  impurities,  particularly  sulphur,  more 
quickly  than  can  be  done  in  induction  furnaces.  This  deduction, 
combined  with  the  fact  that  large  units  can  be  more  readily  built, 
points  to  the  conclusion  that  for  large  charges  of  comparatively  crude 
material,  such  as  liquid  pig-iron  or  Bessemer  steel,  the  arc  furnace  is 
the  better,  while  for  smaller  charges  of  pure  cold  raw  materials,  par- 
ticularly for  alloy  steels,  the  induction  furnace  has  advantages. 

A  comparison  of  different  furnaces  is  in  some  respects,  particularly 
in  the  matter  of  power  consumption,  invidious.  Not  one  of  the 
furnaces  above  described  has  reached  its  final  form,  and  improvements 
are  constantly  being  made  ;  whilst  the  numerous  available  figures  of 
power  consumption  are  not  easily  comparable  owing  to  differences 
in  raw  material,  size  of  unit,  etc.,  etc.  Nevertheless  we  can  discuss 
comparatively  some  of  the  more  obvious  and  fundamental  features 
of  design  of  the  different  furnaces.  It  may  be  noted  in  passing  that 
much  of  the  criticism  which  has  appeared  in  technical  journals  is  of  an 
interested  and  frivolous  nature,  dealing  with  trivialities  or  imagined 
defects.  The  fact  remains  that  all  the  furnaces  above  described  are 
capable  of  regular  and  easy  working,  and  of  turning  out  with  certainty 
high-class  products. 

Induction  furnaces,  and  the  special  features  of  the  Rochling- 
Rodenhauser  furnace,  have  already  been  considered  at  length.  There 
is  little  to  add,  except  to  emphasise  the  fact  that  the  heat  unutilised 
in  the  pole-plate  circuits  must  form  quite  a  considerable  fraction  of 
the  total  generated  in  these  circuits.  Exact  data  on  this  point  would 
be  interesting.  The  advantages  of  this  type  over  other  induction 
furnaces  do  not  lie  in  the  energy  consumption  per  ton  of  steel,  but 
in  the  possibility  of  using  large  units  and  cruder  charges. 

Of  the  different  arc  furnaces,  the  Heroult  must  be  considered  the 
simplest  in  construction,  and  therefore  most  suitable  for  continuous 
working  up  of  large  charges.  Then  comes  the  Girod  furnace,  whilst  the 
Stassano  furnace  is  the  most  complicated  and  requires  most  labour. 
And  although  these  complications  are  unimportant  when  working  up 
small  charges  of  highest  quality  steel,  it  is  unlikely,  other  considerations 
apart,  that  the  Stassano  furnace  will  ever  be  employed  in  the  regular 
manufacture  of  large  quantities  of  steel  of  open-hearth  quality. 

Walls  and  roof  of  the  Stassano  furnace  are,  in  consequence  of  the 
peculiar  position  of  the  electrodes,  more  exposed  to  the  heat  of  the 
arc  than  in  the  other  types,  but,  as  the  arc  does  not  directly  play  on 
the  slag,  less  exposed  to  the  action  of  slag  vapours.  Further,  the  roof 
is  made  of  basic  material  (in  the  other  furnaces  of  acid  material), 
and  we  find  its  life  comparatively  long,  that  of  the  hearth  being 
shorter  than  in  other  furnaces.  The  carbon  (electrode)  losses  and  the 


462    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

metal  losses  are  also  considerably  less,  as  the  hearth  is  so  effectually 
closed  in. 

The  Girod  and  Heroult  furnaces  must  be  regarded  as  equally 
reliable  under  regular  working  conditions.  Experience  has  shown 
that  the  water-cooled  steel  electrodes  in  the  former  make  no  difference 
whatever  to  the  durability  of  the  hearth.  Nor  can  we  discriminate 
between  them  in  the  matter  of  efficacy  of  heating.  Furnaces  of 
equal  load  will  have  the  same  number  of  arcs,  and  will  require  the 
same  carbon  electrode  section.  The  amount  of  heat  abstracted  from 
the  furnace  by  the  Girod  hearth  electrodes  is  very  small — thus  in  a 
3-ton  furnace  only  about  1  per  cent,  of  the  total  fed  in.1  The  wear 
and  tear  of  roof  and  hearth  should  also  be  about  equal.  The  claims 
put  forward  in  favour  of  the  Girod  furnace  because  the  current 
passes  through  the  bath  must  be  regarded  as  unfounded.  It  cannot 
be  too  strongly  emphasised  that  by  far  the  greater  proportion  of  heat 
produced  in  these  furnaces  is  produced  in  the  arcs — only  a  very  small 
fraction  is  generated  actually  in  the  metal  or  slag.  Far  more  is  pro- 
duced in  the  carbon  electrodes. 

6.  Ferro-alloys2 

A  very  important  development  in  the  steel  industry  during  recent 
years  has  been  the  increased  demand  for  alloy  steels — i.e.  steels  which, 
besides  carbon,  contain  another  metal  or  metals  as  constituents. 
Such  steels  often  have  very  favourable  mechanical  properties,  and  are 
now  indispensable  for  the  manufacture  of  high-speed  tools,  machinery 
exposed  to  severe  usage,  armour-plate,  projectiles,  etc.  At  first  these 
steels  were  prepared  by  the  addition  of  the  metal  itself  or  of  ferro- 
alloys produced  in  the  blast  furnace,  but  ferro-alloys  produced  electro- 
thermally  are  now  generally  used.  For  nickel  steels,  metallic  nickel 
is  used  ;  for  manganese  steels,  the  ferro-manganese  employed  is  still 
made  in  the  blast  furnace,  the  high  temperature  of  the  electric  furnace 
causing  too  great  volatilisation  losses.  But  ferro-silicon,  the  most 
important  product  of  all,  as  well  as  ferro-chromium,  ferro-tungsten, 
ferro-molybdenum,  and  ferro-vanadium,  are  chiefly 3  prepared  by 
electrothermal  methods,  by  far  the  best  at  the  high  working  tempera- 
tures necessitated  by  the  difficultly  reducible  oxides  used.  The  products 
are  also  purer  than  those  made  by  other  methods,  containing  much  less 
carbon,  phosphorus,  sulphur,  etc. 

The  chemistry  of  these  processes  is  simple.  It  consists  essentially 
in  the  interaction  at  high  temperatures  of  as  pure  a  form  as  possible 

1  Zeitoch.  Elektrochem.  17,  773  (Wll). 

l.hrtrochem.  Ind.  2,  349,  395,  449  (1<H)4) ;   5,  9  (7.W7)  ;  Zeitsch.  Elektrochem. 
9,  302  (1W3). 

:i  Certain  amounts  of  the  pure  metals  arc  m;ul<-  by  the  QbkUchmidt  Thermite 
process. 


xxiv.]  FERKO-ALLOYS  463 

of  the  refractory  metal  oxide,  carbon,  and  iron,  CO  and  the  ferro-alloy 
resulting.  Occasionally  a  pure  iron  oxide  is  used  instead  of  the  iron. 
But  as  this  necessitates  energy  consumption  for  the  reduction  of  the 
iron  oxide  as  well  as  for  the  reduction  of  the  refractory  metal  oxide, 
this  procedure  is  but  seldom  followed.  As,  instead  of  difficultly 
fusible  pure  metal,  a  more  or  less  rich  low-melting  iron  alloy  results, 
it  follows  that  the  energy  needed  per  unit  weight  of  reduced  sub- 
stance is  less  than  that  necessary  if  the  latter  were  produced  in  the 
pure  state.  The  reaction  takes  place  at  a  lower  temperature.  Thus 
Greenwood  l  found  that  the  reaction  in  a  finely  divided  intimate 
silica -carbon  mixture  commenced  at  1460°,  but  when  iron  was  added 
the  temperature  could  be  as  low  as  1200°. 

The  total  energy  absorbed  during  the  reaction  is  composed  of  that- 
needed  to  reduce  the  refractory  oxide  minus  that  produced  by  the 
burning  of  the  carbon  (both  at  room  temperature)  plus  that  necessary 
to  heat  the  product  to  its  tapping  temperature.  The  heat  effect  of 
the  alloying  of  the  iron  and  second  constituent  must  be  neglected  as 
probably  insignificant.  We  will  carry  out  the  calculation  in  detail 
for  a  50  per  cent,  ferro-silicon. 

The  melting-point  —  per  cent,  composition  diagram  of  the  system  iron- 
silicon  has  been  mapped  by  Tammann  and  Guertler.-  From  their  results  we 
gather  that  the  50  per  cent,  alloy  melts  at  about  1370°.  Suppose  it  to  be  tapped 
at  1600°.  The  CO  will  impart  much  of  its  heat  to  the  fresh  charge,  and  we  will 
assume  it  leaves  the  furnace  at  1000°.  The  equation  is 

Si02  +  2C >Si  +  2CO. 

The  heat  of  combustion  of  a  kilogram  atom  (28'3  kilos.)  of  Si  to  SiO.,  at  room 
temperature  is  180,000  Cals.  The  heat  of  combustion  of  2  kilogram  atoms  of 
carbon  to  CO  is  2  x  29200  =  58400  Cals.  Hence  the  production  of  28'3  kilos, 
silicon  at  room  temperature  requires  180000  —  58400  =  121600  Calories,  i.e. 
2,148.000  Cals.  for  500  kilos.  The  specific  heat  of  silicon  between  0°  and  1600° 
we  will  set  at  0'21.  For  the  heating  of  500  kilos,  to  1600°  are  therefore  necessary 
1600  x  500  x  0'21  =  168000  Cals.  For  the  heating  and  melting  of  500  kilos. 
of  iron  are  necessary  about  310  x  500  =  155000  Cals.  We  will  set  the  mean 
molecular  specific  heat  of  CO  between  0°-1000°  at  7'0,  and  calculate  that  the 
heating  up  of  the  CO  needs  247,000  Cals.  The  total  is 

Cals. 

For  chemical  reaction  2,148,000 

For  heating  the  silicon  168,000 

For  heating  and  melting  the  iron  155,000 

For  heating  the  CO  247,000 


2,718,000 

The  theoretical  quantity  of  energy  necessary  for  the  production  of  1  ton  of 
50  per  cent,  ferro-silicon  is  therefore 


1  Electrochem.  Ind.  7,  120  (1909). 

-  ZeitschSAnorg.  Chem.  47,  163  (1905). 


464    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

The  theoretical  minimum  values  for  other  ferro-alloys  can  be 
similarly  calculated. 

Ferro-Silicon.1 — This  is  the  most  important  of  the  ferro-alloys, 
largely  used  for  deoxidising  steel  during  refining,  in  making  stool 
castings,  and  for  regulating  the  silicon  content  of  cast-iron.  When 
made  in  the  blast  furnace,  the  silicon  percentage  seldom  exceeded 
15  per  cent.,  whilst  the  first  electrothermal  products  were  mostly 
30  per  cent.  Par  richer  alloys  can  now  be  made,  and  there  is  a  tendency 
to  produce  three  grades  only — 25  per  cent.,  50  per  cent.,  75  per  cent. — 
the  relative  demand  for  the  richer  products  steadily  increasing. 

The  raw  materials  for  ferro-silicon  manufacture  are  silica,  carbon, 
and  iron.  (Iron  oxide  is  still  used,  but  only  very  occasionally.)  They 
must  be  very  carefully  selected,  particularly  from  the  point  of  view 
of  impurities.  If  these  enter  the  product  they  lower  its  value  for 
steel  refining.  In  particular,  if  phosphorus  is  present,  then  the  ferro- 
silicon  is  liable  to  evolve  PH3,2  which  has  been  the  cause  of  several 
poisoning  cases.  Apart  from  that,  the  impurities  tend  to  form  more 
or  less  infusible  slags,  which  disturb  the  working  of  the  furnace. 
Formerly  the  charge  contained  lime  as  a  flux,  but  with  pure  materials 
this  is  usually  unnecessary.  As  silica,  95-98  per  cent,  quartzite  is 
used.  A  lower  percentage  material  is  apt  to  cause  disturbances.  The 
chief  impurities  are  A1203,  MgO,  and  CaO,  which  form  the  slag.  Sand 
is  usually  too  impure,  and  contains  too  much  moisture.  The  carbon 
is  present  as  charcoal,  anthracite,  or  a  high-grade  coke.  Generally, 
the  less  dense  the  variety  of  carbon  and  the  lower  the  ash,  the  more 
suitable  it  is.  Unfortunately,  most  cokes  contain  too  much  sulphur 
and  phosphorus,  and  both  coke  and  anthracite  suffer  from  their  high 
content  of  ash.  Phosphorus  and  sulphur  also  debar  the  use  of  cast-iron, 
and  shavings,  etc.,  of  soft  iron  and  steel  are  usually  employed.  In 
small  furnaces  the  charge  should  not  contain  pieces  greater  than  \" 
diameter  ;  in  large  furnaces  bigger  pieces  are  permissible. 

All  ferro-silicon  furnaces  are  of  the  mixed  arc-resistance  type.  The 
current  passes  as  an  arc  between  electrode  and  charge,  through  the 
charge,  and  then  to  the  other  electrode,  sometimes  by  means  of  a  second 
arc,  sometimes  not.  The  part  played  by  resistance  heating  depends 
on  the  arrangement  of  electrodes  and  composition  of  charge.  The 

1  Many  of  the  details  in  this  section,  particularly  those  dealing  with  the  charge 
used  and  with  the  results  obtained  with  large  three-phase  furnaces,  are  taken 
from  the  article  by  Helfenstein  in  Askenasy's  Einfuhrung  in  die  technische  Ele.k- 
trochemie,  vol.  i.  (1910).  See  also  Conrad,  Electrochem.  Ind.  6,  397  (MM).  Further 
Metall.  Chem.  Engin.  8,  133  (W10). 

-  Recent  investigations  indicate  that  only  those  grades  which  lend  1<>  disin- 
tegrate behave  thus — i.e.  alloys  containing  30-65  per  cent.  Si.  Richer  or  poorer 
products  are  innocuous,  and  the  alloys  prepared  in  large  modern  furnaces  are 
stated  to  be  much  freer  from  all  such  impurities.  Metall.  Chem.  Enyin.  8,  133 
(inn*). 


XXIV.] 


FERRO-ALLOYS 


465 


more  heat  produced  this  way,  the  higher  the  voltage  and  the  lower 
the  current  for  a  hearth  working  at  constant  power. 

We  can  distinguish  three  types  of  furnace.  The  first  (Fig.  118), 
of  which  many  are  still  working  in  France,  has  a  hearth  electrode  like 
the  Girod  steel  furnace.  The  working  of  a  furnace  of  this  type  (the 
Rathenau  furnace)  has  been  fully  described  by  Pick.1  Such  furnaces 
consist  essentially  of  a  hearth  or  crucible  of  electrode  carbon  sur- 
rounded by  a  wall  of  acid  firebrick,  the  whole  contained  in  a  strong 
iron  sheath.  The  lining  must  be  very  carefully  constructed,  for  if 
the  ferro-silicon  penetrates  to  any  extent  between  the  bricks,  and 
there  solidifies,  it  will  thrust  them  asunder  and  destroy  the  furnace. 
A  protective  layer  of  solidified  ferro-silicon  forms  the  best  furnace 
lining.  A  250-K.W.  unit  has  a  crucible  1  metre  in  diameter  and 
40  cm.  deep.  The  hearth  forms  one 
electrode,  contact  being  made  with  a 
steel  plate  by  stamping  in  a  mixture  of 
graphite  or  carbon  and  pitch.  The 
other  electrode  enters  the  furnace  from 
above,  and  can  be  regulated.  The  fur- 
nace is  uncovered,  and  the  charge  simply 
shovelled  in,  and  heaped  up  on  top 
round  the  electrode.  The^  ferro-silicon 
is  tapped  every  one  or  two  hours  into 
carbon  moulds,  and  the  process  occasion- 
ally stopped  to  remove  the  viscous  slag. 
Direct  current  can  be  used,  but  alter- 
nating current  is  preferable. 

Such  furnaces  usually  take  200-600  K.W.,  but  in  the  Ugine  works 
1,350-K.W.  units  are  stated  to  be  working.  A  200-K.W.  furnace 
takes  about  5,000  amperes  at  40  volts,  and  generally  the  voltage  of 
a  single  hearth  varies  between  35-50  volts,  the  furnaces  of  bigger 
capacity  taking  larger  currents.  The  kind  of  carbon  used  somewhat 
affects  the  current-voltage  relations.  With  the  feebly-conducting 

•tr  y-w  1-f-o  or  o 

charcoal,  the  ratio  —         '-  is  higher  than  with  coke  or  anthracite,  the 
current 

current  density  in  the  electrode  carbons  showing  corresponding  varia- 
tions between  3-7  amps./cm.2  Owing  to  the  small  size  of  the  units, 
and  to  the  fact  that,  being  uncovered,  the  volatilisation  losses  are 
very  great,  the  yield  of  ferro-silicon  with  these  furnaces  is  low.  Pick's 
figures  for  a  200-K.W.  unit,  using  charcoal,  are 

1  kilo.  25  per  cent,  alloy  requires    5  K.W.H. 

„      50         „  „          „        16  K.W.H. 

„       70  34  K.W.H. 


Fid. 


118.     Rathenau   Ferro- 
Silicon  Furnace. 


Dissertation  (Karlsruhe,  1906). 


2H 


466    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHA! 


The  losses  of  silica  were  respectively  42  per  cent.,  54  per  cent.,  an(J 
69  per  cent. ;  of  carbon,  47  per  cent.,  43  per  cent.,  and  73  per  cent. 

With  larger  units  (e.g.  500  K.W.)  better  results  are  usually  obtained! 
Thus  Helfenstein  gives  1  kilo.  50  per  cent,  alloy  =  9'2  K.W.H.  using! 
charcoal,  and  12  K.W.H.  using  coke  or  anthracite,  where  the  difference 
results  from  the  larger  currents  in  the  second  case  causing  greaJ 
vaporisation  losses,  heat  losses  also  arising  from  the  high  ash  content! 
of  the  fuel.  The  silicon  losses  vary  from  20-25  per  cent.  The  electrodJ 
consumption  can  amount  to  100  kilos,  per  ton  for  the  50  per  centl 
alloy,  or  even  more  ;  for  the  25  per  cent,  alloy,  up  to  60  kilos,  per  tonj 
This  necessitates  renewals  every  few  days.  As  we  shall  see,  lateil 
types  of  furnaces  give  far  better  results,  and  though,  owing  to  thJ 
smaller  quantities  made,  this  type  of  furnace  may  be  retained  foil 
other  alloys,  it  is  highly  improbable  that  new  ones  will  be  built  for] 
ferro-silicon.  Apart  from  other  considerations,  the  bottom  electrode,] 
of  little  importance  in  the  Girod  steel  furnace,  here  seriously  affects] 
the  durability  of  the  hearth. 

The  second  furnace  we  must  notice  is  the  Keller  furnace  (Fig.  119). 
It  consists  of  two  vertical  shafts,  through  which  the  electrodes  and] 
charge    (anthracite   is   used)  are  intro- 
duced, and   a   horizontal  channel  con-, 
necting    them.     Here    the     ferro-alloy 
collects,  being  tapped  every  two  hours.  • 
Usually    the    shafts    are    large  enough 
to  contain  several  electrodes  connected 
in    parallel,    each   independently  regu— 
lated.     The  slag   can    be    tapped,  and 
the  furnace  run   for    months    continu- 
ously.     Units  have  been    constructed 
up    to  900  K.W.       A  500-K.W.   unit 
requires    3'5   K.W.H.  per  kilo.  30  per 
cent,    ferro-silicon.      The    consumption 
of  electrodes  is  small,   owing  to  their  being  much  better  protected' 
than  in  the  first  type.      They  usually  last  two  to  three  weeks.     The 
losses  of  material  amount  to  10  per  cent. 

The  third  and  latest  type  of  ferro-silicon  furnace  (Fig.  120)  is 
derived  from  the  Heroult  type.  The  hearth  is  neutral,  all  the  electrodes 
being  introduced  from  above.  The  great  advance,  however,  is  the 
increased  load.  In  its  latest  form,  the  furnace  is  fed  with  three-phase 
current,  and  takes  from  4,000-9,000  K.W.,  divided  between  three 
hearths.  The  voltage  can  vary  within  wide  limits,  depending  on  the 
amount  of  resistance  heating.  This  in  its  turn  is  chiefly  determined 
by  the  composition  of  the  charge.  For  the  largest  units,  the  normal 
figure  using  anthracite  is  75-90  volts  per  hearth,  and  to  this  corresponds 
a  current  of  40,000  amps,  (star  connections),  cos  6  being  0'8  and  the 


Fio.  119.— Keller  Ferro-silicon 
Furnace. 


XXIV.] 


FERRO-ALLOYS 


467 


electrode  current  density  some  7  amps./cm.2  But  higher  voltages 
(120-130  volts)  can  be  employed  almost  as  easily.  Such  a  furnace 
can  produce  1  kilo.  50  per  cent,  alloy  per  6'9  K.W.H.,  the  losses  being 
15-20  per  cent,  and  the  electrode  consumption  40  kilos,  per  ton. 
Using  charcoal,  in  which  case  the  hearth  voltage  is  higher,  the  current 
lower,  and  the  vaporisation  less,  the  energy  required  is  5-3-6-0  K.W.H. 
per  kilo,  and  the  losses  10  per  cent.  With  still  larger  furnaces  of 
Helfenstein's  pattern,1  still  better  results  may  be  expected.  The  more 
refractory  the  alloy  made,  the  poorer  is,  of  course,  the  yield,  and  to 
obtain  satisfactory  results  the  larger  must  be  the  load  of  the  working 
hearth. 

These  large  modern  furnaces  are  very  simply  constructed  of  fire- 
brick, lined  with  carbon.  They  are  cooled  by  air  or  often  by  water, 
so  that  a  crust  of  solidified  material  forms  the  actual  lining.  For 
purposes  of  tapping  a  hole  is  melted  in  this  wall  by  means  of  a  pointed 
rod,  serving  also  as  an  electrode.  Current  passes  from  the  rod  through 


FIG.  120. — Large  Modern  Ferro-silicon  Furnace. 


the  lining  to  one  of  the  electrodes,  and  the  liberated  heat  effects  the 
fusion.  Slag  is  occasionally  removed  through  a  larger  hole  bored  at 
the  right  height.  Except  when  tapping,  the  load  is  exceedingly 
constant.  The  regulation  of  the  electrodes  is  done  by  hand,  guided 
by  voltmeter  readings. 

Ferro-chromium  is  largely  employed  for  making  chrome  steels, 
which  are  exceedingly  hard  and  extensively  used  for  armour-plate, 
projectiles,  high-grade  castings,  high-speed  tools,  the  best  cutting 
tools,  etc.  Little  is  known  of  the  manufacture  of  ferro-chrome.  It 
is  prepared  in  the  works  of  Girod,  Keller,  and  Heroult,  and  therefore 
in  furnaces  similar  to  those  described  in  the  last  section.  Grades  are 
made  containing  50  per  cent,  and  65  per  cent,  chromium.  The  carbon 
content  varies  considerably,  but  by  suitable  refining  can  be  brought 
down  to  0-25-0-5  per  cent.  In  the  Heroult  furnace  this  is  done  by 
lining  the  bath  with  a  chromium  ore  and  using  a  slag  containing 
Cr203.  Haber 2  briefly  describes  the  manufacture  in  America  of  a 
71  per  cent,  alloy  containing  23  per  cent,  iron  and  5*2  per  cent,  carbon. 


Fig.  47. 


•  Zeitsch.  Elektrochem.  9,  362  (1903). 

Sal 


468       PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY 


:ype. 


The  furnaces  were  small,  averaging  300  K.W.  and  of  the  Girod  t] 
A  chromium  ore  containing  iron  oxide  was  directly  reduced  without* 
the  addition  of  any  metallic  iron.  One  kilo,  of  alloy  needed  8  K.W.H.. 
The  furnaces  employed  by  Girod  himself  atUgine  are  somewhat  larger,, 
400-600  K.W. 

Ferro-tungsten  is  employed  for  making  very  hard  steel  used  foil 
high-speed  cutting  tools.  Nothing  is  known  of  its  manufacture! 
Girod  makes  grades  containing  up  to  85  per  cent.  W.  Ferro-molyb-  • 
denum  is  stated  to  be  even  more  effective  than  the  last  two  alloys  asi 
an  addition  in  the  manufacture  of  steel  for  high-speed  tools,  large  guns, , 
rifle  barrels,  etc.  It  is  made  as  an  80  per  cent,  alloy.  No  details \ 
are  known  of  its  manufacture,  and  the  same  statement  holds  ofi 
ferro-vanadium,  prepared  as  a  50  per  cent,  alloy  very  low  in  carbon,, 
and  said  to  greatly  increase  the  strength  of  steel  without  at  all  impairing  i 
its  ductility. 

Literature 

Rodenhauser  and  Schoenawa.  Elektrische  Of  en  in  der  Eisen-> 
Industrie. 

Helfenstein.     Article  in  Askenasy's  Einfuhruny  in  die  t('chnische 
Elektrochemie,  vol.  i. 


CHAPTER  XXV 

CALCIUM  CARBIDE   AXD   CALCIUM  CYANAMIDE 
1.  Carbide.    General  and  Theory 

ALTHOUGH  previously  discovered,  our  essential  knowledge  of  calcium 
carbide  dates  from  Moissan,1  who  in  1892  prepared  the  substance  in 
quantity  and  comparatively  pure  by  the  interaction  of  lime  and 
sugar  charcoal  in  an  arc  furnace.  Willson  chanced  to  rediscover  it 
independently  in  America  about  the  same  date.  As  soon  as  its  valu- 
able property  of  generating  acetylene  by  interaction  with  water  was 
recognised,  its  commercial  manufacture  was  started  and  developed 
in  Europe  chiefly  by  Bullier  and  others,  in  America  by  Willson  and 
Bradley.  It  was  the  earliest  of  the  electrothermal  industries.  Owing 
to  injudicious  speculation  and  over-production,  a  severe  crisis  was 
experienced  in  Europe  in  1899.  Many  companies  failed,  and  others 
were  compelled  to  manufacture  other  products,  such  as  ferro-silicon. 
Present  conditions  and  prospects  are  far  better.  Business  reorganisa- 
tion, considerable  technical  improvements,  a  purer  and  more  reliable 
product  causing  a  greater  demand,  and  the  rise  of  the  cyanamide 
industry  2  have  all  played  a  part. 

Calcium   carbide   is   produced  in   the   electric   furnace   according 
to  the  reversible  reaction 

CaO-f  3C^±CaC2+CO. 

This  reaction  is  endothermic.  At  high  temperatures  the  tendency 
is  for  carbon  and  lime  to  interact,  furnishing  carbide  and  carbon  mon- 
oxide. At  low  temperatures  the  equilibrium  moves  in  favour  of  the 
left-hand  side  of  the  equation.  To  each  temperature  corresponds  a 
definite  CO  equilibrium  pressure,3  which  must  be  exceeded  before  CaC2 
can  be  converted  into  lime  and  carbon.  The  higher  the  temperature 

1  The  Electric  Furnace,  p.  203. 

•  Together  with  minor  applications  of  acetylene,  e.g.  for  cutting  and  welding 
metals. 

:>  See  p.  4 7'. i. 


470    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

the  greater  this  pressure.    But  CaC2  can  also  decompose  in  the  absence 
of  CO,  dissociating  into  graphite  and  calcium  vapour,  as  follows  : 


The  calcium  dissociation  pressure  is  higher  the  higher  the  temperature. 
These  two  reactions  are  connected  by  a  third,  the  reduction  of  lime 
by  carbon  to  metal  and  CO.  So  that  we  must  finally  consider  the 
three  following  equations,  the  right-hand  side  of  each  being  favoured 
by  increased  temperature  : 


(a) 
(6) 
(c) 

Several  attempts  have  been  made  to  measure  the  CO  equilibrium 
pressure  corresponding  to  the  first  equation.  Rothmund,1  Lampen,2 
and  Rudolphi  3  heated  together  lime  and  carbon  to  different  tempera- 
tures, and  observed  the  temperature  necessary  for  the  resulting  product 
to  give  acetylene  on  treatment  with  water.  Rothmund  found  1620°, 
Lampen  1700°-!  725°,  and  Rudolphi  1810°.  Assuming  that  the  gaseous 
phase  consisted  of  nitrogen  and  CO  produced  by  the  atmospheric 
oxygen,  we  have  as  a  result  that  the  equilibrium  temperature  corre- 
sponding to  a  CO  pressure  of  about  250  mm.  is  between  1620°  and 
1810°. 

These  measurements,  however,  were  very  approximate.  Results 
of  greater  value  were  got  by  Thompson,4  working  with  an  Arsem  5 
vacuum  electric  furnace,  and  using  a  78  per  cent,  pure  carbide,  the 
residue  being  lime  and  carbon.  He  found  that  between  1700°  and 
2000°,  where  the  CO  pressures  would  have  been  easily  measurable,  no 
readings  were  possible.  The  CaC2  dissociated,  the  calcium  vapours 
burnt  in  the  CO  (equation  c),  and  a  deposit  of  lime  formed  on  the  sides 
of  the  furnace.  He  was  therefore  compelled  to  work  below  1500°, 
and  finally,  owing  to  experimental  difficulties,  to  heat  the  carbide  in 
an  atmosphere  of  a  neutral  gas.  Hydrogen  was  used,  the  equilibrium 
not  being  appreciably  disturbed  thereby.  Analysing  large  quantities 
of  gas,  he  obtained  the  pressures 

0'44  mm.  at  1445°, 
0-82  mm.  at  1475°. 

He  calculated  the  heat  absorbed  in  the  formation  of  CaC2  according 
to  the  above  equation  as  121000  —  3-3  0  Cals.,  where  6  is  the  difference 

1  Zeitach.  Anorg.  Chem.  31,  130  (1H02). 

2  Jour.  Amer.  Chem.  Soc.  28,  850  (  HUH;). 

3  Zfilwh.  Annrtj.  Chem.  54,   170  11007). 

''    M.l'ill.  Ch.tn.  /•>////.  8,  1>7!».  \\'1\  (IH  10). 

3  Trans.  Amer.  Electrochem.  Soc.  9,  15:}  (Hum). 


xxv.]  CALCIUM  CAEBIDE  471 

between  the  working  temperature  and  room  temperature.     At  1460°, 
Q  is  116,000  Cals.     Working   as  on  p.  24,  the  ratio  Pu7f>°  becomes 


0*82 

1-79,  whilst  he  had  found  =  1'86.     The  agreement  justified  a 

further  extrapolation,  and  Thompson  gives  as  mean  values  for  the 
equilibrium  pressures 

0-50  cm.  at  1575°, 

2'53  cm.  at  1675°, 

10-7     cm.  at  1775°, 

40-5    cm.  at  1875°, 

133        cm.  at  1975°. 

Rudolphi's  figure  (25  cm.  at  1810°)  shows  the  best  agreement.  The 
others  are  undoubtedly  too  low. 

The  melting-point  of  pure  CaC2  is  unknown.  The  commercial 
product  always  contains  much  lime  together  with  other  impurities, 
often  including  carbon,  and  consequently  melts  lower.  The  tempera- 
ture at  which  it  collects  in  the  furnace  is  probably  about  2000°,  perhaps 
lower.  From  Thompson's  figures  we  see  that  under  these  circumstances 
a  CO  pressure  of  100-150  cm.  would  be  necessary  to  convert  it  into  lime 
and  carbon.  As  the  gas  leaving  a  carbide  furnace  averages  75  per  cent. 
CO,  corresponding  to  a  pressure  of  57  cm.,  there  is  at  these  temperatures 
no  fear  of  such  a  reverse  action  taking  place.  At  lower  temperatures, 
however,  this  is  possible.  But  it  is  unlikely  to  assume  any  importance. 
The  CaC2  is  produced  at  higher  temperatures  in  the  neighbourhood  of 
the  arc,  and  falls  in  the  liquid  state  to  the  bottom  of  the  furnace,  the 
arc  still  playing  on  its  surface.  For  re-formation  of  carbon  and  lime, 
contact  with  CO  is  necessary,  and  the  area  presented  to  the  action 
of  the  gas  is  small. 

The  quality  of  the  carbide  somewhat  affects  its  stability.  The 
poorer  it  is,  the  lower  the  temperature  to  which  it  can  be  brought 
under  a  given  CO  pressure  without  the  reverse  reaction  commencing. 
Commercial  carbide  is  usually  about  85  per  cent,  pure,  a  better  material 
than  Thompson's,  and  therefore  more  liable  to  be  decomposed  than 
the  latter.  But,  apart  from  any  interaction  with  CO,  there  is  no  doubt 
that  carbide  whilst  in  the  furnace  must  continually  dissociate  into 
its  elements,  giving  graphite  and  calcium  vapour  (equation  &).  The 
latter,  as  it  passes  away,  will  be  oxidised  by  the  CO,  giving  finely- 
divided  lime  and  carbon  dust  (equation  c),  which  will  be  largely  carried 
off  by  the  issuing  gases.  This  fact  is  responsible  for  a  large  proportion 
of  both  the  power  and  the  lime  losses.  If  carbide  were  not  formed 
in  the  arc  far  more  quickly  than  it  dissociates,  there  would  be  none 
at  all  produced  in  the  furnace.  It  should  be  added  that  a  certain 
re  -formation  of  carbide  also  probably  takes  place  in  the  melt  itself 


472    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

between  the  graphite  resulting  from  the  dissociation  and  the  excess  of 
lime  always  present. 

Heated  CaC2  can  also  undergo  another  change.1  If  kept  at  1000° 
for  some  time  it  slowly  separates  carbon  (its  colour  turning  from  grey 
to  black),  and  apparently  forms  a  lower  carbide.  This  new  product 
gives  no  acetylene  with  water,  but  can  absorb  nitrogen,  forming 
cyanamide.  The  change  is  accelerated  by  the  presence  of  certain  sub- 
stances such  as  CaF2,  etc.  No  equilibrium  is  observed.  This  change, 
which  has  not  yet  been  fully  studied,  is  obviously  of  the  greatest  interest. 
Apart  from  other  reasons,  it  is  probable  that  it  forms  a  preliminary 
stage  in  the  conversion  of  carbide  to  cyanamide  by  nitrogen.2 

In  calculating  the  theoretical  energy  expenditure  for  the  production  of  one 
ton  of  carbide,  we  will  assume  with  Haber  that  it  is  tapped  at  2000°,  and  that 
the  gases  leave  the  furnace  at  2500°.  Then  the  energy  necessary  to  produce  64 
kilos,  of  carbide  is  the  sum  of 

(a)  The  energy  necessary  at  room  temperature  for  the  reaction 

CaO  +  3C >  CaC2  +  CO 

using  amorphous  carbon ; 

(b)  The  energy  necessary  to  heat  64  kilos,  of  carbide  from  0°-2000°. 

(c)  The  energy  necessary  for  the  heating  up  at  constant  pressure  of  one  gram, 
molecule  CO  from  0°-2500°. 

We  have— 

(a)  114,550  Cals. 

(6)  The  specific  heat  of  CaC2  between  0°-2000°  can  be  taken  roughly  as  0'28. 
From  that  we  calculate  an  expenditure  of  0'28  x  64  x  2000  =  35840  Cals. 

(c)  Cp  (mean)  for  CO  between  0°-2500°  is  about  7*4.  The  necessary  heat 
expenditure  is  therefore  7'4  x  2500  =  18500  Cals. 

A  total  of  169,000  Cals.  is  therefore  necessary  for  the  production  of  64  kilos. 
of  carbide.  We  have  then — 

169000  x  1000  x  4-19 

1  ton  requires  64^3600 K'W'H- 

=  3100  K.W.H. 


2.  Carbide.    Technical3 

Raw  Materials.— These  are  essentially  lime  and  carbon.  The 
question  of  their  purity  is  important.  If  they  contain  too  large  quan- 
tities of  impurities,  then,  with  small  furnaces,  thick  crusts  are  formed 
which  are  a  great  hindrance  to  regular  and  effective  working.  With 
larger  furnaces  a  viscous  melt  results,  not  easily  tapped.  But  usually 
all  the  impurities  dissolve  in  the  carbide,  and  the  question  becomes 
—which  of  these  lower  the  value  of  the  product  ? 

1  Erlwein,  Warth,  and  Beutner,  Zeitsch.  Elektrochem.  17,  177  (/•'>//). 

2  P.  481. 

3  Many  details  in  this  section,  particularly  those  dealing  with  the  charge  and 
with  the  large  three-phase  furnaces,  have  been  taken  from  Helfenstein's  article 
in  Aakenasy'8  AY////V///-///I,/  in  die  techni&che   Elektrochemie,  vol.  i.  (1!)10).      See 
Also  Conrad,  Klictnu- //»///.  //,//.  6,  \VM 


xxv.]  CALCIUM  CARBIDE  473 

It  is  of  prime  importance  that  the  phosphorus  content  should  be 
at  a  minimum.  Subjected  to  the  strongly  reducing"  action"  of  the 
carbide  furnace,  any  phosphorus  present  will  be  as  phosphide,  from 
which  water  liberates  PH3.  It  is  well  known  that  in  the  impure  state 
this  gas  is  spontaneously  inflammable,  and  there  is  little  doubt  that 
many  of  the  earlier  explosions  with  acetylene  lamps  and  generators 
were  caused  by  it.  Less  likely  to  be  present,  but  equally  harmful, 
is  arsenic.  Sulphur,  though  not  dangerous,  should  also  be  avoided. 
These  impurities  are  likely  to  occur  in  the  raw  materials  as  Ca3(P04)2, 
CaS04,  pyrites,  arsenical  pyrites,  etc.,  and  must  not  exceed  a  certain 
low  limit.  From  the  point  of  view  of  regularity  and  ease  of  furnace 
working,  both  magnesia  and  alumina  must  be  kept  low,  as  it  is  they 
which  thicken  the  carbide  and  form  crusts.  Silica,  alkali,  iron,  etc,  the 
other  constituents  of  the  lime  or  of  therfuel  ash,  are  of  quite  minor 
importance. 

The  lime  used,  besides  being  of  good  quality  chemically,  must 
also  have  a  certain  mechanical  strength,  and  not  tend  to  crumble  into 
dust.  The  carbon  can  be  used,  as  in  the  ferro-silicon  furnace,  as  either 
charcoal,  coke,  or  anthracite.  Charcoal  is  usually  more  expensive  and 
a  larger  quantity  is  needed,  but  its  purity  and  voluminous  structure 
render  it  the  best  form.  Coke  is  often  used,  but  its  high  ash  content 
is  a  disadvantage.  Anthracite  is  perhaps  most  often  employed.  It 
can  be  got  with  a  low  ash  content  (3  per  cent.),  and  fairly  free  from 
sulphur  and  phosphorus.  Its  dense  structure  renders  it  less  reactive 
than  charcoal  or  coke,  and  though  this  decreases  the  combustion  losses, 
it  is  otherwise  not  an  advantage  in  the  furnace  process.  Lime  and 
fuel  are  mixed  and  charged  in  pieces  up  to  a  few  inches  diameter, 
depending  on  the  size  of  the  furnace. 

Furnaces. — Carbide  furnaces  are  essentially  of  two  types,' those  in 
which  the  carbide  leaves  the  furnace  as  a  solid  block,  and  those  from 
which  it  is  tapped  as  liquid.  Considerable  difficulties  retarded  the 
development  of  this  second  type  of  furnace,  owing  to  the  hardness  of 
the  solid  carbide,  its  high  melting-point,  and  the  viscosity  of  the  fused 
mass.  These  have  now  been  overcome,  and  the  disappearance  of  the 
'  block '  furnaces  is  only  a  matter  of  time.  All  carbide  furnaces  are 
arc  furnaces.  Resistance  heating  also  takes  place,  its  extent  depending 
on  the  conductivity  of  the  carbon  used,  and  current  and  voltage  are 
somewhat  affected  thereby.  But  the  essential  part  of  the  heating  comes 
from  the  arc.  Coke-fed  furnaces,  because  of  the  higher  conductivity 
of  the  charge,  have  more  resistance  heating  than  others. 

The  simplest  type  of  carbide  furnace  is  the  low  power  discontinuous 
block  furnace  with  hearth  electrode,  still  much  used  in  France.  A 
box  (Fig.  121)  of  iron  plate  mounted  on  wheels  is  lined  with  electrode 
carbons.  The  bottom  of  this  lining  is  connected  electrically  with  one  of 
the  terminals  of  the  generator  or  transformer,  and  the  current  passes 


474    PKTNCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

through  it,  through  the  layer  of  carbide  in  the  crucible,  and  then  as 
an  arc  to  the  carbon  electrode,  fed  in  from  above.  The  furnace  is 
provided  on  top  with  a  funnel-shaped  sheet-iron  extension,  lined  with  a 
refractory.  The  charge  is  shovelled  into  this  and  heaped  up  round  the 
electrode.  During  a  run  the  electrode  must  be  gradually  raised  as 
the  carbide  collects  in  the  crucible,  and  this  is  done  by  hand.  When 
sufficient  material  has  collected  (the  furnace  can  be  almost  filled), 
the  current  is  stopped,  the  hearth  electrode  disconnected,  the  furnace 
wheeled  away,  and  the  whole  allowed  to  cool  for  a  few  hours.  The 
block  formed  is  then  lifted  out,  and,  after  further  cooling,  separated 
from  the  large  quantities  of  adhering  lime  and  carbon,  broken  up  and 
packed. 

Such  a  furnace  does  not  consume  much  power.  The  first  ones  made 
only  took  100  K.W.  King  describes  early  furnaces  at  Niagara  working 
with  65-70  volts  and  1,700-2,000  amperes.  The  units  now  generally 
used  are  of  200-250  K.W.,  requiring  40-70  volts.  Several  such  furnaces 

are  connected  up  and  run  together. 
The  block  produced  can  weigh  up  to 
400  kilos.,  and  is  taken  out  after  a  four- 
hour  run.  To  a  very  great  extent  (50 
per  cent,  or  more)  it  consists  of  substances 
other  than  carbide,  chiefly  lime  and 
fuel,  but  also  containing  crusts  of  im- 
purities and  half-converted  material. 
The  removal  of  these  impurities  is  a 
great  source  of  loss  in  the  block  process. 
As  far  as  possible,  of  course,  all  is  re- 

turned  tothe  furnace'  but 


F,o.  121.-Carbide  Furnace.  ., 

Block  Type.  away  or  is  lost   as   dust.      Then  large 

quantities  of  fuel    are   burnt  away  and 

lime  vaporised  during  the  process  itself.  We  must  further  add 
the  lime  losses  due  to  dissociation  of  CaC2.  The  heat  losses  are  high, 
both  because  of  the  small  sizfe  of  the  furnace  and  because  of  the  fact 
that,  after  each  block  has  been  taken  out,  the  furnace  itself  and 
the  unchanged  material  removed  from  the  block  must  both  be 
reheated.  There  are  usually  further  large  losses  resulting  from  bad 
electrical  contacts,  particularly  at  the  hearth  electrode,  which  is 
disconnected  after  each  run. 

In  view  of  the  above  it  is  not  surprising  that,  for  the  production 
of  1  ton  of  85  per  cent,  commercial  carbide,  0-8-1-5  tons  of  fuel 
(depending  on  its  nature)  and  1-08-1-2  tons  of  lime  are  necessary 
(Helfenstein).  The  theoretical  figures  are  0-75  ton  lime  and  0-48  ton 
carbon,  assuming  that  all  the  energy  for  the  reaction  is  supplied  by  the 
current.  The  energy  consumption  amounts  to  about  6-7  K.W.H. 
per  kilo,  of  85  per  (cut.  carbide.  Tlir  energy  efficiency  is  therefore 


XXV.] 


CALCIUM  CARBIDE 


475 


3*1 

85  X      _  =  40  per  cent.     Apart,  however,  from  low  energy  efficiency 

and  high  consumption  of  raw  materials,  the  discontinuous  block 
process  has  other  disadvantages.  Of  these  the  chief  is  the  high  labour 
charge  entailed  in  the  frequent  uncoupling  of  the  furnaces,  removal 
and  cleaning  of  the  blocks.  The  output  of  the  plant  is  also  small 
for  its  size,  and  the  whole  method  of  working  cumbersome.  On  the 
other  hand,  the  furnace  itself  is  simple  and  easy  to  repair,  and,  if  the 
power  available  happens  to  be  very  small,  is  the  only  kind  which 
can  be  used. 

Continuous  Block  Furnace. — The  Union  Carbide  Co.  works  a 
continuous  block  process  at  Niagara,  employing  the  Horry  furnace 
(Fig.  122).  In  appearance  this  furnace  resembles  a  large  vertical 
wheel.  It  consists  essentially  of  two  broad  disc-rings  of  sheet-iron. 
These  are  placed  in  a  vertical  position,  parallel  to  one  another  and 


FIG.  122. — Horry  Carbide  Furnace.     Continuous  Block  Type. 

3'  apart.  By  means  of  two  drums,  AA'  and  BB',  on  the  circumferences, 
these  are  combined  together  into  a  ring-shaped  box.  The  inner  drum 
BB'  is  continuous  and  connected  with  the  axle  on  which  the  furnace 
revolves.  The  outer  drum  is  not  made  in  one  piece,  but  consists  of  a 
number  of  segmental  plates  about  V  in  length  (and  naturally  3'  broad). 
These  are  bolted  on  to  the  outside  edges  of  the  two  rings.  A  fixed 
vertical  shaft  C  fits  into  the  box  tangentially  at  one  of  its  horizontal 
diameters.  It  is  clear  that  when  the  furnace  revolves  (in  the  direction 
indicated  by  the  arrow)  the  plates  forming  the  outer  periphery  of  the 
box  must  be  removed  before  the  corresponding  segment  comes  opposite 
to  the  shaft.  In  practice  these  plates  are  only  kept  bolted  on  to  what 
is  at  any  time  the  lower  half  of  the  furnace,  the  upper  half  being 
always  open. 


476    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY     [CHAP. 

Two  electrode  bundles  are  placed  in  the  shaft  C,  and  the  raw 
materials  charged  in  above.  The  current  passes  from  one  electrode 
to  the  surface  of  the  carbide  and  back  to  the  other  electrode.  On 
starting  the  current  and  slowly  revolving  the  furnace,  a  continuous 
block  of  carbide  (sausage-shaped)  is  produced  in  the  middle  of  the  box, 
surrounded  with  unchanged  raw  material.  By  the  time  it  has  reached 
the  other  side  of  the  furnace  it  is  sufficiently  cooled,  and  is  withdrawn. 
During  the  process  the  outer  segmental  plates  must  be  continually 
bolted  on  at  D  and  taken  off  at  E.  The  regulation  and  rotation  of 
the  furnace  are  effected  by  a  screw  working  in  the  cogwheel  F,  and 
controlled  automatically  by  voltage  fluctuations  in  the  hearth. 

According  to  Haber,1  the  furnace  revolves  once  in  three  days.  It  is 
fed  with  110  volts  and  consumes  150-250  K.W.  The  electrodes  are 
each  a  bundle  of  four  rods  (1  m.  X  10  cm.  X  10  cm.).  The  current 
density  in  the  carbon  is  about  5  amps. /cm.2  J.  W.  Richards  2  states 
that  the  load  is  375  K.W.,  and  that  2  tons  carbide  are  produced  daily. 
This  means  a  consumption  of  4-5  K.W.H.  per  kilo.,  a  far  better  result 
than  that  given  by  the  discontinuous  furnace. 

Tapping  Furnaces. — The  introduction  of  furnaces  from  which 
the  CaC2  could  be  tapped  as  liquid  was  rendered  difficult  by  the  very 
high  temperature  necessary  before  the  carbide  becomes  really  fluid,  not 
viscous.  Most  early  attempts  failed.  The  melt  flowed  out  with  great 
difficulty,  and  the  tapping-hole  quickly  became  choked  with  a  hard 
mass  of  solidified  carbide.  In  the  last  few  years  these  difficulties 
have  been  overcome.  The  first  step  was  a  large  increase  in  the  power 
consumed  by  the  furnace.  The  200-250-K.W.  unit  of  the  discontinuous 
block  type  was  doubled,  and  then  still  further  increased.  The  effect 
was  to  raise  the  temperature,  to  increase  the 'fluidity  of  the  melt,  and 
to  prevent  the  formation  of  crusts.  The  second  improvement  consisted 
in  a  modified  method  of  tapping.  The  tapping-hole  was  made  larger 
than  before,  and  closed  with  a  large  iron  plug  whilst  the  carbide  was 
still  flowing  freely  out.  The  formation  of  a  thick  resistive  wall  of 
solidified  carbide  was  thus  prevented.  Further,  when  necessary,  the 
Helfenstein  method  of  tapping  with  the  aid  of  an  auxiliary  electrode  3 
was  adopted. 

By  these  means,  and  by  the  general  use  of  purer  raw  materials, 
the  tapping  of  liquid  carbide  has  been  rendered  easy,  considerable 
advantages  being  thereby  secured.  There  is  a  great  saving  in  labour. 
Tin-  furnaces  are  no  longer  uncoupled  every  few  hours,  and  there  is  no 
block  to  be  cleaned.  Then  there  is  a  considerable  economy  in  heat 
and  material  losses.  The  furnaces  being  larger,  the  relative  heat  losses 
become  less.  And  after  each  batch  of  carbide  has  been  removed,  it  is 

.  KI,  Ltrcchem.  9,  3f,7  (/ 

,„/.  lt  -2'2  (/W-').  :1  See    ).  -Hi7. 


XXV.] 


CALCIUM  CARBIDE 


477 


no  longer  necessary  to  heat  up  the  furnace  and  a  large  quantity  of 
raw  material  which  has  already  been  once  heated.  The  wastage  of 
carbon  and  lime  resulting  from  the  cleaning  of  the  blocks  is  avoided, 
and  the  losses  in  the  furnace  due  to  burning  and  vaporisation  are 
relatively  less. 

We  can  distinguish  two  types  of  furnace  from  which  the  carbide  is 
tapped,  those  with  a  hearth  electrode  and  one  electrode  only  above,  and 
the  larger  new  three-phase  furnaces  with  either  three  or  six  distinct 
arcs.  Of  the  former  type  the  Bullier  furnace  was  one  of  the  earliest, 
and  of  similar  design  to  the  simple  ferro-silicon  furnace  of  Fig.  118. 
A  unit  consumed  200-250  K.W.,  taking  about  3,000  amps,  at  an 
electrode  current  density  of  4-5  amps./cm.2  An  excess  of  lime  was 
used  to  lower  the  melting-point  of  the  product,  and  the  tapping 
difficulties  were  thus  evaded  at  the  cost  of  making  a  lower  grade 


FIG.  123. — Alby  Carbide  Furnace. 

material.  The  energy  consumption  was  6-7  K.W.H.  per  kilo.,  thus 
no  lo.wer  than  that  necessary  with  the  small  block  furnace.  But 
there  was  a  considerable  saving  of  raw  material. 

As  a  type  of  the  modern  furnace  with  hearth  electrode  the  Alby 
furnace  may  be  taken  (Fig.  123).  The  hearth  consists  of  a  steel 
grating  into  which  a  mixture  of  carbon  and  tar  is  compressed.  The 
electrical  contact  is  permanent  and  water-cooled.  The  sheet-iron 
side  walls  have  a  comparatively  thin  refractory  lining.  The  tapping- 
hole  is  in  one  of  the  end  walls.  A  single  unit  usually  consumes 
750-1,200  K.W.  At  Odda  *  the  furnaces  take  28,000  amps,  at  about 
50  volts.  The  electrode  bundle  consists  of  five  carbon  electrodes, 
each  of  section  40  X  40  cm.,  corresponding  to  an  electrode  current 
density  of  3*5  amps,  cm.2  The  furnaces  are  tapped  every  45  mins. 
into  special  ladles,  and  yield  carbide  at  an  expenditure  of 

1  Electrochem.  Ind.  7,  212,  309  (1909). 


478    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAL>. 

4*2-4-5  K.W.H.  per  kilo.  For  every  ton  of  product  some  900  kilos, 
of  lime  (poor  in  MgO  and  A1203)  and  600  kilos,  anthracite  (3  per  cent, 
ash)  are  used. 

The  large  three-phase  carbide  furnaces  are  very  similar  to  that  in 
Fig.  120.  They  have  been  built  for  loads  up  to  9,000  K. W.,  3,000  K.W. 
per  phase  and  per  hearth.  Above  that  limit  it  was  found  impossible 
to  go,  owing  to  heat  and  fumes,  but  a  larger  furnace  was  built  by 
including  two  separate  three-phase  systems  in  one  furnace  channel. 
The  resulting  unit  contains  six  electrode  bundles,  and  consumes  18,000 
K.W.  at  cos  6  =  0*8  (about).  Such  furnaces  consist  of  rectangular 
iron  boxes  lined  with  a  thin  layer  of  refractory,  and  with  a  hearth  of 
carbon  (water-cooled).  A  layer  of  solidified  carbide  coats  the  inside  of 
the  walls,  and  the  electrode  bundles  are  so  spaced  that  a  bridge  of 
solid  carbide  separates  them.  The  arcs  all  pass  between  electrode  and 
carbide.  Opposite  to  each  hearth  is  a  tapping-hole.  From  these  the 
carbide  is  tapped  into  open  cast-iron  ladles,  cooled  for  24  hours,  broken 
up  and  packed.  The  electrode  bundles,  which  may  each  carry  up  to 
45,000  amps.,  are  dimensioned  so  that  the  current  density  does  not 
exceed  6  amps./crn.2  x  The  electrode  holders  are,  of  course,  water- 
cooled. 

Such  furnaces  produce  carbide  at  an  expenditure  of  4-4*2  K.W.H. 
per  kilo.  Using  charcoal,  the  figure  may  even  fall  to  3*8  K.W.H. 

3*1 
per  kilo.,   corresponding  to  an  energy   efficiency  of    85  X     •    =  69 

o'o 

per  cent.  These  figures  are  better  than  those  given  by  other  types 
of  furnace.  The  consumption  of  lime  and  carbon  is  almost  the  same 
as  that  of  the  Alby  furnace.  The  chief  advantages  of  these  large 
furnaces  is,  however,  their  greater  simplicity  of  construction  and 
working.  With  the  introduction  of  the  Helfenstein  closed  furnaces,2 
these  advantages  will  be  still  further  extended. 

3.  The  Nitrogen  Problem  3 

The  last  decade  has  seen  an  enormously  increased  demand  for 
combined  nitrogen  in  available  form,  a  demand  resulting  from  the 
wider  appreciation  of  the  value  of  artificial  manures  for  crop-raising. 
The  only  two  sources  of  combined  nitrogen  for  this  purpose  until  very 
recently  have  been  the  Chile  deposits  [NaN03]  and  the  distillation  of 
coal  [(NH4)2S04].  The  world's  consumption  of  the  former  substance 
is  now  over  2,000,000  tons  per  annum,  of  the  latter  about  1,200,000 

1  Thus  a  7,000  H.P.  three-phase  furnace  takes  45,000  amps,  at  80-100  volts 
per  phase,  cos  B  being  about  0*75  and  the  electrode  current  density  5  amps./cm.J 

2  Fig.  47. 

a  Guye,  Jour.  8oc.  Chem.  Ind.  25,  567  (lim);  Haber,  Zeitsch.  Angew.  Chem. 
23,  684  (1910). 


xxv.J  CYANAMIDE  479 

tons,  and  the  demand  is  likely  to  continue  rapidly  to  increase.  On 
the  other  hand,  the  Chile  deposits  are  becoming  exhausted,  and,  even 
including  the  less  pure  material  which  has  yet  hardly  been  touched, 
will  probably  not  last  out  for  more  than  fifty  years,  assuming  a  normal 
rate  of  increase  of  consumption.  The  production  of  (NH4)2S04  is 
limited  by  the  coal  consumption,  and  although  far  more  nitrogen  might 
be  obtained  from  this  source  if  a  greater  proportion  of  the  coal  were 
gasified  in  low  temperature  gas  producers,  yet  the  amount  available 
will  never  be  capable  of  coping  with  the  present  increasing  demand. 
In  the  future  it  is  indeed  possible  that  the  enormous  dissipation  of 
the  combined  nitrogen  of  sewage  which  takes  place  may  be  checked,  but 
at  present  there  is  nothing  to  expect  in  that  direction. 

Under  these  circumstances  it  has  become  imperative  to  secure 
fresh  sources  of  combined  nitrogen.  The  atmosphere  furnishes  a 
practically  unlimited  supply  in  an  uncombined  state ;  the  problem  is 
therefore  the  conversion  of  this  nitrogen  into  a  useful  form.  Three 
methods  of  doing  this  are  in  the  field.  The  first,  which  has  been  worked 
technically  for  some  years  and  which  we  shall  presently  discuss, 
consists  in  separating  pure  nitrogen  from  air  and  allowing  this  tp 
act  on  CaC2,  when  calcium  cyanamide  (CaCN2)  results.  This  product 
can  either  be  used  directly  as  a  fertiliser,  or  its  nitrogen  readily 
converted  into  ammonia. 

The  second  method  has  been  recently  worked  out  by  Haber  and 
Le  Rossignol,  and  is  being  developed  by  the  Badische  Anilin  und  Soda 
Fabrik.  It  consists  in  the  direct  synthesis  of  ammonia  from  nitrogen 
and  hydrogen  at  relatively  low  temperatures  under  pressure,  using  a 
suitable  catalyst,  and  will  probably  assume  considerable  importance 
in  the  near  future. 

The  third  method  consists  in  passing  an  electric  discharge  through 
air.  Nitrogen  oxides  result,  and  by  absorption  in  water  or  in  alkaline 
solutions  are  converted  into  HN03,  nitrates,  or  nitrites.  We  shall 
consider  this  method  in  detail  in  Chapter  XXVII. 

4.  Cyanamide.    General :  and  Theory 

It  was  early  discovered  that,  on  strongly  heating  a  mixture  of 
carbon  and  an  alkali  or  alkaline  earth  in  nitrogen,  the  latter  was 
absorbed,  giving  cyanides  or  similar  substances  from  which  ammonia 
could  be  obtained.  Many  investigators  attempted  to  carry  out  this 
reaction  technically,  but  without  success.  Thus  Erlwein  heated  the 
charge  under  pressure  in  a  resistance  furnace.  At  best  he  got  a  product 
with  20  per  cent,  nitrogen,  but  generally  only  half  as  much  was  absorbed 
and  the  power  costs  were  high.  Frank  suggested  in  1895  the  action 
of  nitrogen  on  CaC2  previously  prepared,  and,  with  Caro,  succeeded 

1  Trans.  Farad.  Soc.  4,  99  (1908) ;  Zeitsch.  Angeu:  Chem.  22,  1178  (1909). 


480    PKINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

finally  in  developing  a  successful  commercial  process  on  these  lines. 
Finely-divided  technical  carbide  rapidly  absorbs  nitrogen  at  1000°, 
giving  calcium  cyanamide,  according  to  the  equation 


This  product  can  be  used  directly  as  a  fertiliser  with  great  success, 
except  in  certain  abnormal  soils.  Steam  gives  ammonia  almost 
quantitatively  (97  per  cent.);  and,  by  suitable  treatment,  cyanides 
and  products  such  as  urea  and  guanidine  salts  can  be  made.  The 
heating  necessary  during  the  nitrogen  absorption  was  at  first  carried 
out  in  gas-fired  retorts.  This  method  has  now  been  replaced  by 
electric  heating. 

The  chemistry  of  the  relations  between  CaC2,  CaCN2,rand  connected 
substances  is  complex,  and  by  no  means  yet  cleared  up,  in  spite  of  the 
work  recently  devoted  to  the  subject.  Two  formulae  have  been 
proposed  for  CaCN2,  viz.  : 

Ca  =  N-C^N, 
and 


The  first  corresponds  more  closely  to  the  name  of  the  substance,  the 
latter  renders  its  relation  to  Ca(CN)2  more  apparent.  Thus  we  can 
write  the  general  equation 


where  R  is  an  atom  of  a  monovalent  (alkali)  metal  or  a  half-atom  of  a 
divalent  (alkaline  earth)  metal. 

According  to  this  equation,  carbon  and  a  cyanamide  can  react, 
giving  a  cyanide,  and  vice  versa.  It  is  found  experimentally  that,  if 
R  be  an  alkali  metal,  the  right-hand  side  of  the  equation  is  favoured. 
Thus  CaCN2  heated  with  carbon  and  NaCl  or  KC1  as  a  flux  gives  large 
quantities  of  NaCN  or  KCN.  If  CaCl2  be  used  instead,  only  quite 
small  quantities  of  Ca(CN)2  result.  The  equilibrium  in  that  case  lies 
over  in  favour  of  the  left-hand  side  of  the  equation.  A  barium  salt 
gives  a  mixture  of  cyanide  and  cyanamide  in  more  or  less  equal  pro- 
portions. In  all  cases  the  higher  the  temperature  the  larger  the 
proportion  of  cyanide. 

Commercial  CaC2  and  BaC2  commence  to  absorb  nitrogen  appre- 
ciably at  about  700°.  CaC2  gives  almost  entirely  cyanamide,  BaCa 
also  large  quantities  of  cyanide,  corresponding  to  the  above  statement. 
Until  quite  recently  it  was  assumed  that  the  reaction  between  the 
nitrogen  and  the  carbide  took  place  according  to  the  equation 

CaC2-f-N2  —  *CaCN2  +  C 
without  the  formation  of  any  intermediate  product.     On  that  view 


xxv.]  CYANAMIDE  481 

there  should  be  a  definite  nitrogen  equilibrium  pressure  corresponding 
to  each  temperature.  If  that  be  exceeded,  any  carbide  present  must 
be  azotised  ;  if  the  pressure  be  kept  below  that  value,  cyanamide 
and  carbon  must  react  (or  tend  to  react)  and  reproduce  carbide  and 
nitrogen.  As  the  absorption  of  the  nitrogen  is  strongly  exothermic, 
rise  in  temperature  should  favour  the  left-hand  side  of  the  equation. 

According  to  Caro,1  the  equilibrium  temperature  corresponding  to 
one  atmosphere  pressure  is  1360°.  Matignon 2  calculates  it  under 
certain  assumptions  to  be  1100°.  Attempts  have  been  made  to 
measure  this  equilibrium,  e.g.  by  Le  Blanc  and  Eschmann.3  The 
investigators  have  attributed  their  non-success  to  a  variety  of  causes  ; 
viz.  (a)  disturbance  of  the  equilibrium  by  sublimation  of  the  cyanamide  ; 
(6)  gradual  alteration  in  the  properties  of  the  carbon  present  ('  ageing  ')  ; 
(c)  impurities  which  may  take  part  in  the  reaction  ;  (d)  chemical 
inertness  of  the  nitrogen.  But  the  results  of  Erlwein,  Warth,  and 
Beutner  4  indicate  a  still  more  fundamental  cause,  i.e.  that  the  above 
equation  does  not  fully  express  the  azotisation  of  CaC2.  Two  con- 
secutive reactions  are  probably  involved,  the  decomposition  of  the  CaC2 
by  heat  into  carbon  and  a  lower  carbide,  and  the  absorption  of  nitrogen 
by  the  latter.  Assuming,  for  example,  CaC  to  be  formed,  we  should 
have 

(i)  CaC, — >CaC+C; 

(ii)  CaC  +  N, — >  CaCN2. 

Any  equilibrium  involving  nitrogen  would  then  correspond  to  equation 
(ii),  and  it  is  clear  that  its  investigation  must  be  preceded  by  a  thorough 
study  of  the  change  undergone  by  CaC2  when  heated. 

It  is  true  that  Thompson  and  Lombard 5  claim  to  have  measured 
true  equilibrium  values  (1  atmosphere  at  1700°  [extrapolated]).  But 
it  is  an  unlikely  assertion.  The  straight  line  relation  they  give  con- 
necting pressure  and  temperature  between  1050°-1450°  necessitates 
an  enormous  and  improbable  variation  of  heat  of  reaction  with 
temperature,  and  their  criteria  of  equilibria  are  not  quite  free  from 
objection.  Further,  one  interesting  experiment  confirms  the  work 
of  Erlwein,  Warth,  and  Beutner.  A  sublimed  sample  of  cyanamide 
was  mixed  with  graphite  and  heated.  Much  gas  (assumed  to  be 
nitrogen)  was  given  off  at  1287°.  But  no  carbide  resulted.  They 
suppose  oxygen  to  have  accidentally  entered  their  furnace.  It  is 
more  likely  that  the  cyanamide  simply  decomposed  thus  : 

CaCN2 — ^CaC-f-N2. 
Carbide  would  first  be  formed  at  a  far  higher  temperature. 

1  Zeitsch.  Angew.  Chem.  22,  1178  (1909). 

2  Ann.  Chim.  Phys.  [viii],  14,  51  (1908). 
J  Zefoch.  Eleldrochem.  17,  20  (1911). 

4  Zf  it  *<•!>.  El'kirochem.  17,  177  (1911),  and  p.  472. 

5  Metatt.  Chem.  Encjin.  8,  617,  682  (1910). 

2  i 


482    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP! 


1100 


Time  la  Hours. 

FIG.  124. 


We  must  next  deal  with  the  dynamics  of  the  reaction.  Moissanj 
showed  that  pure  CaC2  is  quite  inert  towards  N2  at  1200°,  and  that  pure! 
BaC2  only  absorbs  traces.  That  commercial  CaC2  absorbs  nitrogen] 
comparatively  easily  and  below  1200°  is  due  to  the  fact  that  it  contains! 
10-15  per  cent,  lime  and  other  impurities.  But  even  then  the  action! 
is  not  rapid.  Foerster  and  Jacoby 1  investigated  the  rate  at  which  the] 
gas  is  absorbed  at  different  temperatures,  and  Fig.  124  contains  theiii 

results.      The    maximum     nitrogen! 
content   possible    with    the    sample! 
of  carbide  used  was  25*5   per  cent.! 
One  notices   that   the    temperature! 
must   be    at   least    1000°   for   com-J 
plete    and    rapid     absorption.      At 
lower     temperatures     an     apparent] 
saturation  limit  is   reached,  beyond 
which    the   percentage    of   nitrogen] 
very   slowly   rises.     But   these    are] 
not    true     equilibria,    for     carbide 
azotised  at  1100°  loses  no  nitrogen! 
when  heated  for  four  hours  at  900°.] 
The  above  absorption  velocities  hold] 

for  nitrogen  at  atmospheric  pressure.  The  rate  of  azotisation  is  pro-J 
portional  to  the  pressure  up  to  about  two  atmospheres  ;  above  thatl 
limit,  further  increase  of  pressure  makes  little  difference.2 

Under  technical  conditions,  using  large  quantities  of  carbide,  the 
heat  liberated  during  the  initial  stages  of  the  absorption  serves  to! 
heat  the  charge  up  to  1000°-!  100°,  i.e.  to  the  temperature  necessary! 
for  satisfactory  absorption.  A  product  with  20-22  per  cent,  nitrogen! 
usually  results.  External  heating  is  required  only  for  raising  the  charge! 
to  the  initial  reaction  temperature,  and  for  compensating  subsequent! 
conduction  and  radiation  losses.  It  has  been  found  that  various! 
substances  admixed  with  the  carbide  are  capable  of  lowering  this! 
minimum  initial  reaction  tempeiature,  and  their  technical  use  has  been! 
proposed.  It  was  claimed  that,  as  the  reaction  takes  place  at  a  loweil 
temperature,  the  heat  losses  during  the  azotisation  would  be  less,  and] 
less  energy  would  be  required  for  the  initial  heating  up  of  the  charge.! 
The  best  known  of  these  additions  are  CaCl2  (Polzenius)  and  CaFJ 
(Carlson). 

This   subject   has   been  investigated  in  detail  by  a   number  of 
workers,  including  Bredig,  Fraenkel,  and  Wilke,3  Foerster  and  Jacoby,4] 
and  Rudolphi.6    A  very  large  number  of  substances  have  been  found  to 

1  Zefach.  Elektrochem.  13,  101  (/.'*//). 

•  Pollacci,  ZeitAch.  Elektrochem.  14,  "><>.">  ( 1W8). 

3  Zeitsch.  Elektrochem.  13,  69,  605  (/M7). 

4  Loc.  cit.,  and  Zeitsch.  Elektrochem.  15,  820 
6  Zeilsch.  Anorg.  Chem.  54,  170  (/M7). 


xxv.]  CYANAMIDE  483 

thus  facilitate  azotisation.  Such  are  CaO  (which  accounts  to  a  large 
extent  for  the  properties  of  the  technical  carbide),  CaC03,  Na2C03, 
NaCl,  LiCl,  A1C13,  K2C03,  etc.,  etc.,  as  well  as  CaCL,  and  CaF2.  With 
the  exception  of  LiCl,  CaCL,  is  by  far  the  most  effective  addition. 
Thus,  Bredig  heated  a  sample  of  carbide  at  800°  for  two  hours  and 
obtained  the  following  results  : 

TABLE  LXVIII 

Addition.  Per  cent,  nitrogen  absorbed. 

3'2 

10  per  cent.  CaO  4'0 

10  per  cent.  NaoCO;,  8'5 

10  per  cent.  NaCl  13 

10  per  cent.  Cad*  22 

Rudolphi  found  that  a  carbide  which  only  took  up  1*4  per  cent,  nitrogen 
during  two  hours  at  800°  absorbed  13'05  per  cent,  when  18*7  parts 
CaCl2  were  added  to  62  parts  CaC2.  Foerster  showed  that,  under  his 
working  conditions,1  CaC2  with  15  per  cent.  CaCl2  absorbed  in  two  hours 
at  700°  seven  times  as  much  nitrogen  as  pure  CaC2  did  at  800°.  At 
900°  the  effect  of  10  per  cent.  CaF2  during  a  two-hours  experiment  was 
equal  to  that  of  15  per  cent.  CaCi2  at  800°. 

The  effect  of  the  quantity  added  is  very  irregular  in  the  initial  stages 
of  the  absorption.  With  CaCl2,  the  maximum  effect  at  800°  is  produced 
with  30  per  cent.  CaCl2  in  the  reaction  mixture  ;  with  CaF2  the  best 
proportion  is  5  per  cent,  at  800°  and  2  per  cent,  at  900°.  With  K2C03 
the  best  effects  are  produced  with  4  per  cent,  added.  But  in  some 
cases  (e.g.  CaF2 — Foerster)  these  differences  disappear  in  course  of  time, 
the  tendency  being  for  a  constant  percentage  of  the  carbide  to  be 
azotised,  whatever  the  amount  of  addition  present.  This  only  deter- 
mines the  velocity  of  absorption.  The  final  limit  differs  in  different 
cases.  Thus,  with  CaF2,  the  limit  at  860°  is  14*9  per  cent,  nitrogen,  as 
compared  with  about  5  per  cent,  without  the  CaF  . 

The  ultimate  cause  of  the  effect  produced  by  these  additions 
(including  the  lime  always  present  in  commercial  carbide)  appears  to 
be  connected  with  the  lowering  of  melting-point  brought  about.  Other 
possible  explanations  were  eliminated  by  Bredig  and  his  co-workers. 
Both  with  and  without  any  special  addition,  the  azotised  mass  is 
found  to  have  sintered  (i.e.  to  have  undergone  surface  fusion).  The 
immediate  cause  is  more  obscure,  but  is  probably  connected  with  the 
rate  of  decomposition  of  the  CaC2  to  carbon  and  that  lower  carbide 
which,  as  we  have  seen,  is  probably  the  substance  that  actually 
absorbs  the  nitrogen. 

It  has  already  been  indicated  that  technically  these  additions  are 

1  Differences  in  composition  of  starting  material  and  in  degree  of  fineness 
of  division  account  for  the  discrepancies  between  the  results  of  the  different 
experimenters. 

2  i  2 


484    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

not  used.  Foerster  and  Jacoby  were  able  to  show  that,  in  the  case  of 
CaCl2,  though  the  reaction  commenced  at  a  lower  temperature,  the 
heat  liberated  soon  raised  the  mass  to  1000°.  Thus  the  gain  would 
be  limited  to  the  small  amount  of  energy  required  to  heat  up  the  charge 
to  the  reaction  temperature.  But  with  CaF2  the  reaction  took  place 
more  slowly  and  the  temperature  remained  at  about  900°.  The  heat 
losses  would  in  that  case  be  somewhat  lower,  but  it  is  doubtful  whether 
the  advantage  would  amount  to  much.  The  energy  consumed  in  the 
cyanamide  furnaces  is  small  in  any  case. 

5.  Cyanamide.    Technical1 

The  published  details  concerning  this  process  are  somewhat  scanty. 
The  carbide  is  first  finely  ground  in  an  air-tight  apparatus  filled  with 
pure  dry  nitrogen,  and  then  transferred  into  a  number  of  drum-shaped 
retorts.  These  are  of  iron,  take  from  300-500  kilos,  carbide,  and  are 
provided  with  a  central  resistance  in  the  shape  of  a  carbon  rod  (or 
several  in  series).  These  retorts  are  now  placed  in  position  in  the 
'  furnace,'  which  consists  of  a  gas-tight  horizontal  channel,  the  roof 
of  which  can  be  removed  to  allow  of  the  handling  of  the  retorts,  and 
the  carbon  resistances  of  the  latter  are  connected  electrically  to  suitable 
permanent  terminals  in  the  furnace.  Pure  nitrogen  is  then  admitted 
into  the  channel  under  a  slight  plus  pressure  in  order  to  overcome  any 
tendency  of  air  to  enter.  This  nitrogen  is  usually  prepared  by  fractional 
distillation  of  liquid  air  (Linde  process),  but  can  also  be  made  by 
passing  air  over  heated  copper.  It  must  be  as  free  as  possible  from 
02,  H20,  CO,  C02.  Thus  02  should  on  no  account  exceed  0'4  per  cent. 
These  substances  act  on  the  carbide  and  lower  the  nitrogen  content 
in  the  final  product. 

\Vht-n  all  air  has  been  expelled,  the  current  (usually  alternating)  is 
switched  in.  Each  retort  takes  some  65-75  volts.  It  is  probable  that 
the  amperage  is  low  (e.g.  20  amperes),  and  that  a  number  of  units  are 
connected  together  in  parallel.  The  process  lasts  30-40  hours,  longer 
with  th<>  larger  units.  During  this  time  the  temperature  rises  to  about 
1000°.  When  this  value  has  been  reached  in  the  outermost  layers  of 
the  retort  (i.e.  those  furthest  from  the  resistance)  the  current  is 
stopped,  and  the  reaction  allowed  to  complete  itself.  The  end  of  the 
-s  is  indicated  by  a  manometer  in  tin-  nitrogen  pipe-line.  After 
cooling  the  retorts,  the  cokr-like  mass,  which  lias  somewhat  shrunk 
and  sintered  together,  is  crushed  and  packed.  It  usually  contains  20 
per  cent,  nitrogen,  sometimes  a  little  more,  sometimes  only  15  per  cent. 
if  the  quality  of  the  carbide  is  bad. 

The  use  of  the  small  units  described  is  necessary  because  of  the 

1  Zeitsch.  Elektrochrm.  15,  820  (1909)  ;  ElrcfrarJirm.  Intl.  7,  212.  :«)<».  ::•'.(> 
(1909)  ;  Metall.  Chem.  A'//j////.  9,  100  (1911). 


xxv.]  CYANAMIDE  485 

difficulty  of  otherwise  getting  a  reliable  product,  which  demands 
uniform  conditions,  particularly  of  temperature.  The  electric  method 
of  heating  avoids  the  difficulties  otherwise  arising  from  the  strong 
sintering  of  the  material,  particularly  its  adhesion  to  the  sides  of 
the  externally-heated  clay  retorts.  The  energy  consumption  is  small. 
According  to  Caro,  the  total  energy  necessary  for  the  fixation  of  one  ton 
of  nitrogen  by  this  method,  including  the  production  of  the  carbide, 
azotising,  machine  driving,  grinding  and  crushing,  mechanical  charging 
and  air  liquefaction,  amounts  to  <  3  H.P.  years.  The  share  of  the 
cyanamide  furnaces  in  this  amount  must  be  of  the  order  of  O'l  K.W.H. 
per  kilo,  of  product  (0'5  K.W.H.  per  kilo,  of  nitrogen). 


Literature 

Helfenstein.     Article    in    Askenasy's  Einfukrunrj  in  die  technische 
Elcktrochcmie,  vol.  i. 


CHAPTER  XXVI 
OTHER  ELECTROTHERMAL  PRODUCTS 

IN  this  chapter  we  shall  treat  certain  electrothermal  products  relatively 
less  important  than  those  already  discussed.     They  include  : 

(a)  Carborundum  and  allied  substances  ; 

(b)  Graphite; 

(c)  Alundum  :   fused  silica  ; 

(d)  Carbon  bisulphide  :  phosphorus  :  zinc. 

1.  Carborundum  and  Allied  Products 

When  silica  is  reduced  by  carbon  at  high  temperatures,  it  is  possible 
by  varying  the  conditions  to  obtain  a  number  of  different  products. 
We  can  distinguish  at  least  four  :  (a)  silicon  monoxide  ('  monox  '), 
SiO  ;  (6)  metallic  silicon  ;  (c)  silicon  carbide,  SiC  ('  carborundum  ')  ; 
(d)  an  oxycarbide  of  rather  indefinite  composition,  but  approximately 
Si2C20  ('  siloxicon  ').  Silicon  and  carborundum  are  technical  products, 
and  attempts  have  been  made  to  put  monox  and  siloxicon  on  the 
market. 

The  two  conditions  chiefly  determining  the  product  are  temperature 
and  the  composition  of  the  charge.  According  to  Greenwood,1  an 
intimate  mixture  of  Si02  and  carbon  first  evolves  CO  at  1460°,  and  in 
absence  of  further  knowledge  we  can  suppose  that  at  this  temperature 
the  reaction 


becomes  appreciable.  At  higher  temperatures  the  course  of  the 
reaction  depends  on  the  proportion  of  carbon  present.  If  this  is  not 
in  excess,  the  SiO  will  be  reduced  to  metallic  silicon  only  ;  witk 
sufficient  carbon  present,  siloxicon  or  carborundum  will  result.  The 
latter  can  also  directly  arise  from  the  action  of  silicon  vapour  on  carbon, 
and,  at  still  higher  temperatures,  will  dissociate,  liberating  its  silicon 

1  Elcctruchem  hid.  7,  111)  (HM). 
480 


CARBORUNDUM  487 

as  vapour  and  leaving  a  grpahite  residue.     The  corresponding  equations 
are 

(a)  Si02+ C^SiO  +  CO; 

(b)  SiO  +  C^±Si  +  CO; 

(c)  2SiO  +  3C  ^±Si2C20  +  CO  ; 

(d)  Si  +  C^SiC; 

(e)  SiO  +  2C^±SiC  +  CO; 

(d)  SiC  ^±  Si  +  C  (graphite). 

All  are  reversible.  CO  passed  over  silicon  or  carborundum  at 
suitable  temperatures  gives  carbon  and  silica.  Our  knowledge  of  the 
temperatures  corresponding  to  these  different  reactions  is,  however, 
scanty.  We  know  that  the  preparation  of  silicon  demands  a  furnace 
of  high  power  and  a  high  temperature,  exceeding  that  necessary  for 
the  production  of  50  per  cent,  ferro-silicon,  which  we  have  assumed 
to  be  tapped  at  1600°.  Larnpen  and  Tucker x  have  investigated  the 
temperatures  of  production  and  dissociation  of  carborundum.  At 
1600°  the  first  signs  of  siloxicon  formation  were  observed.  Between 
1600°  and  1900°  more  and  more  was  produced.  At  1920°  its  conversion 
to  carborundum  commenced,  and  was  complete  at  1980°.  The  longer 
the  charge  was  heated  and  the  higher  the  temperature,  the  larger  were 
the  crystals  produced.  Finally,  at  2220°  dissociation  into  graphite 
and  silicon  commenced,  and  was  complete  at  2240°.  Gillett 2  investi- 
gated the  matter  more  accurately.  He  gives  1540°  +  30°  and  1820° 
+  20°  as  the  temperatures  of  formation  of  siloxicon  and  carborundum 
respectively,  and  2220°  +  20°  as  the  temperature  of  dissociation  of 
the  latter.  The  first  two  values  are  lower  than  those  of  Lampen  and 
Tucker,  the  last  agrees  well.  These  figures  give  us  some  idea  of  the 
necessary  technical  conditions. 

Silicon  Monoxide  (Monox). — It  is  unnecessary  to  consider  this 
substance  at  length.  A  red- brown  powder,  its  use  as  a  pigment,  as 
a  reducing  agent,  and  for  polishing  purposes,  was  suggested  by  Potter, 
who  discovered  it.3  For  a  short  time  it  was  on  the  market,  but  met 
with  no  great  success,  and  its  technical  production  soon  ceased. 

Silicon.— The  production  of  this  substance  (95  per  cent,  pure) 
has  been  developed  both  by  the  ferro-silicon  and  the  carborundum 
manufacturers.  The  former  look  on  it  as  essentially  a  high-percentage 
ferro-silicon,  whilst  the  latter  regard  it  as  one  of  the  products  of  the 
reduction  of  silica  by  carbon.  Indeed  the  first  attempts  4  to  make  it 
took  place  in  a  resistance  furnace  (such  as  is  used  in  the  manufacture 
of  carborundum  (p.  489).  This  consisted  of  a  vertical  cylinder  of 

1  Jour.  Amer.  Chem.  Soc.  28,  850,  853  (1906). 
-  Jour.  Phrjs.  Chem.  15,  213  (1911). 

3  Trans.  Amer.  Electrochem.  Soc.  12,  223  (1907). 

4  Tone,  Trans.  Amer.  Electrochem.  Soc.  7,  243  (1905). 


488    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY     [CHAP. 

firebrick,  open  at  the  top,  and  containing  a  vertical  carbon  core  as 
resist er.  The  charge  was  a  thoroughly  incorporated  mixture  of 
5  parts  silver  sand  -f-  2  parts  finely-ground  coke.  It  is  stated  that 
various  silico-carbides  were  first  obtained  and  subsequently  decom- 
posed, giving  drops  of  silicon,  which  gradually  collected  in  the  bottom 
of  the  furnace,  from  which  the  melt  was  tapped  off.  If  this  view1 
is  correct,  it  follows  that  the  temperature  of  formation  must  have 
exceeded  2200°,  the  silicon  vapours  condensing  in  the  cooler  parts  of 
the  furnace. 

This  method  evidently  worked  unsatisfactorily,  as  we  learn  l  that 
the  Carborundum  Co.  now  uses  an  arc  furnace,  apparently  similar 
to  the  large  furnaces  used  in  Europe  for  the  same  purpose  and  for  the 
manufacture  of  high-percentage  ferro-silicon.2  The  unit  employed 
consumes  1200  H.P.  and  is  constructed  of  firebrick  lined  with  carbon. 
It  has  two  vertical  electrodes,  dipping  well  down  into  the  charge  of 
coke  and  sand.  A  pig  of  250-350  kilos,  is  tapped  every  few  hours. 
It  is  90-97  per  cent,  pure,  the  impurities  being  iron  and  aluminium, 
with  some  carbon.  The  yield  (per  K.W.H.)  is  small,  and  in  Continental 
practice  the  furnaces  are  larger — at  least  1500  H.P. — with  a  very 
high  electrode  current  density.  The  tapping  offers  difficulties,  and 
the  electrical  method  is  invariably  used.  The  product  is  a  dense 
crystalline  substance  with  a  dark  silver  lustre.  According  to  Tone 
it  melts  at  1430°,  but  at  this  temperature,  and  probably  for  a  good  deal 
higher,  it  will  certainly  be  very  viscous.  It  is  used  for  making  silico- 
steels,  etc. 

Carborundum.3— This  substance  is  the  most  important  of  the 
interaction  products  of  silica  and  carbon.  It  was  discovered  in  1891 
by  Acheson,  who  was  attempting  to  dissolve  carbon  in  molten  clay, 
hoping  that  on  cooling  it  would  crystallise  out.  Instead  of  diamonds 
he  obtained  hard  blue  crystals,  and,  believing  their  constituents 
to  be  carbon  and  alumina,  coined  the  name  '  carborundum  '  from 
*  carbon '  and  'corundum.'  The  product  was  quickly  marketed  as 
an  abrasive,  and  its  importance  in  this  direction  has  since  continually 
increased. 

Carborundum  is  prepared  by  heating  a  suitable  charge  of  carbon 
and  sand  in  a  resistance  furnace.  As  carbon  is  used  anthracite  or 
good  quality  coke,  containing  not  less  than  85  per  cent,  carbon.  The 
ordinary  impurities  do  not  seriously  affect  the  process,  but  sulphur 
is  unpleasant  on  account  of  the  S02  produced,  and  iron  and  aluminium 
appear  to  catalyse  the  dissociation  into  graphite  and  silicon.  Moisture 
is  ae  far  as  possible  avoided.  The  finely-ground  fuel  is  mixed  with 
silver  sand  (98-99'5  per  cent.  pure).  The  proportions  of  the  consti- 
tuents correspond  roughly  to  the  equation  Si02  -f  3C  — >  SiC  -f  2CO. 

1  Electrochem.  Ind.  7,  189  (1!H>!>).  "  P.  4Wi. 

;  7,  189(/W.'y). 


XXVI. J 


CARBORUNDUM 


489 


A  slight  excess  of  coke  is  used.     In  addition  some  sawdust  and  some 
salt  are  added.     The  purpose  of  the  sawdust,  which,  of  course,  soon 
lecomposes,  is  to  increase  the  porosity  of  the  charge  and  thus  allow 
he  evolved  CO  to  stream  away  more  easily.      The  salt  attacks  the 
dfs  of  some  of  the  metallic  impurities  and  forms  volatile  chlorides 
e.g.   FeCl3,  A1C13),  which  distil  away  from  the  reaction  zone.     One 
of  charge  contains 

(Fitzgerald)  (Tone) 

Ton  Ton 

Sand  0-522  0*544 

Coke  0'354  0'351 

Sawdust  0-106  0'070 

Salt  0-018  0-035 

The  construction  of  the  furnace  is  as  follows :  The  two  ends  are 
permanent  stout  brick  walls  (Fig.  125),  lined  with  refractory  car- 
borundum bricks.  Through  them  enter  the  electrode  bundles.  Each 
bundle  consists  of  a  number 
of  rectangular  carbon  blocks 
aaa,  firmly  gripped  in  an  iron 
frame  set  in  the  end  wall. 
Between  the  electrodes  are 
copper  strips,  which  pass 
through  the  iron  frame  and 
make  connection  externally 
with  the  cables.  As  the 
technique  of  making  large 
carbon  electrodes  improves, 
this  electrode  construction 
will  be  considerably  simplified. 
A  shallow  firebrick  channel 
connects  these  two  ends, 

and  the  sidewalls  consist  of  firebrick,  mounted  in  sections  in  curved  iron 
frames  (convex  side  outwards)  to  facilitate  their  easy  removal.  Like 
the  furnace  bed,  they  are  not  strongly  heated,  and  merely  serve  to  keep 
the  charge  in  position.  The  charge  c  is  filled  into  the  furnace  to  the 
lower  level  of  the  electrodes.  Then  the  heating  core  d  is  introduced  along 
the  length  of  the  furnace.  It  consists  of  granulated  coke,  the  pieces 
having  a  diameter  of  about  an  inch.  As  far  as  possible  the  material 
of  an  old  core  is  used,  which  very  largely  consists  of  graphite,  and  has 
therefore  a  much  higher  conductivity  than  fresh  coke.  To  secure 
good  contact  between  core  and  electrodes,  the  intervening  space  is 
filled  with  finely-divided  coke  powder  (e),  well  pressed  down.  Finally 
more  charge  is  heaped  in  all  round  (not  too  tightly,  as  gases  must 
escape),  and  the  core  is  covered  above  by  a  thick  layer  of  material. 
There  is  no  roof  of  any  kind.  In  consequence  of  the  curved  sides  the 
furnace  has  a  roughly  circular  cross-section. 


FIG.  125. — End  of  Carborundum  Furnace. 


490    PKINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

Furnaces  have  been  built  taking  up  to  2,000  H.P.  Their  dimensions 
and  the  relative  sizes  of  electrodes,  core,  etc.,  vary  correspondingly. 
A  2,000-H.P.  furnace  is  30'  long  and  10'  in  diameter.  The  core  is  3'  in 
diameter.  When  current  first  passes  the  resistance  of  the  furnace  is 
high.  A  2,000-H.P.  unit  requires  about  230  volts  (alternating  current), 
and  will  therefore  take  some  6,000  amperes.  As  the  core  heats  up  its 
resistance  decreases,  and,  the  furnace  being  worked  at  constant  power, 
voltage  will  fall  and  current  rise.  At  the  end  of  the  process  some 
20,000  amperes  will  be  flowing  at  75  volts.  Not  all  will  traverse  the  core 
— some  will  pass  through  the  graphite  and  carborundum  surrounding 
it.  In  a  2,000-H.P.  furnace  we  can  assume  the  maximum  core  current 
density  not  to  exceed  3  amps. /cm.2  In  the  electrodes  it  will  be  still 
lower,  and  this  fact,  together  with  the  absence  of  air,  accounts  for 
their  long  life.1 

These  relations  of  current  density,  size  of  furnace,  and  disposition 
of  charge  are  of  the  utmost  importance  in  determining  the  efficient 
working  of  the  furnace,  and  the  best  proportions  in  any  case  can  only 
be  decided  by  trial.  The  reason  is,  of  course,  that  the  current  density 
determines  the  temperature  and  the  rate  at  which  the  heat  travels 
outwards  from  the  core,  and  as  the  temperatures  of  formation  and 
dissociation  of  carborundum  lie  comparatively  close  together  (1820°- 
2220°)  the  limits  \vithin  which  current  density,  etc.,  can  be  varied  are 
narrow.  In  ideal  working  the  mass  of  charge  which  has  reached  the 
temperature  of  carborundum  formation  must  be  as  large  as  possible 
in  comparison  with  the  quantity  which  has  reached  the  temperature  of 
graphite  formation,  and  in  practice  it  is  found  that  if  the  current  varies 
beyond  the  limits  of  +3  per  cent,  of  the  most  favourable  value  the 
results  are  bad.  Collins  has  published  some  interesting  considerations 
on  this  point.2 

An  enormous  quantity  of  CO  is  liberated  during  the  process,  and  is 
burnt  at  the  sides  and  top  of  the  furnace.  At  the  same  time  a  consider- 
able contraction  takes  place,  and  Aniberg  states  that  to  allow  for  this 
it  is  usual  to  build  the  core  sloping  up  somewhat  from  the  two  electrodes 
towards  the  middle  of  the  furnace.  When  the  requisite  amount  of  energy 
has  been  passed  in,  the  current  is  cut  off  and  the  furnace  rapidly  dis- 
mantled. Immediately  surrounding  the  core  is  a  ring  of  graphite. 
The  carbon  has  been  first  converted  to  carborundum  by  the  vapours 
of  SiO  or  silicon,  and  this  has  subsequently  dissociated.  Then  comes 
a  layer  of  carborundum,  the  crystals  being  larger  the  nearer  they  are 
to  the  core,  and  this  passes  imperceptibly  into  a  region  of  siloxicon. 
Finally  there  is  the  half-converted  and  unchanged  material,  its 
amount  minimised  by  the  circular  furnace  cross-section.  The  outermost 
layer  of  this  is  caked  together  owing  to  the  fusion  of  the  salt  present, 

1  Their  connections  with  the  copper  leads  are  water-cooled. 

2  Trans.  Aincr.  Elcctrochem.  Soc.  9,  31 


xxvi.]  CARBORUNDUM  491 

much  of  which  has  distilled  out  from  the  interior  of  the  furnace.  The 
thickness  of  the  carborundum  layer  in  a  2,000-H.P.  furnace  is  some 
18"-20". 

The  different  products  are  carefully  separated,  and  the  carborundum 
is  broken  up,  sieved,  and  the  best  grades  washed  free  from  graphite, 
metallic  oxides,  etc.  Five  furnaces  are  worked  together,  being  at 
different  stages  of  heating,  charging,  discharging,  etc.  A  large  modern 
furnace  requires  some  8'5  K.W.H.  per  kilo,  of  purified  carborundum, 
and  produces  a  comparatively  small  proportion  of  siloxicon.  The 
smaller  earlier  furnaces  gave  less  favourable  results. 

We  can  calculate  the  theoretical  quantity  of  energy  necessary  for 
carborundum  production  as  follows.  The  equation  is 

Si02  +  3C — >SiC  +  2CO. 

We  have  [Si,OJ  =  180000  Gals. 

[Si,C]  =  2000  Gals. 
[C,0]  =  29200  Gals. 

Hence  the  production  of  one  kilo.-mol.  (40'3  kilos.)  of  carborundum  at 
room  temperature  requires  180000  -  2000  -  58400  =  119600  Gals.  We 
will  assume  that  the  mean  temperature  to  which  the  carborundum  is 
heated  is  2100°,  and  that  the  CO,  while  usefully  warming  the  next  layer 
of  charge,  cools  down  on  an  average  to  1400°.  (Although  it  doubtless 
leaves  the  furnace  at  a  much  lower  temperature,  much  of  the  heat  it 
gives  up  is  unutilised.)  Then  as  G  (mean)  for  carborundum  between 
0°-2100°  is  about  11 '3,  and  Cp  (mean)  for  CO  between  0°-1400°  about 
7*1,  we  have 

Heat  required  to  heat  up  carborundum 

=  11-3  X  2100  =  23700  Cals. 
Heat  required  to  heat  up  CO 

=  2  X  7'1  X  1400  =  19900  Gals. 
Hence,  total  amount  of  heat  required 

=  119600  +  23700  +  19900  =  163200  Gals., 
and  1  kilo,  carborundum  requires 

163200  X4'19  =  4-7  K.W.H. 


40-3  X  3600 

This  is  the  necessary  irreducible  minimum.  Under  technical 
conditions  there  are  radiation  losses,  losses  due  to  vaporisation  of 
salt,  reduction  of  impurities,  to  heating  up  unchanged  charge,  to 
producing  graphite  and  siloxicon,  to  heating  up  the  core,  and  also  in 
the  electrodes.  Further,  the  material  must  be  kept  at  a  high  tempera- 
ture for  a  long  time  to  facilitate  the  formation  of  large  crystals. 

Pure  carborundum  is  white.  The  commercial  material  has  gener- 
ally an  iridescent  surface  coloration,  due  to  a  trace  of  oxidation.  If 
this  be  removed,  the  colour  is  seen  to  be  usually  a  dark  green.  Its 


492     PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP 

exceedingly  resistive  chemical  properties,  particularly  to  oxygen.  are 
well  known.  It  is  practically  nnattacked  below  its  dissociation 
temperature.  All  acids  are  without  action  on  it.  Chlorine  attacks 
it  with  difficulty,  but  fused  alkalies  and  alkaline  carbonates, 
and  certain  metallic  oxides  and  salts,  such  as  PbCr04,  dissolve  it. 
Its  extreme  hardness  makes  it  a  very  important  abrasive,  both  tor 
grinding  and  polishing.  It  is  used  as  a  top-dressing  for  pavements 
and  stairs,  etc.  Its  indifference  to  oxygen  and  to  fused  metals  and 
its  infusibility  render  it  a  valuable  refractory. 

It  has  also  a  high  electrical  conductivity,  and  its  use  in  electrical 
cooking  apparatus,  etc.,  has  often  been  suggested.  Recently  a  special 
modification  suitable  for  this  and  other  purposes,  arid  called  silundum, 
has  been  prepared  by  Boiling.1  This  material  is  apparently  a  solid 
solution  of  silicon  in  carbon  of  variable  composition,  and  is 
formed  by  the  action  of  silicon  vapour  on  carbon.  Articles  of  silundum 
are  prepared  by  shaping  them  out  of  graphite,  embedding  them  in 
what  is  practically  the  charge  of  a  carborundum  furnace,  and  subject  in.u 
them  to  the  action  of  silicon  vapours.  The  extent 'of  the  silicitieation 
depends  on  the  duration  of  heating.  It  can  be  either  superficial  or 
extend  through  the  whole  mass.  The  article  produced  retains  its 
original  shape,  cohesion,  and  strength,  and  possesses  the  hardness, 
resistivity,  and  electrical  conductivity  of  carborundum.  All  kinds  of 
vessels  and  tubes  can  be  made,  and  its  power  of  conducting  currents 
at  high  temperatures  renders  it  particularly  serviceable  for  electrical 
cooking  apparatus. 

Siloxicon  was  also  discovered  by  Acheson,  and  some  is  always 
obtained  in  the  manufacture  of  carborundum.  The  so-called  '  amor- 
phous carborundum  '  is  merely  a  variety  of  siloxicon.  It  is  a  greyish- 
green  powder,  approximating  to  the  composition  Si2C20.  How  far 
it  is  a  definite  compound  is  not  clear.  Like  carborundum,  it  is 
exceedingly  refractory,  and  makes  a  very  useful  furnace  lining  if 
the  atmosphere  is  of  a  reducing  nature.  But  it  is  much  more 
readily  oxidised  than  carborundum,  and  this  naturally  restricts  its 
application. 

2.  Graphite 

The  successful  development  of  the  artificial  production  o!  graphite 
we  also  owe  to  Acheson.  It  was  a  direct  outcome  of  his  carborundum 
work.  Carbon  exists  in  three  forms.  Two  are  crystalline — diamond 
and  graphite.  The  third  is  amorphous  and  unstable  at  all  temperatures 
with  respect  to  one  of  the  other  two.  Of  the  two  crystalline  forms  the 
diamond  is  stable  at  low  temperatures,  graphite  at  \\\»\\  temperatures, 
the  former  changing  into  the  latter  on  heating.  The  exact  temperature 
above  which  this  change  tends  to  occur  is  unknown,  but  may  be  as 

1  Zcittcli.  Elcktrochcm.  15,  7i>:>  (1HW)  ;   Electrochtm.  Ind.  7,  24  (U>OH). 


xxvi.]  GKAPHITE  493 

low  as  450°.  Below  this  temperature,  whatever  it  is,  amorphous  carbon 
will  tend  to  slowly  change  into  diamond,  above  this  temperature  into 
graphite.  The  rate  of  change  into  diamond  is  immeasurably  slow, 
and,  unless  the  temperature  be  high,  the  rate  of  change  into  graphite 
is  slow  also.  We  know,  however,  that  furnace  electrodes  of  carbon 
heated  to  high  temperatures  become  gradually  graphitised,  as  also 
do  the  tips  of  arc  lamp  carbons. 

It  has  already  been  mentioned  that,  after  a  carborundum  furnace 
run,  the  conducting  core  is  found  surrounded  by  a  ring  of  graphite, 
which  has  resulted  from  decomposition  of  previously  formed  carborun- 
dum. Acheson  followed  up  this  point,  and  soon  found  that  the  rapid 
conversion  of  coke  or  anthracite  into  graphite  does  not  necessitate 
the  presence  of  sufficient  silica  to  form  carborundum  with  all  the  carbon 
present.  A  far  smaller  quantity  suffices,  and,  moreover,  other  oxides  are 
active,  some  more,  some  less  so  than  silica.1  A1203,  Fe203  and  B203 
are  examples.  The  more  oxide  present  up  to  a  certain  limit,  the  lower 
the  temperature  at  which  the  reaction  velocity  becomes  measurable. 

It  is  evident  that  the  function  of  the  oxide  is  catalytic.  The 
exact  way  in  which  it  acts  is  less  clear.  The  most  natural  assumption 
is  that  the  stability  of  the  metallic  carbide  is  intermediate  between 
the  stabilities  of  the  systems  metal  -[-  amorphous  carbon  and  metal 
+  graphite,  and  that  when  the  metal  has  been  liberated  from  its  oxide 
by  the  carbon  it  first  forms  carbide,  which  subsequently  decomposes, 
giving  graphite.  The  metallic  vapour  liberated  forms  more  carbide 
with  the  amorphous  carbon,  and  the  process  continues.  When  all 
the  amorphous  carbon  present  has  become  graphite,  the  metal  simply 
volatilises  away.  This  hypothesis  agrees  with  the  fact  that,  after 
complete  graphitisation,  the  charge  can  be  almost  entirely  freed  from 
impurities  by  further  heating. 

Graphite  2  is  produced  in  two  forms,  as  powder  and  as  finished 
articles,  ready  for  use.  These  include  bars  and  plates,  lamp  carbons, 
electrodes,  brushes  for  electrical  machinery,  metallurgical  crucibles,  etc. 
The  furnace  used  for  the  production  of  graphite  powder  is  similar  to 
the  carborundum  furnace.  It  has  the  same  permanent  end  walls  and 
arrangement  of  electrodes,  a  shallow  firebrick  bed,  and  movable  side- 
walls  of  carborundum  bricks.  When  charging,  a  layer  of  carborundum 
is  first  placed  on  the  bed  to  protect  it  from  fusion.  Then  the  charge 
is  filled  in  to  the  lower  edge  of  the  electrodes.  Its  nature  depends  on 
the  quality  of  the  product  required.  Usually  it  consists  of  anthracite 
ground  to  the  size  of  rice,  but  far  larger  pieces  can  be  used.  The 
anthracite  contains  5-15  per  cent,  ash, -and  the  silica,  alumina,  and 
Fe203  present  suffice  to  catalyse  the  reaction.  For  the  finest  quality 

1  Borchers  and  Mogenburg,  Zrir*rli.  E\>  klrorhem.  8,  743  (1902);   Borchers  and 
Weekbecker,  Metnll.  1,  137  (1904). 
-  Electrochem.  Ind.  1,  52  (1902). 


494    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

of  lubricating  graphite,  petroleum  coke  is  used  with  1-2  per  cent. 
Fe203  added.  The  core  consists  of  graphite,  originally  coke.  It  is 
longer  and  of  smaller  cross-section  than  in  a  carborundum  furnace 
of  equal  load.  This  is  due  to  the  far  higher  conductivity  of  the 
charge.  Finally,  more  charge  is  heaped  around  and  above  the  core, 
and  the  furnace  covered  with  a  siloxicon  layer. 

A  1,000-H.P.  unit  (the  largest  constructed)  is  30'  long,  and  has  an 
internal  cross-section  2'  in  diameter.  When  starting,  200  volts  are  used. 
The  current  varies,  and  is  comparatively  low.  As  the  temperature  rises 
the  resistance  decreases.  The  voltage  is  kept  constant  until  the  current 
has  so  increased  that  the  furnace  is  taking  its  maximum  power — 
i.e.  until  the  current  is  about  3,700  amperes  (assuming  cos  6  =  1). 
Then  the  voltage  gradually  falls  with  simultaneous  increase  in  current, 
the  final  values  being  80  volts  and  9,000  amperes.  The  total  duration 
of  a  run  is  20-24  hours.  At  its  conclusion  the  furnace  is  allowed  to 
cool  sufficiently,  carefully  dismantled,  and  the  graphite  ground  and 
sieved  or  otherwise  graded. 

During  a  single  run  a  1,000-H.P.  furnace  will  convert  90  per  cent. 
of  a  6-ton  charge  into  graphite.  The  energy  used  is  consequently 

750  x  24 
about  —  =  3*3  K.W.H.  per  kilo.     This  will  be  a  maximum 

O4UU 

figure.  The  theoretical  energy  expenditure  necessary  for  the  forma- 
tion of  one  kilo,  of  graphite  (assuming  a  mean  temperature  of  2200°) 
is  equal  to  the  amount  required  to  heat  up  the  graphite  to  2200° 
minus  the  heat  evolved  at  room  temperature  by  the  graphitisation  of  the 
amorphous  carbon.  Taking  c  (mean)  for  graphite  as  0'45,  the  former 
figure  is  0'45  X  2200  =  990  Cals.  The  heat  evolved  during  the  trans- 
formation of  amorphous  carbon  into  graphite  is  236  Cals.  per  kilo., 
and  hence  the  total  heat  needed  is  990  —  236  =  754  Cals.,  equivalent  to 
0*88  K.W.H.  per  kilo.  The  low  efficiency  of  27  per  cent,  (probably 
somewhat  higher)  is  the  result  of  the  long  duration  of  heating,  the 
large  radiating  surface  of  the  furnace,  the  reduction  and  volatilisation 
of  impurities,  electrode  losses,  etc.,  etc.  As  in  carborundum  manufac- 
ture, several  furnaces  are  run  together,  being  at  different  stages  in 
the  process  at  the  same  moment. 

The  product  got  from  anthracite  has  5-10  per  cent,  ash,  that 
from  petroleum  coke  0*5  per  cent,  or  less.  The  physical  properties  of 
the  graphite — colour,  consistency,  etc. — depend  largely  on  the  duration 
of  heating  and  manner  of  cooling,  and  on  the  kind  of  carbon  and  the 
catalyst  used.  Material  intended  for  lubricating  purposes  is  treated 
with  particular  care.  The  other  grades  are  used  extensively  in  electro- 
technics,  for  cleaning  purposes,  for  pigments,  and  to  some  extent  in 
pencils.  Recently  emulsions  in  oil  and  water,  known  as  '  oil-dag ' 
and  '  water-dag,'  have  been  marketed.  They  are  used  respectively  as 
a  lubricant  and  as  an  anti-corrosion  metal  paint. 


XXVI.] 


ALUNDUM 


495 


For  the  preparation  of  graphite  articles,  such  as  electrodes  and 
motor-brushes,  finely-powdered  petroleum  coke  is  used.  This  is  com- 
pressed and  moulded  with  the  aid  of  some  tar  or  pitch,  the  necessary 
amount  of  a  suitable  oxide  being  added  as  catalyte.  The  furnace  used 
by  the  Acheson  Co.  is  again  of  1,000  H.P.,  and  externally  very  similar 
to  the  carborundum  and  powdered  graphite  furnaces  (Fig.  126).  It 
is  shorter  than  the  latter,  however,  and  differs  from  both  in  having  no 
core.  The  space  between  the  end  electrodes  is  instead  filled  with  the 
objects  to  be  graphitised,  arranged  transversely  across  the  furnace  at 
right  angles  to  the  flow  of  the  current,  each  article  being  surrounded 
and  separated  from  its  neighbours  by  a  layer  of  granulated  coke.  This 
arrangement  increases  the  resistance  of  the  furnace,  and  ensures 
gradual  and  regular  heating.  This  is  necessary  in  order  that  the  gases 
may  escape  from  the  interior  of  the  articles  without  damaging  them. 
The  greater  part  of  the  heat  production  takes  place  in  the  coke  inter- 
layers.  The  furnace  is  finally  covered  with  a  layer  of  siloxicon. 


Dnnnnnnnnnn 
nnnaannDDDD 


FIG.  126. — Graphite  Electrode  Furnace. 

The  duration  of  a  run  is  about  the  same  as  with  the  powdered 
graphite  furnaces,  and  voltage  and  current  vary  similarly.  The  pro- 
duction of  graphite  articles  is  3-3*5  tons  per  run.  Owing  to  the  purity 
of  the  raw  material,  the  product  has  only  O'l-Q'6  per  cent.  ash. 

Another  method — that  of  Girard  and  Street — has  been  to  some 
extent  used  for  making  graphite  electrodes  and  rods.  The  moulded 
carbon  article  (B203  being  the  catalyst)  is  drawn  by  rollers  through 
a  horizontal  arc  passing  between  carbon  electrodes  in  a  closed 
chamber  filled  with  an  indifferent  gas.  This  treatment  graphitises 
them  sufficiently  for  certain  purposes.  It  is  stated  that  the  energy 
used  is  7 '36  K.W.H.  per  kilo,  of  graphite,  a  considerably  higher  figure 
than  that  of  the  Acheson  process. 

3.  Alundum.     Silica 

Alundum.1 — This  is  the  name  given  to  the  fused  alumina  manu- 
factured by  the  Norton  Emery  Wheel  Co.  at  Niagara,  and  extensively 

1  Elecirocfom.  Ind.  1,  15  (1902). 


496    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY     [CHAP. 

used  as  an  abrasive,  for  grinding  and  drilling,  and  recently  for  making 
firebricks,  muffles,  etc.  A  similar  product  made  at  Rheinfelden  is 
called  '  diamantin.' 

The  raw  material  consists  of  the  purest  bauxite  obtainable.  Before 
use  it  is  washed  free  from  various  impurities,  dried  and  calcined.  At 
Niagara  the  fusion  takes  place  in  an  arc  furnace  devised  by  Higgins. 
It  consists  essentially  of  a  circular  hearth  of  carbon  blocks  and  a 
movable  wall.  This  wall  is  of  'sheet-iron,  water- jacketed  throughout. 
In  shape  it  is  a  truncated  cone,  which  fits  into  a  groove  on  the  outside 
of  the  fixed  hearth.  Two  carbon  electrodes,  introduced  from  above, 
convey  the  alternating  current  to  and  from  the  furnace.  The  alumina 
is  charged  in  above,  much  as  in  carbide  furnaces.  The  fused  material 
falls  to  the  bottom,  and  the  current  passes  from  one  electrode  to  the 
melt  and  back  to  the  second  electrode,  the  furnace  thus  containing  two 
arcs.  As  the  alumina  collects,  the  electrodes  are  presumably  raised. 
When  sufficient  has  been  fused,  the  current  is  stopped,  the  electrodes 
lifted  out,  and  the  whole  furnace  allowed  to  cool  for  3-4  hours.  The 
movable  wall  is  then  raised,  and  the  block  of  alundum  removed.  It 
is  broken  up,  crushed,  graded,  and  worked  up  in  various  ways.  In 
colour  it  is  a  dark  brown,  due  to  the  presence  of  iron.  It  may  also 
contain  traces  of  other  substances,  present  in  the  charge,  and  reduced 
by  the  carbon  electrodes. 

Each  furnace  works  at  110  volts  and  2,500  amperes,  and  therefore 
consumes  (at  a  maximum)  275  K.W.  In  24  hours  it  produces  about 
7,000  Ibs.  alundum.  The  energy  expenditure  per  kilo,  amounts  to 

275  x  24  X  2*2 

-  =  2'1   K.W.H.,   about  one-fourth  that  required  for 
/  uuu 

an  equal  weight  of  carborundum.  The  raw  material  is,  however, 
more  expensive,  and  there  must  be  a  considerable  electrode 
consumption. 

Fused  Quartz  Ware. — We  can  only  deal  with  this  subject  very 
briefly  here.  Fused  quartz  glass  is  made  in  England  by  the  Thermal 
Syndicate  and  in  Germany  by  the  Deutsche  Quarz-Gesellschaft  of 
Beuel  a.Rh.1  The  process  of  the  former  is  based  on  work  of  Hutton's,2 
developed  later  by  Bottornley  and  others,  and  the  Quarz-Gesellschaft 
uses  practically  the  same  methods. 

The  principle  employed  is  as  follows  :  Pure  silver  sand  (99 '5  per 
cent.)  is  fused  by  a  current  passing  through  carbon  rods  or  plates 
embedded  in  it.  Its  melting-point  is  1800°  to  1900°,  but  it 
must  be  heated  to  about  2000°  before  it  flows  easily  enough  for 
it  to  be  worked,  and  at  this  temperature  it  is  already  readily 
volatile.  When  we  remember  that  carbon  and  silica  commence  to 

1  Both  these  companies  make  the  translucent  material.     Small  transparent 
quartz  vessels  are  manufactured  by  the  linn  of  Herat  us. 

2  Trans.  Amer.  Electrochem.  Soc.  2,  105 


xxvi.J  CARBON    BISULPHIDE  497 

give  siloxicon  at  1540°  and  carborundum  at  1820°,  we  see  that 
there  are  possibilities  of  complicating  side-reactions.  It  must  be 
remembered,  however,  that  the  area  of  contact  between  the  carbon 
and  silica  is  comparatively  small,  and  moreover  that  a  thin,  dense, 
protective  layer  of  siloxicon  or  carborundum  probably  forms  on 
the  resistor.  If  the  temperature  of  the  latter  exceeds  2220°,  this 
layer  will  decompose,  leaving  graphite  and  evolving  silicon  vapours. 
When  sufficient  sand  has  been  fused  the  carbon  resistor  is  withdrawn, 
and,  by  utilising  the  hole  left  in  the  interior  of  the  melt,  the  latter  is 
blown  out  and  moulded  to  the  desired  form. 

This  is  done  in  various  ways.  Air  is  blown  in.  Or  some  chalk, 
greenwood,  or  similar  material  is  placed  in  the  space  left  by  the  resistor, 
the  ends  of  the  semi-solid  mass  pushed  together  and  closed,  and  the 
whole  put  inside  a  suitable  mould.1  In  this  way  evaporating  dishes, 
muffles,  etc.,  are  produced.  To  make  tubing,  a  wrooden  twig  is  pushed 
into  the  interior,  and  the  mass  drawn  out  with  great  rapidity.  On  the 
speed  with  which  this  is  done  depend  the  thickness  and  diameter  of  the 
tubing.  Sometimes  the  resistor  consists  of  a  perforated  carbon  tube. 
When  the  fusion  is  finished,  the  tube  is  not  taken  out,  but  compressed 
air  is  forced  into  its  interior,  and  the  mass  thus  blown  into  a  mould.2 
Hutton  made  quartz  tubing  by  simply  passing  a  current  through  a 
carbon  pencil  embedded  in  sand.  This  fused  round  the  pencil,  and 
after  cooling  a  tube  resulted,  from  which  if  necessary  the  carbon  could 
be  removed  by  burning  out. 

The  crude  articles  thus  obtained  are  subsequently  trimmed  and 
polished,  use  being  made  of  the  oxyhydrogen  flame,  the  sand  blast,  and 
various  abrasives. 

4.  Distillation  Products.    Carbon  Bisulphide.    Phosphorus.    Zinc 

The  purely  chemical  manufacture  of  the  remaining  substances  which 
are  here  discussed — carbon  bisulphide,  phosphorus,  and  zinc — suffers 
in  each  case  from  certain  common  disadvantages.  The  reacting  mass 
is  distilled  at  a  high  temperature,  and  the  retorts  or  muffles  used  must 
be  small  in  order  to  allow  of  conduction  of  heat  into  the  interior  (in 
the  case  of  CS2  to  diminish  the  danger  resulting  from  a  possible  accident). 
The  results  are  high  labour  charges,  and,  owing  to  the  temperatures  to 
which  the  retorts  are  externally  subjected  and  the  frequently  corrosive 
nature  of  the  charge  or  products,  high  costs  for  depreciation  and  repairs. 
Phosphorus  and  zinc  also  give  poor  yields,  due  to  the  difficulty  of  reaching 
a  sufficiently  high  temperature  and  to  the  strongly  reactive  nature  of 
the  product.  We  have  already  seen 3  that  such  circumstances  favour  the 
development  of  electrothermal  processes,  and  we  find  accordingly  that 

1  Zeitsch.  Angew.  Chem.  23,  1376  (/r^0). 

-  Metall.  Chem.  Engin.  9,  226  (Wll).  *  pp.  4,  168-171. 

2K 


498    PRINCIPLES  OF  \PPLIED  ELECTROCHEMISTRY    [CHAP. 

the  whole  of  the  CS2  used  in  North  America  and  the  greater  part  of  thel 
world's  supply  of  phosphorus  are  so  produced.  With  zinc  the  casd 
is  different.  No  process  hitherto  proposed  has  proved  so  successful 
commercially  as  to  be  widely  adopted. 


FIG.  127. — Taylor  Bisulphide  Furnace. 

Carbon  Bisulphide.— The  electrothermal  production  of  CS2  was 
worked  out  by  E.  R.  Taylor,  and,  as  stated,  his  works  at  Penn  Yann 
(U.S.A.)  supply  the  whole  demand  of  North  America.  A  resistance 
furnace  is  employed1  (Fig.  127)  of  very  ingenious  construction.  It 

1  Zeitsch.  Eleklrochem.  9, 


xxvi.]  CARBON  BISULPHIDE  499 

consists  of  three  parts,  all  circular  in  plan,  the  hearth  A,  the  shaft  B, 
and  the  head  C.  The  whole  is  constructed  of  firebrick,  enclosed  in  a 
stout  iron  sheath.  The  carbon  electrodes  number  four,  placed  horizon- 
tally and  symmetrically  around  the  hearth.  They  enter  through  suit- 
able plates  and  stuffing-boxes,  and  are  insulated  with  mica  and  asbestos. 
They  appear  somewhat  squat  in  shape,  being  4'  long  and  20"  square  in 
cross-section.  The  resistor  consists  of  pieces  of  coke  or  broken  electrode 
carbons.  D,  which  are  introduced  through  four  channels,  EE,  in  the 
hearth  walls  above  the  electrodes.  Carbon  in  this  form  is  only  slowly 
attacked  by  sulphur  under  the  conditions  prevailing  in  the  furnace. 
The  electrodes  also  have  a  long  life.  Of  the  two  reacting  substances, 
the  charcoal  enters  at  the  top  of  the  furnace  by  means  of  a  charging 
arrangement  similar  to  that  in  the  ordinary  blast  furnace.  The  sulphur 
enters  the  hearth  below  the  electrodes  through  channels  in  the  walls 
of  hearth  and  shaft.  The  latter  are  visible  in  the  diagram  (FF) ;  the 
former  are  situated  between  the  electrodes,  with  the  exception  of 
GG,  by  means  of  which  sulphur  is  introduced  behind  the  electrodes. 
The  CS2  leaves  the  furnace  at  H,  is  condensed  under  water,  and 
redistilled. 

The  furnace  construction  would  seem  to  permit  of  an  excellent 
heat  utilisation.  The  charcoal  is  heated  by  the  escaping  vapours 
during  its  descent  to  the  reaction  zone  (immediately  above  the  core), 
which  is  at  a  bright  red  heat.  The  greater  part  of  the  heat,  which  would 
otherwise  be  lost  as  radiation,  serves  to  melt  the  sulphur  charged  in  at 
FF  and  GG,  and  much  of  the  heat  usually  lost  in  the  electrodes  is  here 
utilised,  as,  instead  of  water  being  used  for  cooling,  sulphur  is  employed 
which  afterwards  enters  the  hearth.  The  regulation  of  the  furnace  is 
effected  from  the  generators  used,  by  varying  the  bulk  of  the  carbon 
resistor,  and  to  a  certain  extent  automatically  by  the  entering  sulphur. 
When  the  furnace  becomes  too  hot  the  sulphur  melts  rapidly,  rises  above 
the  level  of  the  electrodes,  and,  being  a  non-conductor,  decreases  the 
current. 

The  furnace  is  fed  with  two-phase  alternating  current.  The  phase 
currents  generally  pass  between  opposite  electrodes,  but,  if  necessitated 
by  a  stoppage,  can  be  easily  made  to  pass  between  adjacent  electrodes. 
Owing  to  the  small  amount  of  the  water-power  available,  the  furnace 
usually  takes  240  K.W.  (60  volts  ;  4,000  amps.),  and  seldom  more 
than  330  K.W.  It  could  easily  take  500  K.W.  The  energy  consump- 
tion amounts  to  about  1-15  K.W.H.  per  kilo.  CS2. 

The  theoretical  amount  necessary  is  readily  calculated.  According 
to  Haber  the  vapours  leave  the  furnace  at  about  100°.  We  can  suppose 
the  CS2  to  be  formed  at  20°,  to  be  heated  in  the  form  of  liquid  to  100°, 
and  at  that  temperature  to  be  vaporised.  The  formation  of  one  mol. 
(76  grams)  CS2  at  20°  requires  19,030  cals.  For  liquid  CS2  between 
20°-100°,  c  (mean)  is  0'245.  L  (vaporisation)  at  100°  is  72  Calories. 

Sxl 


500    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAPJ 
The  heat  necessary  for   the  production  of  one  kilo.   CS2  is  therefore 

19030  -|-  (0-245  X  80)  -f  72  =  342  Calories,  equivalent  to  0'4  K.W.HJ 
76 

The  thermal  efficiency  of  the  Taylor  process  would  thus  be  about! 
35  per  cent.,  an  unexpectedly  low  figure.  The  reasons  for  this  aifl 
not  clear.  A  certain  amount  of  sulphur  is  vaporised  with  the  CSJ 
thus  absorbing  heat.  Perhaps  the  product  really  leaves  the  furnace  at 
a  temperature  considerably  exceeding  100°.  The  electrode  losses  niajj 
be  far  greater  than  is  assumed.  Finally,  though  the  fusion  of  t  lie 
sulphur  in  the  wall-channels  must  considerably  lower  the  radiation;! 
losses,  these  may  be  much  greater  than  they  appear.  The  radiating 
surface  is  very  large  in  comparison  with  the  load  of  the  furnace,  j 

The  furnace  works  exceedingly  smoothly  and  without  explosions. 
It  can  run  for  a  year  without  cleaning.  The  supervision  and  labouJ 
required  are  small. 

Phosphorus.1  —  The  old  chemical  methods  for  the  preparation  of« 
phosphorus  involved  the  reduction  of  HP03  or  of  an  acid  calcium 
phosphate  with  charcoal.  The  disadvantages  were  great,  particularly 
in  the  second  case,  where  corrosive  calcium  silicate  was  produced,  an<j 
where  the  temperature  never  sufficed  to  drive  off  all  the  phosphorusj 
From  various  causes  the  yields  were  exceedingly  low,  an  8  per  cenJ 
phosphorus  recovery  being  considered  good  (Herrmann),  and  a 
preliminary  H2S04  treatment  was  always  necessary. 

For  some  years  past  much  phosphorus  has  been  prepared  electro-.] 
thermally,  and  the  proportion  so  manufactured  steadily  increases.    The 
published  details  concerning  the  processes  used  are  most  meagre.     It,] 
appears,  however,  that  all  methods  employed  are  based  on  the  original 
Readman  and   Parker  patents.     Readman  electrically  heated  boneJ 
ash  or  crude  HP03  with  powdered  coal  or  charcoal  and  sand.     Naturalj 
calcium  phosphate  could  be  used  if  first  calcined.     At  1150°  the  reactioM 
commences,  and  phosphorus  and  CO  distil  off.2    It  is  complete  at  1450°J 
According  to  the  Parker  patent,  wavellite  (A1P04)  is  treated  with  H2SOj 
to  remove  the  aluminium,  and  the  HP03  liquors  mixed  with  coal  and 
reduced  in  the  electric  furnace. 

At  the  present  time,  bone  ash,  calcined  mineral  calcium  phosphate 
or  calcined  wavellite  are  mixed  with  carbon  and  sand  without  any 
preliminary  treatment  with  acid,  and  reduced  electrothermally.  One 
form  of  furnace  is  a  large  gas-tight  iron  cylinder,  lined  with  refractory^ 
and  containing  in  its  upper  part  two  large  carbon  electrodes,  bet  wee  n 
which  an  arc  passes.  The  charge  receives  the  heat  by  radiation. 
Phosphorus  distils  off,  and  collects  under  water  in  copper  vessels,  and 
the  calcium  or  aluminium  silicate  slag  is  drawn  off  intermittently. 


1  Electrochcm.  hid.  5,  107  (/W7)  ;    KM;i,  •<*•!"  >».  /,//.W,.  17,  01  (IHW). 
Ik-mpcl  and  R.  Mullor,  Zeitscli.  Angew.  Chem.  18,  132  (HM). 


xxvi.]  ZINCX  501 

The  charging  is  continuous.  A  solidified  layer  of  slag  protects  the 
lining.  The  percentage  of  phosphorus  recovered  rises  to  92  per  cent, 
and  is  usually  80-90  per  cent.  At  Oldbury  it  is  said  to  be  86  per  cent. 
For  these  favourable  results  the  iron  content  of  the  charge  must  be 
low.  Landis  has  worked  out  a  similar  process.  The  arc  passes  between 
vertical  carbon  electrodes  and  carbon  bricks  in  the  lining.  The  flues 
must  be  constructed  of  non-absorbent  material,  or  else  much  of  the 
phosphorus  is  lost.  The  furnace  is  water-sealed  at  all  the  joints.  The 
slag  is  drawn  off  every  3-4  hours. 

According  to  J.  W.  Kichards,1  the  furnaces  working  at  Niagara  in 
1902  consumed  only  50  H.P.,  and  produced  170  Ibs.  of  phosphorus  daily. 
Assuming  them  to  work  continuously,  we  calculate  that  one  kilo,  requires 
50  X  24  X  3  X  2-2  =  n>6  K  w  H  Tlie  eiectrode  consumption  is 

4X  170 
always  small. 

Zinc.2— As  ordinary  metallurgical  processes  for  the  winning  of  zinc 
furnish  very  unsatisfactory  results,  much  attention  has  been  devoted 
to  different  electrochemical  methods.  We  have  seen  that  neither  the 
electrolysis  of  aqueous  solutions 3  nor  that  of  fused  melts 4  has  met  with 
great  success,  and  up  to  the  present  the  same  statement  holds  applied 
to  electrothermal  process.  But  future  prospects  are  distinctly  more 
favourable.  In  the  ordinary  processes  of  zinc  distillation,  the  charge 
of  calcined  ore  and  carbon  is  heated  in  small  externally-fired  clay 
retorts,  holding  20-40  kilos.  The  reaction  commences  at  about  1030°, 
but  to  expel  all  the  zinc  the  retorts  must  be  heated  to  a  white  heat 
for  perhaps  20  hours.  The  thermal  efficiency  of  the  process  is  very 
low,  4-12  per  cent.,  depending  on  the  size  of  the  retort  and  the  nature 
of  the  ore.  About  2 -5-4  tons  of  coal  are  used  per  ton  of  zinc  pro- 
duced. Particularly  in  the  later  stages  of  the  distillation  is  the  thermal 
efficiency  so  low. 

Many  kinds  of  ore  cannot  be  conveniently  treated  at  all.  A  low- 
grade  ore  must  first  be  concentrated.  An  ore  with  much  iron  and  lime 
forms  a  corrosive  slag  which  attacks  the  retorts.  Mixed  ores  containing 
silver  and  lead  furnish  an  impure  zinc  and  corrode  the  retorts.  If 
smelted  as  lead  ores  in  a  small  blast  furnace,  infusible  slags  result,  the 
furnace  becomes  choked  up,  and  the  zinc  is  lost.  The  retorts  themselves 
are  strongly  attacked,  by  hot  gases  outside  and  by  zinc  vapours  and 
slags  inside.  Their  average  life  is  4-7  weeks.  The  cost  of  renewals 
is  therefore  high,  as  also  the  cost  of  labour,  owing  to  the  small  size 
of  the  units.  The  product  is  not  always  of  a  reliable  and  regular 

1  Eledrochem.  Ind.  1,  17  (1902). 

'  Biwvnand  Oesterle,  Tmn*.  Amer.  Eledrochem.  Soc.  8,  171  (1905)  ;  Johnson, 
Trans.  Amer.  Electrochem.  Soc.  11,  265  (1907) ;  19,  311  (1911) ;  Cote  and  Pierron, 
Electrochem.  Ind.  7,  468  (1909). 

3  P.  281.  4  P.  421. 


502    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY 

composition.  Finally,  the  zinc  recovery  is  poor.  Any  sulphur  in  the 
calcined  ore  keeps  back  twice  its  weight  of  zinc  (hence  the  calcination 
must  be  very  thorough),  zinc  vapours  diffuse  through  the  retorts  and 
pass  through  the  cracks,  and  considerable  losses  are  incurred  in  the 
necessary  concentration  of  the  low-grade  ores.  The  sum  of  these 
losses  amounts  to  10-25  per  cent.,  under  exceptional  conditions  even 
35-40  per  cent. 

A  successful  electrothermal  method  would  mean  the  employment  of 
larger  and  more  durable  units,  and  lower  costs  for  labour  and  repairs. 
The  thermal  efficiency  would  be  higher,  and  some  of  the  sources  of 
loss  of  zinc  would  disappear.  If,  further,  it  were  found  possible  to  treat 
low-grade,  impure,  mixed,  and  uncalcined  ores,  a  great  advance  would 
have  been  made.  Experiments  have  shown  that  all  these  advantages 
can  actually  be  secured  by  using  electrothermal  methods.  Two  main 
causes  have,  however,  prevented  their  general  adoption.  The  first  is 
the  cost  of  the  large  quantity  of  energy  necessary ;  the  second  is  the 
difficulty  of  condensing  the  zinc  vapours  to  liquid  metal  instead  of  to 
zinc  dust. 

This  difficulty  is  universal  in  zinc  distillation,  but  appears  to  be 
particularly  troublesome  in  electric-furnace  processes.1  The  two  chief 
causes  of  the  zinc-dust  formation  are  probably  as  follows,  (a)  The  zinc 
vapours  are  too  dilute  or  the  cooling  is  too  rapid,  and  they  become 
supercooled.  When  the  point  is  reached  at  which  they  condense,  the 
temperature  is  already  below  the  melting-point  of  the  metal,  and  a  fine 
powdery  dust  results,  (b)  The  small  globules  of  condensed  zinc  are 
superficially  oxidised  by  the  gases  present  (C02,  H20,  02,  CO  under 
some  conditions),  or  are  perhaps  coated  with  dust  or  slag  vaporised 
from  the  furnace.  In  that  case  they  are  unable  to  coalesce.  Recent 
reports  from  both  Europe  and  America  state  that  this  condensation 
difficulty  has  been  largely  overcome. 

The  different  processes  proposed  are  divisible  into  two  classes, 
according  to  whether  the  uncalcined  ore  (generally  a  sulphide)  is  directly 
treated,  or  whether  the  calcined  ore  (oxide)  is  used. 

Brown  and  Oesterle  have  investigated  on  a  small  scale  the  electro- 
thermal reduction  of  zinc  blende.  Using  equimolecular  quantities  of 
ore  (59*6  per  cent,  zinc),  lime,  and  coke,  they  successfully  produced  zinc 
and  a  melt  of  CaS  containing  only  0*13  per  cent.  Zn.  The  equation  is 

ZnS  +  CaO  +  C >  CaS  +  CO  +  Zn. 

Attempts  to  form  simultaneously  CaC2  and  CS2  by  using  excess  of 
carbon  proved  unsatisfactory,  as  would  be  anticipated.  Snyder  haa 
devised  commercial  furnaces  for  a  similar  purpose,  but  the  only  process 
t<-< -Imically  working  is  that  of  Cote  and  Pierron,  which  has  bocn  for 
some  years  in  operation  in  a  small  plant  in  the  Pyrenees. 

1  See  for  example,  Snyder,  Trans.  Amer.  Electrochem.  Hoc.  19,  317  (I! HI). 


XXVI.] 


ZINC 


503 


sr  vapours 


The  furnace  consists  of  a  closed  chamber  with  a  graphite  hearth, 
from  which  an  electrode  projects  upwards,  and  an  arched  roof  lined 
with  magnesia,  through  which  a  second  electrode  enters.  An  arc  passes 
vertically  between  the  electrodes,  and  the  charge  of  blende,  lime,  and 
carbon  is  heaped  around  it.  Suitable  openings  are  provided  for 
charging,  for  tapping  slag  and  any  lead  produced,  and  for  the  exit  of 
the  zinc  vapours.  Difficulties  are  evidently  experienced  in  the  con- 
densation, as  the  zinc  is  now  generally  completely  burnt  to  ZnO  and 
collected  as  such.  The  method  of  heating  doubtless  causes  much 
slag  vapour  and  dust  to  accompany  the  zinc  to  the  condensers. 

The  electrothermal  reduction  of  zinc  from  calcined  ores  has  also 
been  much  studied.  In  America,  Johnson  has  designed  several  furnaces, 
one  type  being  provided  with  a  gas-  or  coke-fired  preheater,  in  which  the 
first  part  of  the  reduction  takes  place,  electrical  heating  being  only  used 
for  the  later,  high  temperature,  stages.  In  a  25-H.P.  experimental  arc 
furnace  of  another  type,  he  has 
produced  zinc  at  the  rate  of  2 
tons  per  H.P.  year,  70-80  per 
cent,  of  it  condensing  as  liquid. 
The  furnace  vapours  pass 
through  a  layer  of  heated  char- 
coal before  reaching  the  con- 
densers. In  this  way  any  C02 
present  is  removed  and  the 
condensation  facilitated.  But, 
again,  only  one  furnace  is  in 
technical  operation,  that  of 
de  Laval,  which  is  working  in  three  Swedish  plants,  and  said  to 
be  using  some  6,500  K.W.  It  is  an  arc  furnace  in  which  the 
heat  is  transferred  to  the  active  mass  entirely  by  radiation,  as  in 
the  Stassano  steel  furnace  and  the  phosphorus  furnace.  The  conse- 
quence is  that  the  reduction  proceeds  much  more  quietly,  without  over- 
heating and  vaporisation  of  slag  and  impurities.  Ores  containing 
large  quantities  of  lead  and  silver  furnish  a  very  pure  zinc.  The 
de  Laval  furnace  has  been  used,  indeed,  to  extract  zinc  from  impure 
spelter,  and  the  zinc-silver  alloy x  which  results  from  the  Parkes  lead 
desilverisation  process  could  be  also  treated. 

The  furnace  is  shown  in  Fig.  128,  and  is  of  very  simple  construction. 
It  consists  of  a  closed,  suitably-lined,  thick-walled  chamber,  with  pro- 
vision for  tapping  off  molten  lead,  slag,  etc.  The  zinc  vapours  and  CO 
escape  through  a  flue  in  the  roof.  The  charge,  consisting  of  roasted  ore, 
carbon,  and  fluxes,  enters  through  a  sealed  charging  shaft.  A  hopper 
and  sciew  feed  has  also  been  used.  The  arc  passes  between  carbon 


FIG.  128.— De  Laval  Zinc  Furnace. 


1  See  p.  284. 


504       PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY 

electrodes  inserted  in  the  walls  (at  right  angles  to  plane  of  paper).] 
Owing  to  the  reducing  atmosphere,  the  consumption  of  these  is  lowJ 
Furnaces  are  of  moderate  capacity,  taking  up  to  100  K.W.  As  about] 
50  volts  will  be  used,  the  amperage  may  rise  to  2,000.1  The  utilisation] 
of  the  heat  is  naturally  not  very  good.  One  arc  placed  between] 
two  banks  of  charge  would  be  better.  A  kilo,  of  zinc  requires  about] 
4-8  K.W.H.  (1-37  tons  per  H.P.  year).  This  is  not  as  favourable  as 
Johnson's  figure.  The  zinc  losses  in  the  slags  are  small,  and,  as] 
low-grade  ores  can  be  treated,  the  losses  due  to  concentration  are 
avoided.  The  metal  recovery  amounts  to  at  least  95  per  cent.,  and 
the  product  is  of  excellent  quality.  Two  samples  contained 

Pb    0-06  per  cent.        0-03  per  cent. 

Fe    0-01  per  cent.        0-01  per  cent. 

S  0-00(3)  per  cent. 

Cd,  As     Absent. 

Such  a  metal  can  be  used  for  the  best  qualities  of  brass,  such  as  are ; 
employed  in  cartridge-making. 

In  conclusion  it  may  be  said  that  the  next  few  years  will  probably 
see  a  more  extended  introduction  of  electrothermal  methods  into  non- 
ferrous  metallurgy.  There  are  great  latent  possibilities,  for  the  induction 
furnace  more  particularly  in  refining  and  alloying,  for  the  arc  furnace  in 
the  reduction  of  refractory  or  low-grade  ores.  Hansen  has  already 
mentioned  experiments  on  copper  ;  Harden  has  described  the  electro- 
smelting  of  tin  ores ; 2  and  nickel  ores  have  also  been  treated.3 


Literature. 

Amberg.     Articles   in   Askenasy's   Einfuhrung    in  die  teclinisclie 
EleJctrochemie,  vol.  i. 

1  Assuming  cos  6  —  \. 

2  Metall.  Chem.  Engin.  9,  453  (/"//). 
:t  Metall.  Chem.  Engin.  8,  277  (1910). 


CHAPTER  XXVII 

THE  OXIDATION  OF  ATMOSPHERIC  NITROGEN 
1.  Theoretical 

THE  fixation  of  atmospheric  nitrogen  by  electrical  discharges  has  now 
become  an  established  industry.  Much  laboratory  and  technical 
experimental  work  has  been  devoted  to  the  problem,  and  three  entirely 
different  processes  are  now  in  successful  technical  operation,  all  using 
some  form  or  other  of  the  high-tension  arc  discharge.  We  have  already l 
discussed  the  mechanism  of  chemical  transformation  in  the  electrical 
discharge,  but  it  will  be  interesting  to  consider  at  this  point  the  earlier 
work  done  on  the  oxidation  of  nitrogen  and  the  different  ideas  put 
forward.  Crookes  was  the  first  to  notice  the  formation  of  nitrogen  oxides 
in  a  high-tension  arc  discharge  in  air.  Rayleigh 2  was  the  first  to  study 
the  process  from  the  point  of  view  of  the  economic  production  of 
HN03.  He  used  an  apparatus  consuming  O8  K.W.,  a  N2  —  02  mixture 
with  36  per  cent.  N2,  and  had  an  excess  of  alkali  continually  present. 
Twenty  litres  of  gas  were  absorbed  per  hour.  Assuming  nitrite  to  be 
formed,  a  probable  assumption  as  the  rate  of  absorption  was  rapid, 

2 
1  K.W.H.  fixes  -  X  25  litres  of  nitrogen,  equivalent  to  56  grams  HN03. 

The  next  investigators  were  McDougall  and  Howies.3  They  carried 
out  extensive  experiments,  passing  the  gas  continuously  through  an 
arc  burning  between  two  horizontal  electrodes  in  a  large  earthenware 
vessel.  From  air  they  obtained  a  maximum  yield  of  34  grams  HN03 
per  K.W.H. ,  whilst  the  mixture  used  by  Rayleigh  yielded  67  grams 
per  K.W.H.  The  increased  yield  in  the  latter  case  they  attributed 
to  mass  action.  They  also  found  that  neither  yield  nor  concentration 
of  nitrous  gases  could  be  raised  by  indefinitely  increasing  the  current. 
The  excess  of  electrical  energy  was  simply  dissipated  as  radiation 
and  conduction.  The  NO  formation  therefore  appeared  limited  by 
some  kind  of  equilibrium  condition.  This  view  was  clearly  expressed  by 

i  Chap.  XIV.  -  Trans.  Chem.  Soc.  71,  181  (1897). 

3  Manch.  Mem.  (iv.)  44,  No.  13  (1900). 

505 


506    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 


Muthmann  and  Hofer.1  They  regarded  the  process  as  essentially  a 
thermal  one.  At  low  temperatures  NO  is  really  unstable,  its  equilibrium 
concentration  in  presence  of  nitrogen  and  oxygen  being  exceedingly 
small.  The  reaction  N2  -f-  02  — >  2NO  is  endothermic,  absorbing 
43,100  Cals.  at  room  temperature.  It  follows  that  at  higher  tem- 
peratures the  formation  of  NO  is  favoured,  and  that  at  very  high 
temperatures  its  equilibrium  concentration  in  presence  of  nitrogen 
and  oxygen  may  be  quite  considerable.2 

Nernst,  Jellinek,  and  Finckh 3  have  experimentally  determined  the 
equilibrium  concentrations  of  NO  in  air  at  several  temperatures,  and 
from  their  figures  other  values  can  be  calculated.  In  Fig.  129  absolute 
temperatures  are  plotted  against  per  cent.  NO  in  the  equilibrium 
mixtures.  According  to  the  thermal  theory,  the  incoming  gases  are 
heated  to  the  temperature  of  the  arc,  and  NO  produced  in  equilibrium 
concentration.  On  leaving  the  flame  the  mixture  is  more  or  less 

rapidly  cooled,  during  which 
time  the  tendency  will  be 
for  NO  to  decompose  into 
nitrogen  and  oxygen  in  ac- 
cordance with  the  lower 
equilibrium  concentrations  at 
lower  temperatures.  The 
more  effective  the  chilling,  the 
quicker  the  gases  will  pass 
through  the  range  of  tempera- 
ture in  which  the  velocity  of 

decomposition  is  appreciable,  and  the  higher  then  will  be  the  concen- 
tration of  NO  or  its  oxidation  products  in  the  cooled  gases.  The  two 
points  to  be  aimed  at  are  consequently  (a)  a  high  temperature  in  the 
discharge  ;  (b)  very  rapid  cooling  of  the  gases. 

Muthmann  and  Hofer  worked  very  similarly  to  McDougall  and 
Howies.  They  obtained  concentrations  of  3'7-6'7  per  cent.  NO  in 
the  end  product,  the  figure  being  higher  the  smaller  the  arc,  and 
therefore,  as  they  argued,  the  higher  the  temperature.  Starting  with 
NO,  they  found  that  approximately  the  same  final  concentrations  were 
reached,  a  strong  support  for  the  thermal  theory.  The  concentrations 
they  obtained  would  correspond  to  temperatures  of  2800°-3400°  C. 
They  estimated  the  minimum  temperatures  of  their  arcs  at  1600°- 
1800°.  The  actual  ones  were  probably  several  hundred  degrees  higher. 
The  work  of  Nernst  and  his  pupils  on  the  NO  —  N2  —  02  equilibrium  had 
not  then  been  published,  so  the  theory  of  Muthmann  and  Hofer  was  * 
generally  accepted,  and  their  experimental  work  regarded  as  supporting 

1  Ber.  86,  438  (1903). 

'  P.  24  and  fig.  2. 

3  Zeitach.  Anorg.  Chem.  45,  11(»  (Wo:>)  ;  49,  21»,  220  (HMi). 


Volume  per  cent  of  NO 

OK>*.«cooE£S 

^ 

X 

x 

/ 

x 

X 

_J 



^ 

1000° 


2000°  3000*  4000°  6000° 

Absolute  Temperature. 

Fia.  129. 


XXVIL]      OXIDATION  OF  ATMOSPHERIC  NITROGEN  507 

it.  Erode,1  working  with  a  similar  apparatus,  but  chilling  the  gases 
by  a  water-cooled  tube,  reached  a  concentration  of  8  per  cent.  NO, 
which  would  correspond  to  3700°  C.,  assuming  the  formation  to  be 
purely  thermal.  These  calculated  temperatures  are  of  course  all 
minimum  values.  A  certain  amount  of  decomposition  is  unavoidable. 
Erode  accepted  the  thermal  explanation.  The  arc  he  used,  distorted 
by  the  passage  of  gases  through  it,  consisted  of  three  distinct  zones,  and 
according  to  him  each  zone  corresponded  to  a  distinct  stage  in  the 
reaction.  In  the  one,  NO  and  ozone  were  formed,  in  the  second  ozone 
decomposed,  and  in  the  third  NO  dissociated. 

The  results  of  these  three  researches  all  supported  the  thermal  con- 
ception of  NO  formation,  even  if  the  concentrations  obtained  seemed 
rather  high.  But  in  1906  it  was  conclusively  shown  by  Warburg  and 
Leithauser 2  and  by  Berthelot 3  that  nitrogen  oxides  could  be  directly 
produced  from  air  by  means  of  the  silent  discharge,  and  Warburg4 
expressed  the  opinion  that  electrical  phenomena  would  doubtless  also 
play  a  part  in  the  formation  of  NO  in  the  arc  discharge.  Next  appeared 
the  valuable  work  of  Grau  and  F.  Russ.5  In  order  more  certainly  to 
obtain  equilibrium  conditions  in  their  arc,  they  caused  the  latter  to 
burn  vertically  between  platinum  electrodes  in  a  tube  of  silica  or 
water-cooled  glass.  The  distortion  of  the  arc  to  a  flame,  inevitable 
with  the  horizontal  arrangement  of  the  previous  investigators,  was  thus 
avoided,  and  steadily  burning  arcs  up  to  several  cm.  long  resulted. 
From  these  the  gases  were  sucked  off  through  a  water-cooled  platinum 
capillary,  the  equilibrium  thus  being  rapidly  frozen.  With  a  3-cm. 
arc  a  product  with  5  per  cent.  NO  resulted,  and  with  a  longer  arc  a 
gas  with  5*6  per  cent.,  concentrations  corresponding  to  temperatures 
of  3000°  C.  and  3100°  C.  respectively.  Sucking  the  gases  off  a  few  mm. 
away  from  the  arc  gave  much  lower  concentrations,  e.g.  2'65  per  cent., 
indicating  that  decomposition  had  already  largely  taken  place.  Their 
yields  were  high.  A  3-cm.  arc  gave  454  kilos.  HN03  per  K.W.  year 
(52  grams  per  K.W.H.),  and  a  5-cm.  arc  539  kilos,  per  K.W.  year 
(62  grams  per  K.W.H.).  With  longer  arcs  they  anticipated  even  better 
results.  Grau  and  Russ  clearly  recognised  the  possibility  of  the  arc 
having  some  specific  electrical  effect,  but  did  not  nevertheless  discard 
the  thermal  hypothesis. 

We  owe  to  Haber  and  Koenig6  the  considerations  and  the  work 
which  finally  showed  that  electrical  phenomena  without  doubt  play  an 

I  Vber  die  Oxydation  des  Stickstoffes  in  der  Hochspannungsflamme  (1905). 
-  J>rud.  Ann.  20,  743  (1906) ;  23,  209  (1907). 

3  Ann.  Chim.  Phys.  (viii.),  8,  9,  145  (1906). 

4  Zeitsch.  Elektrochem.  12,  540  (1906). 

5  ExperimentaluntersucJiungen  iiber  die  Luftverbrennung  im  elektrischen  Flammen- 
bogen  (1906). 

II  Zeitsch.  Elektrochem.  13,  725  (1907) ;     14,  689  (1908) ;     (with    Platou)     16, 
789  (1910). 


508    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 


exceedingly  important  part,  and  that  thermal  effects  are  probably 
simply  superposed  on  them,  and  of  a  secondary  nature.  They  critically 
examined  the  experimental  researches  just  described,  more  particularly 
comparing  the  temperatures  deduced  from  the  NO  concentrations 
with  the  temperatures  which  appeared  probable  for  other  reasons. 
Concentrations  of  7  per  cent.  NO,  such  as  were  obtained,  correspond 
with  temperatures  of  3500°  C.,  and  from  their  considerations  they 
conclude  that  such  temperatures  were  probably  never  reached,  even 
supposing  that  the  conditions  in  the  arc  remained  constant  with 
respect  to  time  and  position.  But  they  certainly  do  not  in  a  high- 
tension  alternating  current  discharge.  At  every  half-period  the  arc 
is  extinguished,  there  will  be  great  local  cooling  and  reheating,  and 
consequently  all  parts  of  the  arc  will  be  in  constant  rapid  motion. 
There  is  little  doubt  that  the  composition  of  the  arc  gases  must  vary 
very  considerably  with  respect  to  time  and  place,  and  that  the  mean 
temperature  calculated  from  the  NO  concentration  obtained  has  no 
real  significance  or  application.  Further,  it  would  seem  absolutely 
impossible  to  cool  down  NO  from  a  temperature  such  as  3500°  so 
rapidly  as  to  avoid  extensive  decomposition.  Thus  Jellinek1  has 
calculated  that  even  at  2800°,  pure  NO  at  one  atmosphere  will  bo  half 
decomposed  in  2  X  10""  minutes. 

The  experimental  work  of  Haber  and  Koenig  was  decisive.  They 
burnt  a  vertical  arc  inside  a  narrow  tube,  cooled  outside  with  water. 
Various  electrodes  were  used— platinum,  iron  and  Nernst  filaments. 
In  order  to  keep  the  arc  burning  steadily,  the  gases  were  introduced 
and  withdrawn  between  and  not  behind  the  electrodes.  Gas  pressure, 
current,  and  voltage  were  varied.  A  few  typical  results  will  suffice. 

TABLE    LXIX 


Original 
gas 

Pressure 
in  mm.        Current 
of  Hg.'     in  &m^' 

Arc 
voltage 

Litres 
passing 
per  hour 

Per 

cent. 
NO 

Electrode 
material 

Air 

<)4        230    .    |i>   •= 

4800 

8-5 

9-5 

Iron 

Air 

51        205 

2050 

1-0 

^5'2 

Iron 

Air 

Kii      350 

3700 

1-0 

9-8 

Nernst  filament 

Air 

180       290 

5400 

3.V  1 

7-5 

Platinum 

50N«  :  500., 

103      3110 

•4800 

0-8 

14-4 

Nernst  filament 

18N,  :  820., 

62      wo 

2100 

0-91 

11-1 

Nernst  filament 

25N2:  7502 

99       MQ 

4COO 

0-75 

12-8 

Nernst  filament 

These  investigators,  using  cooled  arcs,  could  thus  produce  No  at 
concentrations  corresponding  to  temperatures  up  to  4750° C.,  and, 
moreover,  without  making  any  attempt  to  rapidly  (-hill  the  gases, 
which  simply  passed  slowly  through  the  arc.2  The  results  obtained 


1  Zmte*.  Anorg.  <'!„„,.  49,  229  (/.'**;). 
'I  In-  yield*  obtaim-d  unc  very  low. 


xxvii.]      OXIDATION  OF  ATMOSPHEKIC  NITROGEN  509 

depended  on  the  gaseous  pressure,  and  different  concentrations  were 

given  by  mixtures-  — _2  and  -    -.     If  the  reaction  were  a  thermal  one, 
4JN2  IJN2 

simply  depending  on  the  law  of  mass  action  and  the  temperature,  these 
two  results  would  be  impossible.  That  the  concentrations  obtained 
really  represented  an  equilibrium,  though  not  a  thermal  equilibrium, 
they  showed  by  starting  with  NO,  and  getting  the  same  mixture  as 

they  obtained  from    — -.     The  decomposition  of  the  NO  took  a  measur- 
1N2 

able  time,  and  this  in  itself  proves  that  the  temperatures  were  not 
very  high.  From  certain  other  observations,  they  calculate  them  to  be 
about  2000°-2200°  C. 

Accounts  have  also  been  published1  of  experiments  with  a  short 
direct-current  arc,  using  a  cooled  anode.2  Gases  containing  up  to  9 
per  cent.  NO  were  obtained  from  air.  The  temperature  of  the  arc 
was  about  2700°  C.,  corresponding  to  an  equilibrium  concentration  of 
NO  in  air  of  about  4  per  cent.  only.  The  highest  yields  of  NO  were 
about  90  grams  per  K.W.H.  At  60  grams  per  K.W.H.  the  NO  concen- 
tration was  3'5-4  per  cent.  In  technical  work  it  seldom  exceeds 
2  per  cent.3  This  difference  is  easily  explained  under  the  electrical 

theory,  as  in  technical  arcs  the  voltage  gradient  hardly  exceeds  10          , 

cm* 

whilst  in  the  arcs  used  by  Holwech  it  was  about  200          ' .     The 

cm. 

ionisation  should  be  much  greater  in  the  latter  case. 

Certain  chemists  still  believe  the  thermal  theory  to  be  tenable.  Thus 
Briner  and  Durand 4  suggest  that,  as  our  knowledge  of  the  NO  —  N2—  02 
equilibrium  at  high  temperatures  rests  only  on  an  uncertain  extra- 
polation of  values  experimentally  obtained  at  lower  temperatures,  it  is 
possible,  assuming  the  thermal  theory,  that  the  high  NO  concentrations 
obtained  by  Haber  and  Koenig  may  in  reality  not  correspond  to  such 
excessively  high  temperatures.  Further,  above  3000°  there  may  be 
considerable  dissociation  of  N2  and  02  molecules  into  N  and  0  atoms, 
and  these  may  partly  combine  on  cooling  to  give  NO  instead  of  entirely 
N2  and  02.  Guye  is  of  a  similar  opinion.  In  this  connection  may  also 
be  mentioned  the  recent  work  of  Strutt,5  who,  by  submitting  pure 

1  Morden,  Trans.  Amer.  Electrochem.  Soc.  14,  113  (1908);  Holwech,  Zeitsch. 
Elektrochem.  16,  369  (1910) ;  Holwech  and  Koenig,  Zeitsch.  Elektrochem.  16,  803 
(1910). 

-  In  some  cases  the  cathode  was  specially  heated,  but  this  made  little  difference. 

:i  There  are  two  difficulties  in  applying  these  results  to  technical  work.  Firstly, 
the  apparatus  would  be  exceedingly  complicated.  Secondly  the  series  resistances 
necessary  with  the  direct  current  would  involve  a  loss  of  power,  not  merely  a 
lowering  of  cos  6. 

4  Jour.  Chim.  Phys.  7,  1  (1909). 

&  Proc.  Roy.  Soc.  A.  85,  219  (1911). 


510    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

nitrogen  at  low  pressure  to  the  action  of  a  high-voltage  intermittent  jar 
discharge,  has  produced  a  gas  containing  a  small  fraction  of  an  active 
nitrogen,  with  very  remarkable  though  fleeting  properties,  which  have 
no  connection  with  the  presence  of  charged  particles  or  ions.  Strutt 
supposes  nitrogen  atoms  to  be  present.  This  active  nitrogen  is 
destroyed  by  oxygen,  but  no  NO  results.  It  seems  that  though 
further  investigation  of  this  very  interesting  point  may  perhaps 
throw  fresh  light  on  the  subject  of  the  production  of  NO  in  the 
high-voltage  arc,  yet  Strutt 's  results,  obtained  with  quite  extraordi- 
nary electrical  means,  cannot  be  regarded  as  affecting  the  views  of 
Haber  and  Koenig.1 

These  depend  for  their  justification  on  the  experimentally  ascer- 
tained fact  that  high  NO  concentrations  are  best  obtained  by  slowly 
drawing  gases  through  a  low-temperature  discharge,  not  by  rapidly 
sucking  them  off  from  a  very  high- temperature  arc,  as  the  thermal 
theory  would  demand.  Confronted  by  this  direct  evidence,  one  is 
compelled  to  accept  the  electrical  theory  of  NO  formation  as  sketched 
on  pp.  190-192.  In  the  ordinary  technical  arcs,  as  in  some  of  the  arcs 
used  by  experimenters  prior  to  Haber  and  Koenig,  thermal  effects 
doubtless  play  a  very  important  part  in  causing  the  decomposition 
of  NO  into  nitrogen  and  oxygen,  but  it  is  a  secondary  one. 

2.  Early  Attempts  at  Technical  Apparatus 

The  designers  of  technical  apparatus  have  been  chiefly  guided  by 
the  following  considerations.  Assuming  the  thermal  theory  to  be 
true,  they  have  wished  to  get  very  high  temperatures  with  subsequent 
sudden  cooling.  Then  it  was  early  found  that,  as  the  consumption 
of  electrical  energy  in  a  given  space  increased,  the  NO  concentra- 
tion did  not  increase  as  quickly,  but  soon  reached  a  limit,  and  any 
further  increase  in  the  energy  used  was  wasted.  Hence  inventors  have 
avoided  short  thick  arcs,  and  have  tried  many  devices  for  making 
long  thin  stable  arcs  which  would  come  into  contact  with  a  lam»- 
quantity  of  air.  Then,  to  get  good  NO  concentrations,  it  is  advan- 
tageous for  all  the  gas  fed  into  the  furnace  to  pass  through  the 
discharge,  avoiding  'false  air*  as  much  as  possible.  Finally,  units 
of  large  capacity,  consuming  much  energy,  are  of  importance. 

Naville  and  Guye  worked  on  this  subject  for  years,  and  devised 
many  pieces  of  apparatus.  In  some  the  air  enters  through  one  of 
the  electrodes,  which  is  hollow.  In  others  a  direct-current  arc  passes 
between  electrodes  of  various  shapes,  and  this  is  rotated  rapidly  by 
means  of  a  rotating  magnetic  field.  Just  like  any  other  conductor 
carrying  a  current,  the  electric  arc  is  deflected  by  a  magnetic  field. 

1  Professor  Haber  has  kindly  informed  the  author  that  this  ia  essentially  his 
view. 


xxvn.]      OXIDATION  OF  ATMOSPHERIC  NITROGEN  511 

With  such  an  apparatus,  up  to  400  kilos,  of  HN03  per  K.W.  year  has 
been  obtained. 

Bradley  and  Lovejoy i  proceeded  on  the  principles  that,  for  a  given 
quantity  of  energy,  as  much  air  as  possible  should  come  under  the 
influence  of  the  discharge,  and  that  the  gases  should  be  cooled  very 
quickly.  They  employed  an  apparatus  containing  a  multitude  of  thin 
arcs,  which  were  rapidly  extinguished  and  restarted.  Through  the  wall 
of  a  vertical  iron  cylinder  were  inserted  large  numbers  of  electrodes, 
arranged  in  rows,  insulated  by  porcelain  sleeves,  and  terminating  in 
platinum  wires.  The  cylinder  contained  a  vertical  concentric  axle 
from  which  projected  rows  of  radial  arms,  corresponding  in  number 
and  position  to  the  electrodes  in  the  side  wall  and  provided  with  short 
platinum  tips.  The  vertical  axle  was  rapidly  revolved,  and  a  direct- 
current  voltage  of  8,000-10,000  volts  applied  to  the  electrodes.  When 
two  platinum  points  came  opposite  to  one  another  a  short  arc  was 
struck,  rapidly  lengthened  out  to  4"-6",  and  broke. 

In  the  unit  used  (10  K.W.)  some  414,000  arcs  were  made  and 
broken  per  minute.  The  results  were  very  good — a  gas  with  2-3  per 

cent.  NO,  and  a  HN03  yield  of  86  J         *-  (750  kilos,  per  K.W.  year),2 

Jv.VV.H. 

better  than  any  figures  at  present  reached  technically.  Nevertheless, 
a  larger  unit  than  the  above  was  never  constructed.  The  reason  is 
clear,  the  apparatus  being  far  too  complicated  for  technical  use,  and 
having  a  high  initial  cost  and  heavy  repairs  charges.  (In  addition 
to  the  many  platinum  electrodes,  each  separate  arc  was  provided 
with  a  large  series  resistance  [in  oil]  for  regulating  the  discharge.) 
Moreover,  the  size  of  the  unit  was  excessive  for  its  load— 10  K.W. ; 
a  commercial  unit  should  take  hundreds  of  kilowatts. 

An  early  apparatus  of  Moscicki  and  v.  Kowalski 3  also  failed  because 
of  its  complications,  though  a  75-K.W.  plant  was  run  for  some  time. 
These  investigators  used  an  alternating  current  arc  of  high  voltage 
and  high  frequency.  A  large  number  of  arcs  discharged  in  parallel 
towards  a  common  earthed  conductor.  Current  of  ordinary  com- 
mercial frequency  was  primarily  used,  and  the  high  frequency  (6,000- 
10,000  periods)  obtained  by  a  suitable  arrangement  of  condensers.  The 
phase  difference  thus  produced  was  compensated  by  suitably-arranged 
inductances.  A  maximum  output  of  412  kilos.  HN03  per  K.W.  year 

(^  ^          r  )  resulte(l.     Haber  and  Platou4  have  shown  since  that 
Jv.\V  .H./ 

on  a  small  scale  the  use  of  high  frequencies  gives  rather  worse  results, 

1  Elrrtrochcm.  2nd.  1,  20  (1902) ;  Zeitech.  Elektrochem.  9,  381  (1903) 

"  Cf.  results  of  Morclen,  Hohvech,  and  Koenig  (p.  509). 

3  Electrochem.  Ind.  5,  491  (1907). 

1  Zcit-ich.  Elektrochem.  16,  797  (1910). 


512    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

but  express  the  opinion  that  in  large  arcs  a  change  of  frequency  has 
no  appreciable  effect. 

Moscicki  and  v.  Kowalski  have  more  recently  adopted  quite  a 
different  kind  of  apparatus,  resembling  that  of  Birkeland  and  Eyde 
(see  below).  A  high-tension  arc  (either  direct  or  alternating  current) 
plays  between  two  concentric  cylinders  or  rings.  Then,  by  means  of 
a  magnetic  field  parallel  to  the  axis  of  the  cylinder,  this  arc  is  made  to 
rotate  radially  around  the  intervening,space.  In  plan  the  appearance 
of  the  arrangement  is  that  of  a  luminous  ring  enclosed  between  two  ring 
electrodes.  The  air,  of  course,  passes  through  this  space.  Using  a  27- 
K.W.  unit,  60  grams  per  K.W.H.  (525  kilos,  per  K.W.  year)  of  HN03 
were  obtained.  With  a  larger  plant  still  better  results  are  anticipated. 
Brion  *  describes  elaborate  experiments  carried  out  with  a  very  similar 

apparatus.     With  a  1  per  cent.  NO  concentration  he  got  55  Jf 

K.  W.H. 

HN03,  with  1-5  per  cent.  50  -        f--,  but  with  2  per  cent.  NO  only 

Jv.  VV.H. 

35  J  •     His  apparatus,  however,  only  consumed  2-4  K.W.     Alter- 

K.W.H 

nating  discharges  gave  distinctly  the  better  results. 

The  three  processes  actually  in  successful  technical  operation  must 
now  be  discussed.  The  Birkeland-Eyde  process  is  worked  at  Notodden, 
Saaheim,  and  Zinnfoss,-  and  the  Schonherr  process  at  Saaheim  and 
Christiansand,  all  in  Norway;  while  the  Pauling  process  is  operated 
near  Innsbruck,  and  in  the  Hautes-Alpes  department  of  France. 

3.  Birkeland-Eyde  Process 

The  difficulty  of  producing  stable  high-current,  high-voltage  arcs  has 
already  been  mentioned.2  In  order  to  design  apparatus  with  a  high 
electrical  energy  consumption  per  unit,  inventors  have  therefore 
attempted  to  break  up  this  single  heavy-current  discharge  into  a  large 
number  of  low-current,  high-voltage  arcs.  We  have  seen  that  such 
attempts  have  generally  failed,  on  account  of  the  complications  thereby 
introduced.  Birkeland  and  Eyde  succeeded  for  the  first  time  in 
evolving  a  technically  successful  apparatus  in  which  this  is  effected,  an 
apparatus  of  very  simple  construction  and  giving  a  perfectly  stable 
discharge. 

Imagine  two  electrodes  1  or  2  cm.  apart  in  air,  and  connected 
in  series  with  a  resistance  and  a  high-voltage  source  of  direct  current. 
The  high-tension  arc  first  produced  quickly  breaks  down  to  a  low- 
voltage  arc  carrying  a  heavy  current,  a  type  of  discharge  useless  for 
HN03  synthesis.  Suppose  now  that  the  arc  be  put  between  the  poles 

1  Zeitsch.  Elektrochem.  18,  761  (1907).  -  P.  187. 


xxvn.]     OXIDATION  OF  ATMOSPHERIC  NITROGEN  513 

of  a  powerful  electromagnet.  As  we  have  seen,  an  electric  arc  is  de- 
viated in  the  magnetic  field,  just  like  any  other  conductor  carrying 
a  current.  Consequently  the  path  of  the  discharge  will  be  bent  out- 
wards from  the  electrodes  in  semicircular  form.  The  resistance  of  the 
arc  thereby  increases,  the  current  falls,  and  the  voltage  rises.  When  this 
has  risen  sufficiently  to  strike  again  across  the  electrodes,  a  second 
arc  will  be  formed,  which,  before  it  has  time  to  break  down  to  the  low- 
tension  arc,  will  be  similarly  bent  away  from  the  electrodes  parallel 
to  and  concentrically  with  the  first.  By  suitable  regulation  it  will 
be  possible  to  produce  a  large  number  of  parallel  semicircular  arcs 
travelling  outwards  from  the  electrodes. 

The  total  current  will  then  be  the  sum  of  the  currents  in  all  these 
arcs,  whilst  the  voltage  will  be  that  of  an  ordinary  high-tension  arc.  An 
unlimited  number  of  arcs  cannot  be  produced,  as  they  will  finally 
extinguish  when  so  long  and  of  such  a  resistance  that  the  applied 


FIG.  130.— Birkeland-Eyde  Arc. 

voltage  no  longer  suffices  to  keep  them  playing.  This  is  the  very 
simple  principle  used  by  Birkeland  and  Eyde.  Only,  instead  of  direct, 
they  employ  alternating  current.  The  arcs  are  thus  extinguished  at  the 
end  of  every  half -period,  even  supposing  this  not  ^to  occur  during 
the  half -period.  Further,  the  arcs  are  not  all  bent  out  on  one  side  of  the 
electrodes,  but  alternately  on  either  side,  according  to  the  direction  of 
the  current.  The  discharge  appears  then  as  in  Fig.  130.  Owing  to^the 
extreme  rapidity  of  the  making  and  breaking  of  the  arcs,  what  is 
seen  is  apparently  a  disc  of  light.  It  is  obvious  thafc  such  an  arrange- 
ment permits  a  great  concentration  of  power  in  a  limited  space,  and 
that  air  introduced  near  the  electrodes  and  removed  at  the  periphery 
of  the  disc  will  be  very  fully  exposed  to  the  action  of  the  discharge. 

In  the  technical  unit,1  the  electrodes  are  copper  tubes,  water-cooled, 
and  placed  about  1  cm.  apart.  The  flamingr 'disc,  its  temperature 
estimated  at  about  3200°  C.,  burns  vertically  in  a  furnace  cased  with 


Trans.  Farad.  Soc.  2,  98  (1906);  and  private  communication. 


2  L 


514    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP! 


air 


steel  and  lined  with  firebrick,  perforated  by  holes  through  which  the 
enters     The  furnace  chamber  in  a  750-K.  W.  unit  (often,  however,  loaded] 
in  practice  with  850-900  K.W.)  is  about  5'  in  length  and  breadth! 

corresponding  to  the  dimensions  of  j 
the  disc,  but  very  narrow  (2"-6"i 
in  the  direction  of  the  magnetic! 
field.  The  two  electromagnets! 
themselves  are  connected  with] 
the  external  casing.  They  absorb! 
O'35-l  per  cent,  of  the  total! 
furnace  load.  Fig.  131  is  a  sec-J 
tional  elevation  at  right  angles  toj 
the  disc.  The  passage  of  the  airj 
is  indicated  by  the  arrows.  It 
enters  the  chamber  through  the 
channels  shown  in  the  lining. 

Such  a  unit  requires  about 
5,000  volts,  of  which  3,300-3,900 
volts  are  across  the  arc,  the  re- 
mainder being  absorbed  by  the 
series  inductances.  Cos  6  is  0'66- 
0-68  (frequency  50  periods)  and 
the  current  190-200  amperes. 


A,  Core  ;  B,  Winding  ;  C,  Gas 
entrance ;  D,  Gas  exit. 

Fia.  131. — Birkeland-Eyde  Furnace. 


But  Birkeland-Eyde  furnaces  of1 
far  greater  capacity  are  now 
constructed,  each  taking  3,200- 
4,000  K.W.,  the  voltage  relations 

being  the  same.  Thus  the  Notodden  factory  contains  four  3,500  K.W. 
units  working  with  a  current  of  940  amps.  The  discharge  burns  very 
steadily,  hardly  needing  attention.  The  electrodes  last  some  300  hours 
without  repair,  and  can  be  rapidly  changed.  The  furnace  lining  is 
very  durable.  The  furnace  gases  contain  about  1-1'2  per  cent.  NO. 

The  yield  of  HN03  is  about  67    grams   (580-590  kilos,  per  K.W.  year). 

K.W.H. 


4.  Pauling  Process1 

Fig.  132  represents  diagrammatically  the  arrangement  employed. 
The  two  electrodes  aa  resemble  horn  lightning  conductors.  They  are 
of  cast  steel,  water-cooled,  and  30  cm.  long.  The  air,  preheated  by  the 
hot  furnace  gases,  is  blown  up  into  the  intervening  space  through  the 
nozzle  e.  The  electrodes  are  provided  with  vertical  slits  at  the  points 
nearest  to  one  another  (4  cm.  apart),  and  through  these  slits  pass  thin 
strips  ('  ignition  knives  ')  of  iron,  bb.  Their  distance  apart,  2-3  mm., 

1  Zeitsch.  Elektrochcm.U,  544  (190!>);  17,  431  (1911). 


xxvii.]     OXIDATION  OF  ATMOSPHERIC  NITROGEN          515 


is  regulated  by  mechanism  at  dd,  which  is  insulated  by  cc  from  the 
electrodes.  The  strips  are  so  thin  that  they  do  not  affect  the  current 
of  air  from  e.  To  start  the  arc,  a  suitably-connected  auxiliary  current, 
of  low  wattage  but  of  the  necessary  high  voltage,  is  used.  The  alter- 
nating current  arc  strikes  between  bb.  The  rapid  stream  of  air  deforms 


FIG.  132.— Pauling  Arc  and  Electrodes. 

it,  and  carries  it  up  and  along  the  electrodes  aa  in  the  form  of  a  flame, 
about  3'  high  in  actual  practice.  At  every  reversal  of  current  a  fresh 
arc  strikes,  and  the  whole  appearance  is  that  of  a  large  flaming  arc, 
such  as  was  used  by  Erode  and  others.  The  temperature  is  high  and 
the  flame  intensely  white,  owing  to  the  presence  of  iron  vapours. 

The  life  of  the  electrodes  is  short — 200  working  hours  for  the  cooled 


FIG.  133.— Pauling  Nitric  Oxide  Furnace. 

main  electrodes,  and  20  hours  for  the  *  ignition  knives,'  which  must 
constantly  be  pushed  up  towards  one  another.  They  can  be  readily 
replaced.  Two  such  arcs  in  series  are  contained  in  each  furnace  (front 
and  side  elevation,  Fig.  133).  Each  is  in  a  separate  chamber,  these 
being  connected  with  a  common  flue.  The  hot  reaction  products 

2  L  2 


516    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

are  cooled  by  introducing  through  A  cold  gases  which  have  already 
been  through  the  discharge.  The  whole  leaves  the  furnace  at  about 
1000°.  Such  a  furnace  consumes  400  K.W.,  200  K.W.  per  arc,  and 
takes  4,000  volts.  Assuming  cos  6  =  0*7,  the  current  passing  will  be 
about  140  amperes.  Several  furnaces  are  worked  in  parallel.  Lately 
their  load  has  been  increased  to  600  K.W.,  and  1,000  K.W.  units  are 
expected  to  be  shortly  in  operation.  The  gases  obtained  contain  1-1-j 


1-5  per  cent.  NO;  60  (525  kilos,  per  K.W.  year)  of  HN03  are 

guaranteed,  and  up  to  70  jf^srg-  nave  Deen  obtained. 

5.  Schonherr-Hessberger  Process  l 

Quite  a  different  principle  is  here  used  for  the  construction  of  a 
commercial  unit  carrying  a  heavy  load.  As  has  been  emphasised,  a 
long,  heavy-current,  high-tension  arc  is  a  very  unstable  thing.  It  is 
easily  extinguished,  particularly  if  cold  air,  impinging  on  it,  deforms  or 
cools  it.  In  the  two  processes  above  discussed  this  fact  is  accepted, 
and  no  attempt  made  to  circumvent  it.  The  electrodes  are  placed  very 
close  together,  and  the  air  supplied  is  caused  to  enter  at  right  angles 
to  the  path  of  the  arc  —  in  the  direction,  in  fact,  in  which  it  is  most 
capable  of  extinguishing  the  arc.  By  suitable  means  (magnetic  field 
or  strong  current  of  air)  the  arc  is  deformed  or  lengthened  until  it 
finally  goes  out,  either  because  of  the  deformation  or  because  of  the 
current  reversal.  If  for  the  latter  reason,  it  cannot  restrike  at  the 
same  point  when  the  voltage  again  increases,  because  the  air  passing 
through  lowers  the  temperature  and  increases  the  resistance  too  much. 
A  fresh  arc  strikes  across  the  electrodes  between  their  nearest  points, 
and  is  in  its  turn  also  deformed  and  extinguished.  The  discharge 
therefore  consists  of  a  number  of  arcs  rapidly  succeeding  one  another, 
striking,  and  becoming  extinguished. 

Schonherr,  on  the  other  hand,  directly  attacked  the  problem  of 
producing  a  stable,  high-current,  high-voltage  arc.  He  recognised  that,- 
if  the  air  were  so  introduced  that  it  did  not  deform  the  discharge 
or  cool  it  excessively,  a  stable  arc  should  be  possible  with  both  direct 
and  alternating  current.  Using  the  latter,  it  should  be  possible  to  keep 
the  temperature  so  high  that  after  reversal  the  conductivity  suffices 
for  the  arc  to  re-form.  He  achieved  these  conditions  very  simply.  As 
in  the  Pauling  furnace,  it  is  the  air  current  which  draws  the  arc  out 
from  a  short  to  a  great  length  ;  yet  it  is  introduced,  not  at  right 
angles  to  the  discharge,  but  so  that  it  surrounds  the  latter  and  travels 
along  in  a  parallel  direction.  It  is  not  forced  into  the  discharge,  but 
enters  it  by  diffusion  and  through  eddy  currents,  and  from  all  sides 

1  Trans.  Amer.  Electrochem.  Soc.  16,  131  (1909);  also  private  communication. 


XXVIL]      OXIDATION  OF  ATMOSPHERIC  NITROGEN  517 


Water 


simultaneously.     The  arc  is  neither  deformed  nor  unduly  cooled,  and 
burns  quietly  and  stably. 

Fig.  134  shows  the  cross-section  of  a  furnace.1  The  discharge 
chamber  (4)  consists  essentially  of  a  vertical  iron  pipe,  connected 
to  earth  and  serving  as  one  electrode.  It 
is  20/-25/  in  length,  a  few  inches  in  diameter, 
and  water-jacketed  at  its  top  end.  Its  base 
is  slightly  widened  out,  and  contains  the 
other  electrode,  also  vertically  placed  and 
carefully  insulated.  This  consists  (E)  of  a 
rod  of  iron,  passing  through  a  water-cooled 
copper  block.  The  iron,  which  takes  the 
total  current,  is  slowly  consumed,  and  must  be 
occasionally  pushed  up  through  the  copper 
block.  The  central  discharge  chamber  is 
surrounded  by  other  concentric  vertical  iron 
pipes,  and  finally  cased  with  iron  lined  with 
firebrick.  The  air  enters  at  C,  traverses  the 
annular  spaces  (2)  and  (3)  between  the  ver- 
tical pipes,  where  it  becomes  preheated, 
enters  (4)  near  the  bottom  electrode,  passes 
out  of  the  same  at  the  top,  and  finally  leaves 
the  furnace  at  D.  Its  course  throughout  is 
indicated  by  the  arrows.  GGG  are  peep- 
holes. One  pole  of  the  source  of  current,  in 
series  with  a  regulating  inductance,  is  con- 
nected with  E.  The  other  pole  is  earthed. 

To  start  the  arc,  the  iron  bar  Z,  actuated 
by  the  lever  shown,  is  moved  down  until  it 
makes  contact  with  E.  The  discharge  com- 
mences, Z  is  withdrawn,  and  a  short  arc 
plays  between  E  and  the  adjoining  wall  of 
the  furnace.  The  air  is  now  introduced 
through  holes  regularly  placed  in  the  wall  of 
the  furnace  round  E.  These]  air  entrances 
are  directed  tangentially  to  the  furnace  wall, 
and  the  incoming  air  consequently  ascends 
the  latter  with  a  helical  motion,  carrying  up 
the  end  of  the  arc  with  it.  The  number  of 
entrances  thus  opened  can  be  regulated. 
The  length  of  the  arc  is  determined  by  the 
applied  voltage  and  by  the  conductivity,  hence  by  the  temperature, 
hence  by  the  velocity  of  the  surrounding  air  layer.  By  adjusting 
the  air  supply,  therefore,  the  point  at  which  the  arc  strikes  across  to 


FIG.  134.— Hessberger 
Furnace. 


Designed  by  Hessberger. 


518    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

the  furnace  wall  can  be  regulated,  and  it  is  arranged  that  this  occurs 
somewhere  in  the  water-cooled  part  of  the  latter. 

The  result  of  this  regular  directed  introduction  of  air  is  a  perfectly 
stable  arc  some  20'  long,  which  plays  between  E  and  some  point  in  the 
water-jacketed  furnace  top.     This  arc  is  surrounded  by  a  cushion  o: 
air  which  ascends  the  pipe  in  a  more  or  less  spiral-like  path.     Constanl 
interchange  of  gases  necessarily  takes  place  between  air  layer  anc 
discharge,  owing  to  the  temperature  difference  between  them,  and  to 
the  eddy  motions  set  up  by  the  continual  temperature  changes  in  the 
arc  itself,  due  to  the  alternating  current  variations.     But  as  the  amoun 
of  cool  air  entering  the  discharge  in  a  short  space  of  time  is  small 
compared  with  the  mass  of  heated  air  present,  the  temperature  and 
conductivity  of  the  arc  gases  are  not  materially  lowered,  and  the 
discharge  continues  passing  despite  the  half-period  reversals  of  voltage. 
The  distance  between  E  and  the  furnace  wall  is  sufficient  to  prevent 
the  arc  ever  preferably  striking  at  that  point  across  the  incoming 
cold  air.     Should,  however,  the  arc  go  out,  the  voltage  rises,  it  auto- 
matically relights  at  the  bottom,  and  is  again  carried  up. 

The  Hessberger  furnaces  are  usually  fed  with  three-phase  current, 
three  being  combined  together  in  star.  The  largest  built  take  nominally 
700-750  K.W.,  but  can  be  loaded  up  to  1,000  K.W.  Such  a  unit  has 
voltage  and  current  respectively  about  3,500  volts  and  290  amperes. 
Taking  into  account  the  series  inductance  necessary,  cos  6  is  0'66. 

The  gases  contain  l'5-2  per  cent.  NO,  and  the  yield  is  68T5™!™t  (590- 

Jv.  W.H. 

600  kilos,  per  K.W.  year)  of  HN03.  The  entering  air  is  wanned  to 
500°  during  its  passage  into  the  arc  chamber,  leaves  the  same  at  about 
1200°,  and  cools  to  850°  before  leaving  the  furnace.  Its  residual  heat 
is  suitably  utilised.  Schonherr  calculates  that  about  40  per  cent,  of  the 
energy  supplied  to  the  arc  appears  as  hot  water,  30  per  cent,  as  steam, 
17  per  cent,  is  lost  as  radiation,  10  per  cent,  lost  by  the  final  cooling 
with  water,  and  only  3  per  cent,  used  in  the  formation  of  nitrogen 
compounds. 

Wear  and  tear  and  first  cost  of  the  plant  are  both  low.  The  iron 
core  of  the  middle  electrode  (E)  gradually  oxidises  away,  must  be  pushed 
up,  and  finally  replaced.  This  can  be  rapidly  effected.  The  iron 
walls  of  the  furnace  are  protected  by  an  air  layer,  and  only  come  into 
contact  with  the  arc  at  the  outset.  The  upper  part,  where  the  arc 
plays,  is  water- jacketed  and  only  very  slowly  oxidised.  It  is  made 
in  a  separate  piece  to  facilitate  renewal. 

A  comparison  of  these  three  processes  is  interesting.  The  Schonherr 
and  Birkeland-Eyde  processes  furnish  almost  identical  yields,  but  the 
former  gives  a  richer  gas.  The  furnace  is,  further,  of  very  simple 
construction  and  durable.  The  Birkeland-Eyde  furnaces  are  less 
simple  and  cost  more  initially,  but  far  larger  units  can  be  built, 


xxvn.]     OXIDATION  OF  ATMOSPHERIC  NITROGEN          519 

which  is  [a  great  advantage.  The  Pauling  apparatus  is  exceedingly 
simple,  but  gives  yields  about  10  per  cent,  lower.  The  units  are 
about  the  size  of  the  Hessberger  furnaces.  All  three  furnaces 
appear  to  require  very  little  attention  whilst  working,  and  this  fact, 
of  course,  lessens  the  undoubted  advantages  of  the  large  Birkeland- 
Eyde  units.  Nevertheless,  the  recent  Norwegian  large-scale  tests 
appear  to  have  indicated  that  the  Birkeland-Eyde  furnace  (3,500 
K.W.)  is  at  least  as  satisfactory  technically  as  the  Hessberger 
furnace  (750  K.W.),  if  not  more  so. 

From  the  point  of  view  of  the  theory  of  NO  formation,  the  com- 
parative results  obtained  certainly  do  not  support  the  correctness  of 
the  thermal  hypothesis.  The  NO  concentration  is  lower  in  the  Pauling 
process,  in  which  cooling  gases  are  used,  than  in  the  Schonherr  process, 
in  which  the  air  passes  slowly  along  and  through  the  arc.  We  should 
expect  the  opposite  result.  There  seems  no  doubt  that  the  electrical 
theory  is  primarily  the  correct  one.  That  powerful  thermal  effects 
are  superposed  is  clear,  but  the  more  prominent  they  are,  the  lower 
the  NO  concentration  and  the  yield. 

6.  Working  up  of  Furnace  Gases 

Having  discussed  the  formation  of  NO  in  the  furnace,  we  must 
now  consider  the  working  up  of  the  gas  to  HN03  and  other  products. 
The  gases  leave  the  furnaces  at  temperatures  between  600°-1000°. 
Below  600°  the  combination  of  NO  and  oxygen  commences.  Below 
140°  it  is  complete.  It  is  a  reaction  which  takes  place  with  a  low 
velocity *  that  appears,  according  to  Bodenstein  and  to  Foerster,  to  have 
a  slight  negative  temperature  coefficient,  the  only  known  case.  The 
next  stage  is  the  absorption  of  the  resulting  N204— N02— NO  mixture 
in  water  or  a  suitable  alkaline  solution.  We  owe  to  Foerster  and  Koch 2 
a  detailed  study  of  the  action  of  water  on  N02  gas.  The  gas  at  the 
outset  behaves  as  a  mixture  of  N203  and  N205,  and  gives  a  solution 
containing  HN02  and  HN03. 

2N02  +  H20  — >  HN02  +  HN03. 

But  this  solution  is  only  stable  when  exceedingly  dilute.  Otherwise 
the  HN02  decomposes  as  follows  : 

3HN02 — >  HN03  +  2NO  +  H20. 
Summing  these  two  equations,  we  have 

3N02  +  H20  ^±  2HN03  +  NO, 

expressing  the  total  result  of  the  action  of  N02  gas  on  water.  This 
reaction  is  reversible.  NO  gas,  passed  into  strong  HN03,  evolves  N02. 

1  Holwech,  Zeitsch.  Angew.  Chem.  21,  2131  (1908). 

2  Zeitsch.  Angew.  Chem.  21,  2161,  2209  (1908). 


520    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [c  HAI>. 

Consider  now  the  absorption  of  the  dilute  technical  gases'  by  water 
in  the  light  of  this  equation  and  of  the  mass-action  law.  There  is 
always  a  certain  tendency  for  the  reaction  to  take  place  from  right  to 
left,  as  the  gases  invariably  contain  some  unoxidised  NO.  Further, 
in  a  fairly  strong  HN03  solution,  much  of  the  water  present  is  combined 
with  the  anhydrous  acid  as  hydrate,  and  is  unable  to  react  according 
to  the  above  equation.  Lastly,  the  N02  partial  pressure  in  the  gases 
treated  is  very  low,  and  the  vapour  pressure  of  the  HN03  continually 
increases.  These  facts  show  that  an  equilibrium  concentration  of  HN03 
must  be  reached  which  cannot  be  exceeded  by  further  bubbling  the 
gases  through,  and  this  concentration  will  be  greater  the  greater 
the  N02  partial  pressure  in  the  gases.  It  was  found  to  be  (at  room 
temperature)  68-69  per  cent.  HN03  with  [1NO  :  20  J,  >  55  per  cent. 
HN03  with  5  per  cent.  N02,  52  per  cent.  HN03  with  2  per  cent. 
N025j46  per  cent.  HN03  with  1  per  cent.  N02.  With  technical  furnace 
gases  ^it  should  be  possible,  then,  to  produce  a  46-52  per  cent, 
acid. 

The  behaviour  of  the  gases  towards  an  alkaline  solution  is  equally 
interesting.  As  long  as  they  contain  some  NO,  a  nitrite  is  quantitatively 
and  rapidly  formed  ;  i.e.  they  behave  as  if  the  NO  were  present  combined 
with  N02  as  N203.  Le  Blanc  suggested  this  actually  to  be  so,  and 
Foerster  and  Blich1  subsequently  proved  experimentally  the  sup- 
position to  be  correct.  An  excess  of  N02  acts  more  slowly,  giving  a 
mixture  of  nitrite  and  nitrate. 

On  the  large  scale,  HN03  is  made  by  the  Pauling  plants.  The 
formation  of  N02  is  given  time  to  take  place  by  passing  the  gases 
slowly  through  a  large  reaction  chamber,  much  of  their  heat  being  first 
suitably  utilised.  The  absorption  takes  place  in  towers  of  granite, 
sandstone,  earthenware,  etc.,  packed  with  a  substance  like  quartz.  The 
counter-current  principle  is  used  and  a  35-40  per  cent,  acid  results.2 
This  is  further  concentrated  by  waste  heat  to  60  per  cent.  acid.  Finally, 
by  distilling  this  60  per  cent,  acid  with  twice  its  weight  of  92  per  cent. 
H2S04  (done  by  spraying  the  mixture  into  a  tower  filled  with  lava  and 
heated  from  without)  a  98  per  cent,  acid  results.  The  H2S04  is  con- 
centrated and  used  again.  At  Notodden  a  40  per  cent,  acid  is  got 
in  the  first  tower.  Succeeding  towers  give  about  25  per  cent,  and 
10  per  cent.  acid.  In  all  cases  the  gases,  after  leaving  the  last  acid 
absorption  tower,  pass  through  iron  absorption  towers,  where  they 
react  with  NaOH  or  Na2C03  liquors.  They  usually  contain  enough 
NO  to  give  a  product 3  containing  95  per  cent,  of  its  nitrogen  as  nitrite 
and  only  5  per  cent,  as  nitrate.  Small  quantities  of  nitrogen  compounds 

1  Zeiisch.  Angew.  Chem.  23,  2017  (UUO). 

2  It  is  obvious  that  here,  as  in  the  other  cases,  insufficient  time  is  given  to 
reach  the  equilibrium  state  observed  by  Foerster  and  Koch. 

3  Used  for  the  nitre-pots  of  the  sulphuric  acid  chamber  process. 


xxvii.]      OXIDATION  OF  ATMOSPHERIC  NITROGEN  521 

escape,  up  to  4  per  cent,  of  the  whole.  Of  the  amount  absorbed, 
four-fifths  is  held  by  the  acid,  one-fifth  by  the  alkaline  towers. 

In  the  Norwegian  plants,  the  acid  produced  chiefly  serves  to  dissolve 
limestone  to  Ca(N03)2.  The  solution  obtained  is  concentrated,  and 
finally  allowed  to  solidify.  The  product  contains  12'75-13'1  per  cent, 
nitrogen.  Much  NH4N03  is  produced  by  direct  neutralisation  of  gas- 
works' ammonia  liquors.  At  Christiansand  (Hessberger  furnaces)  the 
gases  are  absorbed  hot  (200°  C.)  by  NaOH  liquors.  About  98  per  cent. 
is  so  retained,  the  sole  product  being  NaN02  (96*4  per  cent.  pure). 

Many  other  methods  have  been  suggested  for  working  up  the 
furnace  gases.  Some  of  these  are  of  considerable  interest.  In  Schloes- 
ing's  patent,  the  nitrous  gases  are  absorbed  at  300°-350°  by  quicklime. 
Anhydrous  Ca(N02)2  and  finally  Ca(N03)2  result.  Guye  proposes  to 
compress  the  moist  gases  to  five  atmospheres,  and  then  suddenly  to 
expand,  when  a  cloud  of  95  per  cent.  HN03  is  claimed  to  result.  Another 
suggestion  is  treatment  with  ozone.1  In  presence  of  water,  concen- 
trated HN03  results  at  once.  But  cheap  ozone  is  needed. 

1  Zeitsch.  ElektrocTiem.  12,  549  (1906) ;  also  see  Foerster  and  Koch,  loc.  cit. 


CHAPTER  XXVIII 

OZONE 
1.  Theoretical1 

OZONE,  a  polymerised  form  of  oxygen,  of  molecular  formula  03,  is,  at 
ordinary  temperatures,  a  strongly  smelling  gas.  It  has  powerful 
oxidising  and  bactericidal  properties,  and  is  extensively  used  for 
water  purification.  Its  formation  from  oxygen  is  an  endothermic 
reaction.  At  room  temperature 

302—  ->  203  -  68200  Cals. 

The  proportion  of  ozone  in  the  equilibrium  mixture  with  oxygen  is 
very  low,  but  increases  as  the  temperature  rises.  At  1300°  it  is  O'l  per 
cent.,  1  per  cent,  at  2000°,  and  at  4500°  about  10  per  cent.  By  heating 
oxygen  to  a  very  high  temperature  and  rapidly  cooling,  it  is  thus  possible 
to  produce  ozone,2  but  only  in  low  concentrations  owing  to  its  very 
rapid  decomposition.  But  the  use  of  a  silent  electric  discharge  at 
room  temperature  avoids  this  difficulty.  Ozone  is  produced  until  a 
definite  electrical  equilibrium  has  been  reached.  This  equilibrium 
corresponds  to  a  thermal  equilibrium  at  a  higher  temperature,3  and 
hence  the  ozone  concentrations  resulting  can  be  quite  considerable. 

When  the  electrical  equilibrium  has  been  reached,  we  must  assume 
equal  rates  of  formation  and  decomposition  of  the  ozone  molecules,  and 
the  yield  of  ozone  per  ampere-hour  or  K.W.H.  drops  to  zero.  Further, 
the  smaller  the  ozone  concentrations,  the  greater  the  yield,  a  limiting 
value  being  obtained  at  zero  concentration.  The  yield  must  always, 
therefore,  be  defined  in  reference  to  the  concentration.  To  characterise 
the  behaviour  of  an  apparatus  under  given  electrical  conditions,  we 
require  to  know  the  yield  per  ampere-hour  at  zero  concentration 
(limiting  yield),  the  limiting  concentration  of  ozone  attainable,  and  the 
rate  of  variation  of  yield  with  concentration. 

In  Chap.  XIV.  the  simplest  forms  of  silent  discharge — positive  and 

I  Warburg,  Jahrb.  der  Radio.  6,  184  (1900). 

I  Fischer  and  pupils,  Ber.  39,  940,  2557,  3631  (1906) ;  40,  443,  1111  (1907). 
P.  191, 

522 


OZONE 


523 


FIG.  135. 
Glass  Ozoniser. 


negative  point  discharge  —  their  appearance  and  their  characteristics, 

were  briefly  discussed.     In  technical  apparatus  we  usually  encounter  a 

rather  more  complex  form  of  discharge,  viz.  alternating 

current  discharge  between  electrodes  of  small  curva- 

ture, of  which  at  least  one  is  an  insulator  or  bad 

conductor.      Such  is    the  discharge  in  an  ordinary 

ozone  tube.     In  the  older  all-glass  apparatus  (diagram- 

matically  shown  in  Fig.  135)  we  have   two  coaxial 

glass  tubes,  the  outer  surface  of  the  outer  and  the 

inner  surface  of  the  inner  being  covered  with  tinfoil 

connected  with  the  poles  of  a  source  of  high  voltage. 

The  discharge  passes  from  one  glass  surface  to  another 

across  the  annular  space  between  the  two  tubes.     In 

later  forms  the  inner  tube  is  of   metal  —  aluminium, 

gilded  brass,  etc.     In  such  ozonisers,  with  their  large 

discharge  surface,  the  current  density  is  at  first  much 

lower  than  with  a  point  discharge  and  the  glowing 

much  weaker.     But  as  voltage  and  current  rise  the 

glowing  becomes  more  pronounced,  and  brush  discharges  begin  to  pass 

between  isolated  points  on  the  two  surfaces.     Ultimately  the  whole 

space  becomes  uniformly  luminous. 

In  all  forms  of  discharge  there  is  a  close  connection  between  the 
luminous  phenomena  observed  and  the  ozone  produced,  as  measured  by 
the  limiting  yield.  Ozone  is  only  appreciably  formed  in  those  regions 
which  are  luminous.  This  is  plainly  seen  on  comparing  the  yields 
given  by  the  different  forms  of  point  discharge  at  different  voltages  with 
the  corresponding  luminous  phenomena.  If,  moreover,  the  distance 
between  a  negatively-charged  point  and  an  earthed  electrode  be  in- 
creased, keeping  the  current  and  the  size  of  the  '  positive  column  ' 
constant,  no  increase  is  observed  in  the  ozone  formed,  showing  that 
it  arises  solely  in  the  luminous  positive  column.  In  ozone  tubes  the 
limiting  yield  is  increased  by  increasing  the  width  of  the  inter-electrode 
space,  but  the  rate  at  which  the  ozone  yield  diminishes  with  increase 
of  concentration  increases  still  more  rapidly,  and  consequently  smaller 
limiting  ozone  concentrations  are  possible  in  such  tubes  than  in  tubes 
with  a  narrow  annular  space. 

The  relation  between  Ac  —  the  yield  of  ozone  per  amp.  -hour  at  an 
ozone  concentration  C  —  and  the  limiting  yield  A0  is,  if  C  is  not  too  high, 

AC  =  A0-/3C. 

/?  is  therefore  a  measure  of  the  rate  of  de-ozonisation.     The  highest 
possible  concentration  C0  is  given  by  1 

c-  A» 

"- 


1  This  and  other  results  must  be  taken  for  granted  here. 


524     PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

In  a  certain  case,  using  oxygen,  Ao  was  548,  p  2*19,  C0  125,  which  gives 
some  idea  of  the  size  of  these  magnitudes.     The  yields  are  in  grains 

and  the  concentrations  in 

metre3 

The  gas  pressure  affects  the  limiting  yield  considerably,  the  latter 
decreasing  rapidly  as  the  pressure  falls.  In  air  the  limiting  yield  is 
less  than  half  that  possible  in  oxygen.  The  effect  of  temperature 
variation  is  rather  complex.  In  all  cases  a  rise  in  temperature  rapidly 
increases  the  rate  of  de-ozonisation  —  i.e.  ft  increases.  The  limiting  con- 
centration is  therefore  greatest  at  low  temperatures.  This  is  particu- 
larly so  with  air,  for  not  only  does  ft  increase  but  A0  decreases  as  the 
temperature  is  raised.  The  reason  is  probably  the  formation  of  oxides  of 
nitrogen,as  A0  for  pure  oxygen  is  practically  independent  of  temperature. 

Not  only  nitrogen  oxides,  but  other  gases  present,  can  lower  the 
yield  of  ozone.  Chlorine  acts  very  strongly,  and  aqueous  vapour 
must  also  be  avoided.  Thus  in  moist  air,  with  an  aqueous  vapour 
pressure  of  7  mm.,  the  limiting  yield  can  easily  fall  to  60-70  per  cent. 
of  that  given  by  the  dried  gases.  The  reason  is  probably  that  the 
gaseous  ions  condense  the  moisture  around  them—  a  well-known 
phenomenon.  Their  velocity  and  their  power  of  ionising  by  impact 
are  thereby  lowered. 

Technically  the  yield  per  K.W.H.,  not  that  per  ampere-hour,  is  all- 
important  ;  and  in  that  connection  the  low  power  factor  of  most 
ozonisers  (they  use  alternating  current)  must  be  noted.  An  ozone 
tube  or  apparatus  can  be  regarded  electrically  as  a  number  of  con- 
densers or  capacities  in  series,  with  an  ordinary  ohmic  resistance  in 
parallel  with  one  of  them.  This  ohmic  resistance  is  due  to  the  air  space, 
which  acts  both  as  a  conductor  and  as  a  dielectric.  In  such  an  appara- 
tus cos  6  will  be  very  low.  (Cf  .  footnote  on  p.  180.)  If  the  air  were  a 
perfect  non-conductor  the  system  would  consist  entirely  of  capacities, 
and  cos  6  would  be  zero.  If,  again,  the  air  were  a  perfect  conductor 
the  same  would  hold  good.  Cos  0  reaches  a  maximum  value  for  a 
certain  intermediate  conductivity  of  the  air  gap.  The  dimensions  of 
the  same  and  the  current  density  used  must  therefore  be  carefully 
planned  in  order  to  secure  the  best  results.  A  low  current  density  is 
used  in  practice.  The  frequency  of  the  discharge  should  be  high,  as 
this  favourably  affects  cos  6.  Finally,  it  is  better  to  use  an  apparatus 
with  one  metal  electrode  than  one  with  both  electrodes  insulators.  A 
small  metal  apparatus  working  with  a  high  frequency  takes  as  much 
power  and  can  give  as  much  ozone  as  a  large  all-glass  apparatus  usm<$ 
a  low  frequency  discharge  at  the  same  voltage. 

Table  LXX  x  contains  some  data  bearing  on  these  and  other  points. 

A  is  the  yield  per  K.W.H.  and  C  the  concentration  in 


metre3 
1  Warburg  and  Leithauaer,  Drud.  Ann.  28,  1 


XXVTIT.] 


OZONE 

TABLE  LXX 


525 


Distance 

A  for 
C=  10 

A  for 
C  =  4 

Ac 

C0 

between 
elec- 
trodes 

Periods 
per 
second 

Voltage 

Carrent 
in  amps. 

Cos  0, 

in  cm. 

38-3 

41-9 

45-5 

36'5 

0-51 

50 

8,050 

0-182 

0-185 

—            — 

— 

— 

1-40 

50 

10,080 

0-102 

0-314 

52-3 

55-1 

56-8 

59'2 

T40 

50 

16,900 

0-193 

0*243 

51-1 

56-1 

60-1 

31-2 

3'72 

50 

17,500 

0-160 

0-415 

72-2 

78'4 

82-6 

40-5 

2-26 

50 

10,800 

0-182 

0-431 

53-3 

62-4 

68-4 

20-2 

4-66 

50 

13,900 

0-169 

0-450 

75-7 

81-4 

85-2 

51-6 

2-26 

100 

9,480 

0-308 

0-451 

54-0 

63-0 

69-0 

16-8 

4-66 

100 

12,300 

0-280 

0-447 

57-1 

66-0 

71-9 

18-3 

2-26 

510 

9,340 

1-58 

0-537 

33-0 

58'0 

74-7 

11-4 

4-66 

510 

12,100 

1-19 

0-704 

(Lines  \-±  for  glass,  5-10  for  metal  ozonisers.) 

These  figures  make  it  clear  that  the  best  design  for  an  ozone  appara- 
tus will  vary  considerably  according  to  the  relative  importance  attached 
to  a  good  power  factor,  a  high  absolute  yield,  or  a  high  concentration. 
Each  single  case  must  be  decided  on  separately. 

2.  Technical 

The  number  of  proposed  technical  ozonisers  is  enormous,  and  we 
must  simply  briefly  describe  a  few  of  those  actually  used  in  water- 
purification  plants.  The  Siemens  and  Halske  apparatus  is  very 


Water 


TTtn 


FIG.  136. — Siemens  and  Halske  Ozoniser. 

important.  In  principle  it  resembles  the  ordinary  ozone  tube.  A 
commercial  unit  (Fig.  136)  consists  of  an  iron  box  provided  with  glass 
windows.  Through  the  bottom  of  this  pass  upwards^  a^number  of 


526    PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY    [CHAP. 

vertical  glass  cylinders,  coated  outside  with  metal,  which  serve  as  one 
set  of  electrodes.  Concentrically  inside  these  tubes  are  placed  the 
other  electrodes,  consisting  of  cylinders  of  aluminium  foil.  The  air, 
after  drying  by  CaCl2,  passes  along  the  annular  spaces  between  the 
electrodes.  Water  is  run  through  the  iron  box  to  cool  the  apparatus, 
and  is  sometimes  also  run  through  the  inner  electrodes.  These  are 
carefully  insulated,  and  connected  with  a  source  of  high-voltage  alter- 
nating current,  which  passes  from  them  across  the  air  gaps,  through  the 
glass  cylinders  and  cooling  water  to  the  iron  box.  This  is  earthed, 
and  there  is  thus  no  risk  to  the  operator.  The  ozonisers  are  kept  in 
a  darkened  room,  it  thus  being  possible  to  tell  from  the  luminous 
appearance  whether  or  not  they  are  running  smoothly. 

The  ozonised  air  is  compressed  and  forced  up  towers,  down  which 
the  water  passes.  The  latter  must  be  first  freed  from  organic  matter, 
and  any  iron  should  be  removed  or  oxidised  to  the  ferric  condition ; 
otherwise  losses  in  ozone  occur.  A  plant  at  Paris1  uses  110  volts, 
transformed  up  to  4,000  volts.  The  energy  consumed  in  the  ozoniser  is 
57  K.W.H.  per  million  gallons  of  sterilised  water.  The  energy  expended 
in  compressing  the  gases  exceeds  this,  the  grand  total  amounting  to  133 
K.W.H.  per  million  gallons.  In  St.  Petersburg2  the  ozonisers  are 
fed  with  current  at  7,000  volts  (500  periods).  An  ozone  of  concen- 
tration 2-5  .  is  produced.  Some  50,000  metre3  of  water  are  thus 
metre3 

purified  daily. 

In  the  Tyndall  apparatus,  as  modified  by  de  Vries,  the  current 
passes  between  metal  electrodes.  To  avoid  sparking  and  an  unstable 
discharge,  a  very  high  resistance  is  put  in  series,  taking  the  form  of  a 
narrow  glass  tube  filled  with  glycerine  or  alcohol.  One  electrode  con- 
sists of  a  metal  chest,  earthed  and  cooled  by  a  current  of  water.  In 
this  are  suspended  a  number  of  semicircular  copper  plates,  carefully 
insulated.  A  very  high  voltage,  40,000-50,000  volts,  is  required,  and 
the  energy  consumption  is  correspondingly  large. 

The  Vosmaer  apparatus 3  also  uses  all-metal  electrodes.  The  ozoniser 
consists  of  a  number  of  long  vertical  copper  tubes,  along  which  the  air 
flows.  Each  of  these  is  provided  inside  with  and  is  connected  with  a 
long  flat  electrode.  The  whole  is  earthed.  Each  tube  further  contains 
a  series  of  point  electrodes,  carefully  fixed  in  position,  and  insulated 
by  porcelain.  The  discharge  is  regulated  by  a  special  arrangement. 
The  current  given  by  a  transformer  at  5,000  volts  passes  through  a 
choking  coil  of  high  inductance,  and  then  through  the  ozoniser,  which 
is  in  parallel  with  a  high-tension  condenser.  This  raises  the  effective 
voltage  on  the  ozoniser  to  10,000  volts.  The  apparatus  appears  to 

1  Electrochem.  Ind.  7,  411  (1909). 
-  Zeitsch.  Elektrochem.  17,  764  (/.'>//). 
3  Electrochem.  Ind.  2,  511  (1904). 


xxvni.]  OZONE  527 

work  very  efficiently.  The  last  type  of  ozoniser  we  will  refer  to  is  that 
of  Otto,  used  extensively  in  France.  Both  electrodes  are  of  metal, 
discharge  taking  place  between  the  inside  of  a  metal  chest  and  a  number 
of  aluminium  discs  attached  to  an  insulated  axle  which  revolves  inside 
the  chest. 

All  the  above  ozonisers  are  big  technical  units,  intended  for  large- 
scale  water  treatment.  But  smaller  units  are  also  designed,  suitable 
for  household  or  portable  use.  They  are  mostly  of  the  Siemens- 
Halske  type. 


APPENDICES 

L  Transport  Number  of  Anion  (NA)  for  different  Aqueous  Salt 
Solutions  at  18° 


[Cl 
•   JEquiv.= 

o-oi 

0-02 

0-05 

I  o-i 

0-2 

|0-5 

1 

11 

2 

3 

5 

7 

10] 

.        'I         *) 

\*ir     r 

0-506 

0-507 

0-507 

0-508 

0-509 

0-513 

0-514 

0-515 

0-515 

0-516 







CN^Q  ) 

0-614 

0-617 

0-620 

0-626 

0-637 

0-640 

0-642 

0-646 

0-650 

Li<  "1 

0-63 

0-65 

0-67 

0-69 

0-71 

0-73 

0-739 

0-741 

0-745     0-752 

0-763 

0-774 

_ 

aroi 

— 

0-497 

0-496 

0-492     0-487 

0-482 

0-479  !     — 

— 

NaNO, 







0-615 

0-614 

0-612 

0-611 

0-610 

0-608  ,  0-603 

0-585 



.  _ 

0-528 

0-528 

0-528 

0-528 

0-527 

0-519 

0-501 

0-487 

0-476       — 

— 

— 

— 

KZUBuu 







0-33 

0-33 

0-33       0-331 

0-332 

0-332  ,  0-333 

0-335 



__ 

V  -\(  '  H  I' 





0-44 

0-43 

0-43       0-425 

0-422 

0-421     0-417 

_ 



.  _ 

J£QjJ      *       a            



— 

0-735 

0-736 

0-738     0-740 





N.i'iH              — 

— 

0-81 

0-82 

0-82 

0-82 

0-825 

— 

—         — 

— 

— 

-  - 

LiulL              — 





0-85 

0-85 

0-861 

0-873 

0-890       —          — 



_ 

HC1                — 

—       0-172 

0-172 

0-172    0-173 

0-176 

0-180     0-18       0-200 

0-238    0-292 

._ 

BaGU        0-56      0-565    0'575 

0-585 

0-595 

0-615 

0-640 

0-650 

0-657       — 



OtoT,        0-58      0-59      0-61 

0-64 

0-66      0-675 

0-686 

0-695 

0-700     0-710 

0-737     0-764 

0-79 

M  •<  '' 

—         —       0-63 

0-66 

0-68     i  0-69 

0-709 

0-718 

0-729     0-747 

0-766     0-79931     —  1 

OSOL        0-57      0-58      0-59 

0-62 

0-65       0-69 

0-72 

0-73       0-745     0-767 

0-865     0-995        — 

CdL,          0-56      0-59       0-64 

0-71 

0-83       1-00 

1-12 

1-18 

1-22       1-25 

»  to 

2-5  •*>       — 

K  i  '  •  . 



—       0-39 

0-40 

0-41       0-435 

0-434 

0-421     0-413     0-404 

0-380    0-355 



N.i  00 



— 

0-52 

0-53 

0-53 

0-54 

0-548 

0-546 

0-643     0-530 

—  1 

\1  '~ii 





0-60 

0-64 

0-66 

0-70 

0-74 

0-75 

0-76       0-760 

^m 

_ 

Oo8O4 

— 

0-62 

0-626 

0-632 

0-643 

0-668 

0-696 

0-714 

0-720 

— 

— 

— 



1 

— 

— 

0-193 

0-191 

0-188 

0-182 

0-174 

0-169 

0-168 

0-170 

0-190 

0-216    0-268 

Current  Densities  used  in  Technical  Practice,  given  in  amps./dm.2. 


Copper  refining  (multiple) 

Do.  (Hayden) 

Copper  extraction  (Laszczynski) 

Do.  (Siemens-Halske) 

(Hbepfner)         

Silver  refining  (Moebius) 

Do.          (Moebius — for  anodes  rich  in  gold) 

Do.  (Balbach) 

Do.  (Balbach — for  anodes  rich  in  gold, 

the    electrolyte  containing  gelatine) 

Do.          (Dictzcl)        

528 


(cathode) 
(cathode) 
(cathode) 


1-4 

1-6-2 

0-5-1 

016 

2 

2-5-3-5 

0-75 

(2-2-6     (cathode) 
(4-5  (anode) 

5-5*5        (anode) 
1-5 


APPENDICES  529 

Gold  refining  (Wohlwill)          . .         . .          . .         . .  10-15 

Do.            (Wohlwill— for  anodes  rich  in  silver)  5-7 
Do.            (Wohlwill  —  modified      process      for 

anodes  rich  in  silver)           . .          . .  12 

Gold  extraction  (Siemens-Halske)      . .                     . .  0'004 

Zinc  extraction  (Siemens-Halske) 1  "5-3  (cathode) 

Do.                        do.                   30-75  (anode) 

Do.             (Laszczynski)             ..          ..          ..  I'O 

Do.             (Hoepfner) I'O 

Detinning  (Goldschmidt)          . .  ..  0-8-1 -0  (cathode) 

Nickel  extraction  (Savelsberg-Wannschaff)  . .          . .  1-1  '2  (cathode) 

Iron  refining  (Merck)     . .          . .          . .         . .         •  •  3-4 

Do.          (Fischer -Pfanhauser) 10-20 

Lead  refining  (Betts) . .  17-20 

Electroplating    ..          ..  ..0-1-2-0 

Electrotyping     . .          . .          . .          . .          . .          •  •  2-8 


Kellner's  hypochlorite  electrolyser    . .         . .          . .  50-75 

Haas-Oettel's                 do 10 

Chlorate  cells •  •  10-2°  (anode) 

Castner  '  rocking '  cell             7  (cathodic  mercury) 

Solvay  cells 10-20  (mercury) 

Whiting  cell       . .          U  (mercury) 

Wildermann  cell            Up  to  60  (mercury) 

Do 10  (anodes) 

Griesheim  cell 1-2  (diaphragm) 

Bell-jar  cell        •  •  2  (bell-jar) 

Billiter-Leykam  cell . .  4'3  (bell-jar) 

Billiter-Siemens  cell 4-6  (diaphragm) 

Hargreaves-Bird  cell 2  (diaphragm) 

Townsend  cell 15  (diaphragm) 

Finlay  cell          -  -  2  (diaphragm) 

White  lead  production                                                . .  0'24  (anode) 
Bromine  (Wiinsche) 

Bromate  production 10-15  (anode) 

lodoform        do ••  1-2  (anode) 

Anthraquinone  (Moest)                                               •  •  5  (anode) 

Chromic  acid  regeneration      . .         . .                     . .  3  (anode) 

Perchlorate  production            8  (anode) 

Permanganate      do.                 ••  8-5  (?)  (anode) 


Sodium  (Castner)          ••  200-250  (cathode) 

Magnesium  production  30  (cathode) 

Calcium  do ••  10,000  (cathode) 

Zinc  (Swinburne-Ashcroft) 43  (cathode) 

Do.  (Vogel)  I6  (cathode) 

Aluminium        80-100  (anode) 

Acker  process 30°  (anode) 


530      PRINCIPLES  OF  APPLIED  ELECTROCHEMISTRY 


Carbon  electrodes  in  electrothermics 


Graphite    do.  do. 

Stassano  steel  furnace  (carbon) 
Heroult  steel  furnace  (carbon) 

Do.  do.     (graphite) 

Girod  steel  furnace  (carbon) 
Limit  for  pinch  effect  about 
Ferro-silicon  furnaces  (carbon) 
Carbide  furnaces  (carbon) 
Carborundum  furnace 


300-400    (Can  be  taken 
much  higher, 
but     rapidly 
burn  away.) 
2,000 
2,000 
400-500 
1,500-1,600 
500 
50,000 
300-700 
350-600 
300  (in  core) 


Yields  and  Energy  Expenditure  in  Technical  Practice 


Product 

Refined  copper  (multiple) 
Do.          (Hayden) 
Extracted  copper  (Laszczynski) 

Do.  (Hoepfner) 

Refined  silver  (Moebius) 

Do.  (Balbach) 

Refined  gold 
Extracted  zinc  (Siemens-Halske)      .  . 

Do.  (Hoepfner)      .. 

Recovered  tin    (Goldschmidt) 
Extracted  nickel  (Savelsberg-Wannschaff) 

Do.  (Browne) 

Refined  lead       ........ 

Extracted  lead    ........ 

KC103  ........ 

NaOH  (see  p.  383)       ...... 

White  lead         ........ 

Bromine  (Wiinsche)      ...... 

Do.       (Kossuth) 
lodoionn 

Anthraquinone  (Moest) 
NaClO4  (from  NaC103) 


Sodium  (Castner) 
Magnesium 
Calcium 

Zinc  (Swinburne-  Ashcroft) 
Aluminium 

NaOH   (Acker  process) 
Pig-iron  (see  p.  441) 
Steel  (from  hot  charges) 
Do.  (from  cold  charges) 


K.W.H.  per 

kilo. 

0*3 

0'15 

2'] 

0-37 

0-42 

0-84 

0*32 
3-3-3-9 

31 

1-9 

4-2 

3-6 

0-098 

1-0 

7-3 

2-0-3-8 
0-25-0-3 

1-6 

2-7-3-0 
1-5-1-9 

2-5 

3-6 

0-7? 

11-7 

17-7 

42 

3-75 

23 

47 

1-7-3-2 
0-12-0-4 
0-6-1-3 


Tons  per 
H.P.  year. 

21-8 

43-7 
31 

17-5 

15-6 
7-8 

20-5 

1-7-2-0 
2-1 
3-4 
1-56 
1-85 

66-9 
6-55 
0-9 

1-6-3-3 

22-26 
4-1 

2-2-2-4 

3-4-4-4 
2'6 
1-9 
9-4 
0-56 
0-37 
0-1 
1-75 
0-29 
1-39 

2-1-3-8 

16-4-55 
5-0-10-9 


APPENDICES 


531 


Product 

50  per  cent,  ferro -silicon 

70  per  cent,  ferro-chrome 

85  per  cent,  carbide 

Cyanamide  (from  carbide) 

Carborundum 

Graphite 

Alundum 

Zinc  (de  Laval) 

Phosphorus 

Carbon  bisulphide 

Nitric  acid 


K.NV.H.  per 

kilo. 
.     5-3-16 

8 
.     3-8-7 

01 

8-5 

3-3 

21 

4-8 
.      11-6  (?) 

115 


Tons  per 
H.P.  year. 
0-41-1-2 

0-82 
0-93-1-6 
65-5 
0-77 
2-0 
31 
1-37 
0-56 
5-7 


14-7-16-7         0-39-0-45 


IV.  Theoretical  Quantities  of  Electrolytic  Products  resulting  from 
the  Passage  of  1  ampere-hour  of  Electricity 

Copper  (from  cupric  salts)         . .          . .          . .         . .          . .  1186  grams 

Do.     (from  cuprous  salts)       . .          . .          . .         . .         . .  2*372 

Silver         4*023 

Gold  (from  auric  salts)  . .          . .          . .         . .         . .  2*451 

Do.    (from  aurous  salts)  . .          . .         . .         . .         . .  7 '354 

Zinc  ..          ..          1-219 

Iron  . .          1-042 

Nickel        1-094 

Lead  3'857 

Tin  (from  stannic  salts)  . .          . .          . .         . .          . .  1109 

Sodium       . .          . .         - 0-859 

Magnesium  . .          . .          . .          . .          . .          . .          . .  0'454 

Calcium  0'747 

Aluminium  . .          . .          . .          . .          . .          . .          . .  0*337 

Hydrogen  0*0376 

Oxygen  (free  or  bound  *  active  '  oxygen)         . .          . .          . .  0*298 

Chlorine  (free  or  bound  *  active  '  chlorine)       . .         . .          . .  1*322 

Bleaching  powder  with  34  per  cent.  '  active  '  chlorine          . .  3*89 

Bromine  2*981 

Potassium  chlorate          0*762 

Sodium  chlorate  0*662 

Potassium  perchlorate  (from  potassium  chloride)        . .          . .  0*646 

Potassium  hydroxide  . .          . .         . .         . .         . .  2*094 

Sodium  hydroxide  . .          . .          . .         . .         . .          . .  1*494 


2M2 


INDEX  OF  AUTHORS  AND  FIRMS 


The  references  are  to  pages 


Abbott,  395 
Abegg,  34,  76 
Abel,  335,  341 
Abel  (book),  341 
Acheson,  488,492,493 

Company,  495 

Adolph,  372,  404 
A.F.A.G.  Berlin-Hagen,  224 
Allmand,  84,  205 
Aluminium  Corporation,  427 
—  und  Magnesium  Gesellschaft,  417 
Amberg,  443,  490 
Amberg  (article),  504 
Andreoli,  278 
Angeli,  402 
Appelberg,  162 

Arndt,  159,  164,  416  ,419,  421 
Arrhenius,  68,  70,  71,  75 
Ashcroft,  414,  415 
Askenasy,  225,  384,  385,  399,  400 
Askenasy  (book),  443,  464,  468,  472, 
485,  504 

Badger,  164 

Badische  Anilin  und  Soda  Fabrik,  385, 

479 

Baekeland,  380 
Bahntje,  126 

Balbach  Smelting  and  Refining  Co.,  295 
Barker,  383 
Barnes,  217 
Baum,  247,  256 
Bechterew,  214 
Becker,  412 
Bein,  55 

Bein  (book),  219,  244 
Bemmelen,  van,  239 
Bernfeld,  260,  306 

(firm,  Leipzig),  155 

Berthelot,  14,  402,  607 

Betts,  155,  302»  307 

Betts  (book),  307 

Beutner,  214,  472,  481 

Bicknell,  420 

Billiter,  153,  297,  362,  370,  373,  375 

Billiter  (book),  385 


Biltz,  147 

Binschnedler,  389 

Birkeland,  512,  513 

Blich,  520 

Block,  63 

Bodenstein,  15,  519 

Bodlander,  277 

Boericke,  132,  390 

Boiling,  492 

Borchers,  212,  214,  261,  416,  417,  418, 

419,  493 

Borchers  (book),  182 
Bose,  273,  391 
Bottomley,  496 
Bradley,  469,  511 
Bran,  151 

Brand,  399,  400,  401 
Bredig,  482,  483 
Breuer,  363 
Briner,  359,  509 
Brion,  185,  188,  512 
Brislee,  134 

British  Aluminium  Co.,  428 
Brochet,  359,  365,  368 
Brochet  (book),  385 
Erode,  409,  410,  411,  507,  515 
Brown,  293,  311,  501,  502 
Browne,  298 
Bruner,  214 
Brunner,  314 

—  Mond  &  Co.,  281,  286 
Bucherer,  212,  214 
Buchner,  155 
Bugarszky,  84 
Bullier,  469 

Bunsen,  201,  412,  416,  418 
Burgess,  299,  301,  310,  316,  389 

Canadian  Copper  Co.,  298 

Cantoni,  351 

Carborundum  Co.,  488 

Carlson,  482 

Caro,  402,  479,  481,  485 

Caron,  417 

Carrier,  412,  414,  415 

Caspari,  118 


INDEX  OF  AUTHORS  AND  FIRMS 


533 


Castner,  350,  351,  408 

Castner-Kellner  Co.,  352,  385 

Catani,  435 

Chattaway,  200 

Chloride  Co.  (accumulators),  225 

Clacher,  428 

Clancy,  279 

Clausius,  68 

Coehn,  36,  118,  131,  265,  271,  272,  314 

Coffetti,  120 

Cohen,  200,  260 

Collins,  172,  490 

Conrad,  182,  464,  472 

Consortium    fur    electroch.   Industrie, 

404 

Corbin,  340 
Cote,  501,  502 
Couleru,  401 
Cowper-Coles,  317 
Cramp,  185 
Crookes,  505 
Cumming,  233 
Czepinski,  163,  164 

Daniell,  57 

Danneel,  125,  415 

Dannenberg,  118 

Darling,  408 

Darmstadter,  396 

Davies,  192 

Denham,  57,  76 

Denison,  55 

Denso,  151 

Deutsche  Quarz-Gesellschaft,  496 

Deville,  417 

Dietzel,  284 

Dolch,  289 

Dolezalek,  227,  229,  231,  233 

Dolezalek  (book),  244 

Donnan,  84,  383 

Durand,  509 

Eckardt,  289 

Edison,  203,  234 

Egli,  260 

Elbs,  127,  147,  303,  394,  403 

Electrolytic  Alkali  Co.,  153,  376,  378 

Engelhardt,  285,  329,  450 

Engelhardt  (book),  341,  406 

Engemann,  294,  300 

Erhvein,  472,  479,  481 

Eschmann,  481 

Ewan,  410 

Eyde,  512,  513 

Fabian,  294 
Faraday,  29,  50,  141 
Farbwerke  Hochst,  396 

vonn.  Meister,  Lucius  und  Bruning, 

396 

Faure,  223 
Faust,  239 


Ferchland,  154,  285 

Ferranti,  454 

Field,  313 

Finckh,  506 

Findlay  (book),  17 

Finlay,  380 

Fischer,  224,  299,  522 

Fitzgerald,  489 

Fleischmann,  213 

Foerster,  105,  120,  121,  132,  133,  134, 
142,  153,  237,  239,  240,  241,  242,  243, 
247,  252,  256,  282,  283,  289,  294, 
295,  299,  306,  314,  319,  320,  323, 
325,  326,  336,  337,  338,  344,  356, 
369,  372,  390,  395,  401,  482,  483, 
484,  519,  520,  521 

Foerster  (book),  116,  147 

Fontana,  396 

Foster,  213 

Fraenkel,  482 

Frank,  479 

Franke,  261 

Frary,  164,  420 

Frauenberger,  141 

Frazer,  41 

Fredenhagen,  141 

Friedberger,  146,  403 

Friedrich,  206 

Fromm,  125,  282,  283 

Gahl,  119 

Gall,  338 

Gay  Lussac,  40,  321 

Geibel,  339 

Geilenkirchen,  443 

Geipert,  429,  430,  431 

Gessler,  159,  421 

Gillett,  389,  487 

Gin,  442,  444 

Girod,  449,  468 

Gladstone,  222,  226 

Glaser,  302 

Goldschmidt,  H.,  289 

Goldschmidt,  K.,  291 

Goodwin,  420,  421 

Gordon,  163 

Grabau,  413 

Grafenberg,  236 

Grau,  192,  507 

Grave,  142 

Greene,  442 

Greenwood,  463,  486 

Griesheim  Elektron  Co.,  412 

Grinberg,  151,  344 

Gronwall,  439 

Grotthuss,  49 

Grove,  201,  211 

Grube,  131,  133 

Griinauer,  422,  423 

Guertler,  463 

Giinther,  E.,  261,  296,  298 

Glinther,  E.  (book),  307 

2M3 


534 


INDEX  OF  AUTHORS  AND  FIRMS 


Gunther,  0.,  282 

Guye,  150,  358,  359,  301,  478,  509,  510, 

521 
Gyr,  320 

Haagn,  03 

Haanel,  437,  439,  440 

Haber,  105,  122,  125,  127,  133,  141, 
140,  151,  192,  201,  209,  213,  214,  218, 
309,  344,  425,  420,  427,  428,  429,  430, 
431,  407,  472,  470,  478,  479,  499, 
507,  508,  509,  510,  511 

Haber  (book),  147,  400 

Hambuechen,  299,  301,  310,  389 

Hansen,  170,  504 

Hantzsch,  283 

Harden,  435,  450,  453,  455,  504 

Hatfield,  34,  35 

Haussermann,  302 

Hayek,  v.,  398 

Helfenstein,  159,  174,  474 

Helfenstein  (article),  404,  408,  472,  485 

Helmholtz,  03 

Hempel,  500 

Heraeus  (firm),  490 

Hering,  173,  170,  178,  453 

Herold,  239,  242,  243 

Heroult,  438,  439 

Herrmann,  500 

Herschkowitsch,  138 

Herz,  394 

Hessberger,  517 

Hevesy,  v.,  101,  411 

Higgins,  490 

Hill,  383 

Hirech,  122 

Hittorf,  52,  54,  55,  50,  57,  140 

Hoepfner,  204,  280,  290 

Hofer,  104,  431,  500 

Hoflf,  van't,  39,  40,  45 

Hohler,  418 

Holland,  234,  238 

Holland  (book),  244 

Hohvech,  509,  511,  519 

Hooker  Electroch.  Co.,  378 

Hopfgartner,  53 

Howies,  505,  500 

Hoyle,  185 

Hulin,  104,  413 

Hurter,  338 

Huth,  285 

Hutton,  450,  490 

Ihle,  202 
Imhoff,  322 
Inglis,  143 
laambert,  19 
Isenburg,  389 

Jacobsen,  271,  272,  314 
Jacoby,  482,  484 
Jacques,  214 


Jahn,  H.,  53,  55,  84 

Jahn,  S.,  30 

Jakowkin,  319 

Janeczek,  409 

Jellinek,  405,  400,  500,  508 

Johnson,  204,  205,  221,  501,  503,  504     ! 

Joost,  134,  152,  153 

Jordis,  315 

Jorre,  320,  350 

Jungner,  214,  210,  234 

Just,  225 

Kailan,  104,  105 

Kalmus,  159 

Keller,  438,  439,  450 

Kellner,  332,  333,  351 

Kemmerer,  100 

Kern,  120,  294 

Kiliani,  250,  282 

King,  474 

Klonowski,  399,  400 

Koch,  519,  520,  521 

Koenig,   192,  507,  508,  509,  510,  511 

Kohlrausch,  50,  03,  05,  GO,  07 

Kossuth,  392 

Kowalski,  v.,  511,  512 

Kretzschmar,  320,  390 

Kiigelgen,  v.,  413 

Lalande,  203 
Lampen,  470,  487 
Landis,  501 
Langbcin,  310,  312,  :}]T> 

-  und  Pfanhauser,  299 
Langer,  213,  214 
Laszczynski,  285 
Le  Blanc,  75,   141,  142,  155,  157,  ]J)2, 

232,   320,   351,   389,   390,   397,   409, 

410,  411,  412,  481,  520 
Le  Blanc  (book),  37,  48,  57,  70,  105 
Le  Rossignol,  479 
Lee,  299,  300 
Leithauser,  507,  524 
Lenz,  205 
Lepsius,  152,  302 
Lindblad,  439 
Loeb,  53 
Loebe,  394 
Lombard,  481 
Lorenz,   158,   159,   101,   102,   104,   1  <>.">, 

100,  204,  200,  388,  418,  421,  422,  423 
Lorenz  (book),  37,  70,  110,  100,  433 
Louis,  175 
Love  joy,  511 
Lucion  (book),  3X5 
Luckow,  388,, 389 
Luther,  134,  335,  341 
Lyon,  439 

McCaughey,  31.", 
McDougall,  505,  500 
Macgregor,  442 


INDEX  OF  AUTHORS  AND  FIRMS 


535 


Magnus,  256 

Marshall,  402 

Mathers,  302 

Matignon,  481 

Matt'hes,  363 

Matthiessen,  418 

Mellor  (book),  25 

Merck  (firm),  301 

Meves,  395 

Miller,  389 

Mitrofanoff,  225 

Moebius,  268 

Moest,  396 

Mogenburg,  493 

Mohn,  306 

Moissan,  469,  482 

Moller,  133 

Mond,  L.,  213,  214 

Mond,  R.,  217 

Montlaur,  338 

Morden,  509,  511 

Morse,  41 

Moscicki,  511,  512 

Moser,  213 

Moser  (book),  146,  147,  406 

Mott,  119 

Mugdan,  385 

Miihlhaus,  376 

Miiller,   E.,    105,    126,    129,    132,    146, 

147,    154,   322,   323,   325,   326,   336, 

337,  338,  394,  397,  403 
Miiller,  R.,  500 
Mustad,  121,  299 
Muthmann,  141,  164,  431,  506 
Mylius,  125,  282,  283 

Naville,  510 

Nernst,  53,  63,  92,  93,  133,  506 

Nernst  (book),  2/i 

Neumann,  251,  278,  296,  427,  429,  430, 

435,  443 

Nobis,  119,  131,  134,  215,  219 
Norton  Emery  Wheel  Co.,  495 
Noyes,  54,  55,  64,  74,  76 
Nubling,  303 
Nuranen,  192 

Oechsli,  401 

Oesterle,  501,  502 

Oettel,  A.,  164,  416,  417,  418 

Oettel,  F.,  32,  33,  330,  417 

Olsen,  429,  430 

Orford  Copper  Co.,  298 

Osaka,  131 

O.sann,  443 

Ost \vald,  73,  209 

Parker,  500 
Patten,  315 
Pauli,  390 
Perkin,  396 
Pfaff,  299,  301 


Pfanhauser,  309 
Pfeffer,  40,  41 
Pfleiderer,  134 
Pick,  465 
Pierron,  501,  502 
Pietzsch,  404 
Piguet,  132,  401 
Planck,  68,  73 
Plante,  223 
Plato,  419,  420 
Platou,  507,  511 
Pohl,  15 

Poincare,  416,  421 
Polzenius,  482 
Potter,  487 
Preuner,  20 
Pring  (book),  11 
Puschin,  137,  138 
Pyne,  426 

Ramsbottom,  399,  400,  401 

Raoult,  45,  71 

Rathenau,  419 

Rayleigh,  505 

Readman,  500 

Reason  Mfg.  Co.,  34 

Regelsberger,  396 

Reinartz,  262 

Richards,  J.  W.,  8,  170,  409,  425,  426, 

427,  429,  435,  476,  501 
Richards,  T.  W.,  32,  159,  163 
Richardson,  237,  429,  431 
Rodenhauser,  458 
Rodenhauser  (book),  182,  468 
Rothmund,  18,  470 
Roush,  395 
Ruckstuhl,  166 
Rudolphi,  470,  471,  482,  483 
Ruff,  419,  420 
Rumpf,  231 
Russ,  F.,  192,  507 
Russ,  R.,  127,  133 

Sacerdoti,  119,  120,  133,  153,  343 

Sacher,  411 

Sack,  122 

Sackur,  141,  142,  213 

St.  Gilles,  14 

Savelsberg,  297 

Schleicher,  225 

Schloesing,  521 

Schlotter  (book),  317,  406 

Schmiedt,  146,  154,  398 

Schoch,  122,  292,  293 

Schoenawa  (book),  182,  468 

Schonherr,  147,  403,  516,  518 

Schoop,  222,  239,  240 

Schultze,  G.,  143 

Schultze,  H.,  422 

Schwab,  247,  256 

Schwabe,  306 

Schweitzer,  121,  292,  293 


536 


INDEX  OF  AUTHOKS  AND  FIRMS 


Seidel,  247,  256 

Senn,  302 

Seward,  413 

Siemens  und  Halske,  285,  307 

Sjostedt,  438 

Snyder,  170,  502 

Soller,  146,  154,  397 

Solvay  Co.,  352 

Sonneborn,  344 

Speketer,  363 

Spitzer,  120,  312 

Sprosser,  153,  346 

Stalhane,  439 

Stansfield  (book),  182 

Stassano,  437 

Steele,  55 

Steiner,  372 

Stockem,  414,  415,  419 

Strasser,  119 

Streintz,  230,  231,  232 

Strutt,  509,  510 

Stull,  159 

Suchy,  422 

Swinburne,  423,  424 

Tafel,  30,  119,  130 

Taitelbaura,  215,  216 

Tammann,  463 

Tardy,  156,  361 

Taussig,  174,  348,  353,  354 

Taylor,  E.,  498 

Taylor,  W.,  143 

Teeple,  395 

Thermal  Syndicate,  496 

Thiele,  323 

Thompson,    24,    237,    261,    262,    264, 

429,  430,  470,  471,  481 
Thomson,  183 
Toepler,  184 
Tombrock,  200 
Tone,  488,  489 
Tower,  .r>f> 
Traube,  M.,  40 


Traube,  W.,  147 
Tribe,  222,  226 
Tronson,  420 
Tscheltzow,  231 
Tucker,  420,  487 
Tudor,  224 

Ulke  (book),  280 

Umbreit  und  Matthes,  203 

Union  Carbide  Co.,  475 

United  States  Steel  Corporation,  174, 

442 
Uslar,  v.  (book),  280 

Veesenrnayer,  217 
Villeroy  und  Bock,  155 
Vogel,  424 
Vries,  de,  40 

Wannschaff,  297 

Warburg,  192,  507,  522,  524 

Warth,  472,  481 

Weber,  363 

Weckbecker,  493 

Wedekind,  203 

Weiss,  164,  431 

Welsbach,  v.,  221 

Weston  Chemical  Co.,  385 

Whitney,  420 

Wilke,  482 

Willner,  164,  419 

Willson,  469 

Wilsmore,  95 

Winteler,  356,  401 

Wohler,  164,  419,  420,  421 

Wohhvill,  248,  272,  274,  275 

Wolff,  161 

Wologdine,  173 

Yamasaki,  132,  289,  390 
Yngstrom,  435 

Zedner,  239 


SUBJECT   INDEX 


The  references  are  to  pages 


'  Absolute '  scale  of  potential,  95 

Accumulators,  220  ff 
-  plates,  223,  234 

Acetic  acid,  224 

Acetone,  393  ff 

Acetylene  chlorides,  385 

Acid,  formation  in  brine  electrolysis, 
344,  361,  366 

Acker  process,  431  ff 

Active  mass,  18 

Addition  agents;  for  metal  deposition, 
126 ;  for  rapid  forming,  224 

A.F.A.G.  plates,  224 

Affinity,  86 

Air,  electric  discharges  through,  183  ff, 
479,  505  ff;  electrode,  215,  216; 
for  fuel  combustion,  168  ff  ;  in  blast 
furnace,  435  ;  ozonisation,  524  ff 

Alby  furnace,  477,  478 

Alcohol,  393  ff 

Alizarin,  395 

Alkali,    electrolytic  production,  342  ff 

Alkali  -chlorine  cells,  318,  342  ff 

classification,  347 ;   comparative, 

355,  383  ;  counter-current,  366  ff ; 
diaphragm,  355  ff,  375  ff ;  mercury, 
347  ff  ;  theory,  342  ff,  355  ff,  366  ff 

Alkaline  bromide  electrolysis,  390  ff ; 
chloride  electrolysis,  318  ff,  342  ff 

Alkaloids,  130 

Alloys ;  anodic  solution  of,  136  ff ; 
cathodic  deposition,  121 

Alternating  current ;  arcs,  187,  508  ; 
discharge  in  gases,  185 ;  for  con- 
ductivity measurements,  60,  electric 
heating,  179  ff,  induction  furnaces, 
167,  451,  ozonisers,  524;  in  gold 
refining,  275  ;  rectifier,  142 

Alumina,  410,  464,  473,  478,  493; 
for  aluminium,  425  ff,  alundum, 
496  ;  from  bauxite,  426  ;  solution  in 
cryolite,  426  ff 

Aluminium ;  electrolytic  production 
of,  425  ff  ;  electroplating  on,  309  ; 
fusion,  454 ;  passivity,  1,42  ;  recti- 
fier, 142 


Alundum,  172,  495 

Amalgams  ;  as  anode,  350  ;  cathodic 
production,  122,  342,  347  ff 

Ammeter  calibration,  35 

Ammonia  ;  in  electric  discharge,  190  ff  ; 
equilibrium,  23 ;  from  cyanamide, 
480  ;  synthesis,  24,  385,  479 

Ammonium  chloride,  199,  206,  208,  301, 
315,  402 

Ammonium  salts,  104,  315,  401  ff,  521 

Ammonium  sulphate,  255,  301,  311, 
403,  404,  478,  479 

Amorphous  carbon,  492,  493,  carborun- 
dum, 492 

Ampere,  6 

Amphoteric  electrolyte,  320 

Aniline,  126,  127,  385 

Anion,  29  ;  cathodic  formation,  126> 
discharge,  130  ff 

Anode,  27;  effect,  160,  164,  420,429  ff; 
fall,  188 

Anode  slimes,  136,  248,  250,  252,  258, 
268,  270,  274,  276,  284,  287,  293, 
295,  304 ;  resistance,  268,  304,  305 

Anodes  (insoluble),  catalytic  action, 
146,  397,  technical,  151  ff,  166 

Anodes  (soluble),  for  primary  cells,  199 

Anodic  current  efficiency,  31,  polarisa- 
tion, 108,  processes,  130  ff 

Anodic  impurities  in  refining  copper, 
249  ff,  gold,  273  ff,  lead,  303  ff, 
nickel,  295,  silver,  265  ff,  zinc,  284 

Anodic  solution  of  metals,  135  ff ; 
effect  of  physical  condition,  135, 
292 ;  of  alloys,  136 ;  with  more 
than  one  cation,  136,  248,  271,  289 

Anolyte,  27 

Anthracene,  395,  396 

Anthracite  for  carbide,  473,  478,  car- 
borundum, 488,  ferro-silicon,  464, 
graphite,  493 

Anthraquinone,  395 

Antimony,  in  refining  copper,  250  ff, 
258,  lead,  303,  305  ;  refining,  307 

Arc,  direct  current,  509,  510,  511  ; 
furnaces,  167,  174,  437  ff,  444  ff, 


538 


SUBJECT  INDEX 


459  ff,  464  ff,  473  ff,  488,  496,  500, 

503,  512  ff ;   heating,  167,  169,  171  ; 

in  gases,  185  ff 
Arsem  furnace,  470 
Arsenic  in  copper  refining,  250  ff,  255, 

258 
Asbestos  diaphragms,  155,  356,  374  ff, 

387 

Ashcroft  process,  414 
Avogadro's  law,  41 
Azobenzene,  127 
Azoxy  benzene,  127 

Back  electromotive  force,  113 

Baker  process,  421 

Balbach-Thum  process,  268,  269 

Barium  salts,  162,  375,  376,  480,  482 

Bauxite,  174,  426,  496 

Bayer  process,  426 

Becker  process,  412 

Becquerel  cell,  210 

Bell-jar  cell,  347,  369,  371,  374,  383, 

397 ;  for  chromic  acid,  397 
Benzal  chloride,  385 
Benzyl  chloride,  385 
Bergsoe  process,  288 
Bernfeld  diaphragms,  155,  383 
Betts  process,  302,  307 
Bichromate  cell,  202 
Billiter-Leykam  cell,  373,  383,  384 
Billiter-Siemens  cell,  155,  375,  383,  384 
Bi-polar  electrodes,  149,  253,  258,  327  ff, 

340,  387,  388,  392,  410 
Birkeland-Eyde  process,  512,  518,  519 
Bismuth ;    in  refining  copper,  250  ff, 

258,    lead,  302  ff ;    passivity,    143  ; 

refining,  306 

Blast  furnace  process,  435,  437 
Bleaching  liquors     (electrolytic),     319, 

384.     See  Hypocldorites 
'  Block  '  furnaces,  473  ff 
Board  of  Trade  unit,  6 
Bone  ash,  500 
Borchers  cell,  216 
Boric  oxide,  493,  495 
Boyle's  law,  40 
Bradley-Lovejoy  furnace,  511 
Brass;     electrodes,    313,     316,    339; 

electrolytic,      309 ;       fusion,      454 ; 

hydrogen  overvoltage,  119;   plating, 

312 
Brine,  375  (see    also  alkaline    chloride 

and  sodium  chloride) 
Bromates ;      electrolytic     production, 

390    ff,    393 ;     from    hypobromites, 

320,    391,    393,    394;   'in    bromine 

cells,  392 
Bromic  acid,  199 
Bromine  ;    anodic  production,  390  ff  ; 

cathodie  reduction,   392 ;    for  elec- 
tricity meter,  35  ;    hydrolysis,  319, 

390;    overvoltage,   I :',-_' 


Bromion,  132,  134,  140 
Bromoform,  393,  394 
Browne  process,  298 
Browne-Neil  process,  288 
Brush  discharge,  185,  186,  523 
Buckling  of  plates,  222 
Bullier  furnace,  477 
Bullion  parting,  269,  273 
Bunsen  cell,  115,  201,  203 


Cadmium,  138,  199,  221,  285  ;  cell,  91 

Calcium,  electrolytic  production,  418  ff  ; 
salts,  160,  401,  402,  418  ff,  424,  444, 
472,  473,  480,  481,  483,  500,  502,  521 

Calcium  carbide  ;  action  of  nitrogen, 
479  ff ;  commercial,  471,  474,  477  ; 
decomposition  by  heat,  472,  481  ; 
dissociation,  470,  471  ;  technical 
production,  472  ff  ;  theory  of  for- 
mation,  469  ff ;  produced  in  steel  fur- 
naces, 444,  447 

Calcium  chloride,  158,  159,  206,  208, 
301,  402,  421,  482  ff,  526  ;  dehydra- 
tion, 418 ;  electrolysis,  418  ff ;  for 
ore  extraction,  264,  286,  296,  297 

Calcium  cyanamide,  action  of  steam, 
480,  for  gold  extraction,  279,  from 
carbide,  479  ff,  technical  produc- 
tion, 484 

Calcium  fluoride  (see  also  fluorspar)  as 
catalyst,  472,  482  ff;  in  fused  salt 
electrolysis,  417,  418,  419,  420, 
425,  427 

Calcium  salts  in  crude  salt,  433  ;  in 
brine  for  alkali-chlorine  cells,  349, 
for  bleaching  liquors,  323,  327,  333, 
for  chlorates,  340 

Calomel  electrodes,  104 

Calorie,  7 

Capacity  of  alternating  current  circuits, 
180,  457,  524,  cells,  201,  204,  209,  220, 
222,  225,  226,  230,  231.  235,  236, 
238,  243,  induction  furnaces,  452, 
454,  457,  458 

Carbon  ;  allotropic  forms,  492  ;  bisul- 
phide. 497,  498,  502  ;  electrodes  for 
furnaces,  175  (see  also  furnace 
electrodes) ;  for  furnace  charges, 
464,  473,  488  ;  hydrogen  overvolt- 
JIL.M-  at  119,  348;  ionisation  of  210; 
furnace  linings  etc.,  173,  425,  465, 
467,  477,  478;  resistors,  484.  I'.ir,. 
497,  499  ;  tetrachloride,  385 

Carbon  anodes,  151,  152;  acid  pro- 
duced at,  361,  367  ;  anode  effect,  164, 
430,  431  ;  attacked  by  oxygen,  345  ; 
disintegration,  345  ;  in  primary  cells, 
210,  214;  manufacture,  152,  428; 
porosity,  345,  346  ;  testing,  346 

Carbon  dioxide,  electric  discharges 
through,  190 ;  for  white  lead,  389  ; 


SUBJECT  INDEX 


539 


in  anodic  chlorine,  345,  346,  349, 
362,  376,  383  ;  in  Hargreaves-Bird 
cell,  377,  378 

Carbon  monoxide,  equilibrium  pres- 
sures (carbide  reactions),  470,  471  ; 
from  blast  furnace,  435  ff,  electric 
furnaces,  435  ff,  438,  443,  489,  490, 
500,  502,  503  ;  in  aluminium  bath, 
428,  429 

Carborundum,  486,  487,  488  ff,  493, 
497  ;  anodes,  151,  153  ;  bricks,  174, 
489;  furnace,  167,  489,  495;  re- 
fractory, 172 

Carmichael  process,  261,  262 

Carnallite,  416,  417 

Caro's  acid,  402,  403 

Cast  anodes,  135 

Cast  iron  (electrothermal),  437,  438 

Castner  mercury  cell,  349,  383  ;  sodium 
process,  408/413 

Castner-KeHner  cell,  352 

Catalyst,  13 

Cathode,  27 ;  catalytic  action  of,  129  ; 
contact,  413,  419,  420;  fall,  188; 
heated,  509;  primary  cells,  199; 
rotating,  270,  286,  287,  316,  317 

Cathodic  current  efficiency,  31,  159, 
depolarisers,  197,  polarisation,  108, 
processes,  117 

Cathodic  metal  deposition,  120  ff  ; 
alloys,  121,  312;  amalgams,  122; 
metals  with  more  than  one  cation, 
122,  245,  272 

Catholyte,  27 

Cation,  29 

Caustic  alkali,  potash,  soda  (see 
alkali,  potassium  hydroxide,  sodium 
hydroxide) 

Cell  constant,  61 

Cement  diaphragms,  155,  356,  363, 
377,  399 

Ceric  sulphate,  146,  199,  221,  395,  396 

Characteristic  curve,  184,  negative, 
185,  187,  189 

Charcoal  for  carbide,  473,  478,  carbon 
bisulphide,  499,  iron,  438  ff,  ferro- 
silicon,  464  ff,  phosphorus,  500, 
zinc  furnaces,  503 

Charge  (of  accumulators),  228,  237, 
241,  243,  244 

Chile  deposits,  478,  479 

Chloral,  385 

Chlorates;   electrolytic  reduction,  126, 
193 ;     in   alkali-chlorine   cells,    344, 
345,  361,  362,  364,  372,  376  ;    from 
hvpochlorite    (chemical),    320,    321, 
338,     (electrochemical),     323,     336; 
technical  production,  338  ff  ;   theory 
of  formation,  318  ff,  335  ff 
Chlorbenzene,  385 
Chloric  acid,  199,  224,  225,  401 
Chlorine,    action    on    alkali,    321    ff ; 


anodic,  286,  297,  298,  346,  349, 
353,  362,  364,  365,  376,  379,  383, 
384,  431,  433;  cathodic  solution, 
219,  286,  349,  385  ;  hydrolysis,  319  ; 
liquefaction,  385 ;  for  nickel  ores, 
297,  298;  overvoltage,  132,  134,  153, 
318  ;  uses,  384 
Chlonon,  132,  140,  146,  271,  290,  292, 

299,  318,  377,  388 
Chloroform,  385 
Choking  coil,  188,  526 
Chromate  for  brine  electrolysis,  322 
Chrome  yellow,  302,  388 
Chromic    acid,    202,    389,    395,    397; 
electrolytic  regeneration,  146,  396  ff, 
401,  404  ;   in  primary  cells,  199,  202 
Chromium,  139,   140,   199 ;    chromate, 

322  ;    sulphate,  395 
Circulation ;    of  electrolyte,  254,  259, 
264,  270,   275,   287,   303,   329,   330, 
338,    339,    392,    402;     in   induction 
furnaces,  452,  458,  460  ;   of  mercury, 
350,  352,  353,  354 
Clancy  process,  279 
Clark  cell,  82,  83,  91 
Cobalt,  131,  221,  250,  320 
Coefficient  of  self-induction,  181 
Coke  for  carbide,   473,   carbon   bisul- 
phide, 499,  carborundum,  488,  489, 
iron,    435    ff,    ferro-silicon,    464    ff, 
graphite  furnaces,  493  ff 
Colby  furnace,  455 
Cold  galvanising,  310 
Colloids,  125 

Complex  ions,  57,  120,  123,  125,  136, 
161,   166,  248,  264,   272,   302,  416, 
salts,  125,  309 
'  Concentrates,'  277 

Concentration ;  cell,  89,  103,  163 ; 
changes  during  electrolysis,  27,  51, 
57,  198,  206,  218,  222,  229,  240,  397  ; 
effect  on  cathodic  deposit,  124, 
electrolytic  dissociation,  68,  electro- 
lytic oxidation,  145,  electrolytic 
reduction,  129,  equivalent  conduc- 
tivity, 65,  specific  conductivity,  64  ; 
polarisation,  114,  115,  119,  135,  165, 
198,  229 

Condenser,  63,  511,  526 
Conduction  losses  (thermal)  from  fur- 
naces, 173 

Conductivity  (electrical);  of  electro- 
lytes, 58,  of  gases,  183,  of  hot 
oxides,  457,  measurement,  59  ff,  159, 
of  molten  electrolytes,  158,  of  work- 
ing cell,  62,  vessels  62 
Conductivity  (thermal)  of  refractories, 

174 

Conductors,  26 
Contact     electrode,     413,     420,     421, 

process,  408,  412,  418,  419 
Continuous  block  furnace,  475 


540 


SUBJECT  INDEX 


Convection  disturbances,  347,  355,  366, 
369,  371,  373,  374,  410 

Copper,  137,  138,  266,  273,  285,  303, 
309,  457;  anodes,  217,  249,  258, 
315,  316,  388  ;  anodic  solution,  248  ; 
cathodes,  129,  151,  217,  254,  262, 
263,  264,  339,  377,  385;  cathodic, 
124,  126,  252,  298  ;  coulometer,  32, 
36;  electro-deposition,  120,  121,  125, 
245  ff,  312;  extraction,  260  ff ; 
fusion,  454,  504 ;  hydrogen  over- 
voltage,  118,  119,  201,  227,  312,  343  ; 
matte  anodes,  260,  261  ;  plating, 
312 ;  refining,  245  ff ;  sub-oxide, 
205;  salts,  123,  327,  339 

Coulomb,  6 

Coulometers,  31  ff 

Counter-current  cells,  366  ff 

Counter-current  principle,  305,  520 

Cryolite,  425  ff 

Cupric  compounds,  57,  147,  264,  296, 
oxide,  199,  203  ff,  214,  221,  388, 
sulphate,  57,  199,  200,  245  ff,  255, 
315,  316 

Cupro-ions,  complex  forming,  123,  136, 
248,  264,  265 

Cupron  cell,  203 

Cuprous  oxide,  205,  250,  388,  389,  salts 
216,  250,  264,  277,  297,  312 

Current,  5,  concentration,  30,  406,  elec- 
trode-potential curve,  113,  measure- 
ment, 31 

Current  density,  5  ;  effect  on  anode 
effect,  164,  anodic  solution  of  alloys, 
139,  bath  voltage,  150,  cathodic 
deposits,  120,  123,  124,  247,  256, 
262,  266,  278,  283,  cell  capacity, 
230,  243,  cell  polarisation,  230,  chemi- 
cal losses  in  mercury  cells,  349,  current 
efficiency  in  diaphragm  cells,  361, 
current  efficiency  in  fused  melt 
electrolysis,  160,  422,  diffusion 
losses,  361,  electrolytic  oxidation, 
145,  electrolytic  reduction,  128, 
hypochlorite  concentration,  325  ff, 
337,  Luckow  reactions,  389,  over- 
voltage,  119,  131,  passivity,  140, 
271,  pinch  effect,  453,  quality  of 
anodic  chlorine,  346,  working  of 
counter-current  cells,  371,  376  (see 
also  Appendix  II.) 

<  un.nt  efficiency,  30,  31,  anodic,  31, 
calculation  of  36,  cathodic,  31, 
influence  of  iron,  162 

Cyanides,  479,  480 

Cyanide  bath,  for  electroplating,  312  ff, 
deterioration,  277,  314 

Cyanogen  chloride,  277,  bromide,  277, 
iodide,  279 

Daniell  coll.  3,  77,  82,  84,  94,  99  106  ff 
200 


Decinormal  calomel  electrode,  105 
Decomposition  voltage,  108,  112 
Decrease  of  free  energy,  78,  81,  85,  total 

energy,  78,  81,  85 
Degree  of  dissociation,  70 
Dehydration  of  fused  salts,  409,  416, 

418,  421  ff 

Deozonisation,  523,  524 
Dephosphorisation  of  steel,  443 
Depolarisation,  115,  121,  128,  134,  197, 

206,  208,  262,  308 
Depolarisers,  115,  197,  199  ;  in  primary 

cells,  199,  secondary  cells,  221 
Design  of  electrodes,  176,  furnaces,  171 
Desulphurisation    of    steel,    443,    444, 

461 
Deterioration  of  electrolyte,  253,  257, 

261,   277,   288,   290,   304,   314,   318, 

primary  cells,  198 
Detinning,  288  ff,  385 
Diamantin,  496 

Diaphragm  cells,  347,  355  ff,  375  ff 
Diaphragms,  154  ff,  369  ff 
Dietzel  process,  269,  284 
Diffusion  coefficient  of  diaphragms,  156 
Diffusion     effects,     114;      in     alkali- 
chlorine  cells,  355,  360  ff,  371,    lead 

accumulator,  229,  230,  primary  cells, 

197,  198,  203,  sodium  cell,  410,  411, 

415 

Dilution  law,  72 

Direct  current  arc,  509,  510,  511 
Discharge  of  accumulators,  228,  237, 

240  ff 

Discontinuous  block  furnace,  473 
Disintegration  of  carbon  anodes,  152, 

345,  graphite  anodes,  152,  345,  349, 

ferro-silicon,  464 
Dissociation   constant   of    water,    1 1 7, 

pressure    17,   of  electrolytes,   47 
Dolomite  (calcined),  172,  446,  447,  457, 

458 

Domnarfvet  experiments,  439 
Dow  cell,  393 
Dry  cells,  208 
Dujardin-Plante  plates,  224 

Eddy  current  losses,  182,  452 

Edison  cell,  221,  234 

Edser-Wildermann  cell,  354 

Efficiency  of  electrolytic  processes,  30, 
110,  furnaces,  8,  170 

Electric  furnaces,  107  ff,  heating,  167  ff, 
losses  in  induction  furnaces,  452,  455, 
shaft  furnace,  439 

Electrical  discharge  in  gases,  183  ff, 
double  layer,  93,  energy,  4,  equili- 
brium in  gases,  191,  509,  522,  units,  4 

Electricity  meter,  34 

Electrochemical  processes,.  3,  4,  com- 
bustion of  fuels,  209  ff 

Electrode,  27  ;    bipolar,  149,  constant, 


SUBJECT  INDEX 


541 


179,  design,  176,  for  furnaces, 
175  ff,  gas,  99,  heat  losses  in,  175  ff, 
in  parallel,  148,  in  series,  149, 
ionising,  92,  loss  (heat),  172,  446, 
460,  462,  500,  manufacture,  152,  175, 
428,  oxidation-reduction,  100,  poten- 
tial, 95,  97,  104,  standard,  104 

Electrodeposited  anodes,  135 

Electrogalvanising,  310 

Electrolysis,  26;  bath,  148,  ff,  causes 
of  loss  during,  30,  concentration 
changes  during,  27,  51,  57,  of  molten 
electrolytes,  159  ff,  407  ff 

Electrolytes,  26  ;  conductivity,  58  ff  ; 
deviations  from  solution  laws,  46  ; 
molten,  158  ff ;  technical,  purity 
desirable,  4,  260,  263 

Electrolytic  cell,  27,  cleaning  of  metal 
surfaces,  309,  conductor,  26,  dis- 
sociation, 67  ff,  oxidation,  34,  144, 
potential,  95  ff,  135,  processes,  87  ff, 
rectifier,  142,  reduction,  34,  126, 
solution  pressure,  92,  121 

Electromotive  force,  6;  induced,  451, 
measurement,  90,  of  accumulators, 
227,  237,  fuel  cells,  210,  212,  primary 
cells,  195  ff,  temperature  coefficient, 
83 

Electron,  183,  187,  189 

Electronic  conductor,  26,  415 

Electroplating,  308  ff 

Electrothermics,  167  ff 

Electrotyping,  308,  315 

El  more  process,  316 

Endosmose,  360 

Endothermic  reaction,  23 

Energy  efficiency,  110 

Equilibrium,  12  ff,  constant,  14 

Equivalent  conductivity,  65,  at  infinite 
dilution,  66 

Equivalent  ionic  conductivity,  66,  71 

Ethylene  chloride,  385 

Exothermic  reaction,  23 


Faraday,  29,  dark  space,  182 

Faraday's  laws,  28  ff,  159 

Faure  plates,  225,  234 

Ferric  oxide,  208,  364,  380,  435,  436, 

443,     493,     494,    compounds,     284, 

288,  380,  417,  426,  489 
Ferro-alloys,  434,  444,  462 
Ferro-chrome,  462,  467,  furnace,  438, 

467,  468 

Ferro-manganese,  454,  462 
Ferro-molybdenum,  462,  468 
Ferro-nickel,  438,  439 
Ferro-silicon,    444,    462,    463,    464    ff, 

469,  487,  anodes,  151,  furnaces,  167, 

172,  173,  438,  464  ff,  473,  477,  488 
Ferro- tungsten,  462,  468 
Ferro-vanadium,  462,  468 


Ferrous  compounds,  239,  260,  288,  300, 
301,  307,  364,  443,  444 

Finlay  cell,  368,  375,  380,  383,  384,  387 

Flaking  of  electrodeposits,  294,  300,  308 

Flaming  arc,  505  ff,  515 

Fluorion,  146,  404,  430 

Fluorspar,  303 

Forming  of  plates,  223  ff 

Free  charged  ions,  68,  75 

Free  energy  decrease,  78,  of  fuel  com- 
bustion, 210  ff 

Frequency,  181,  454,  in  technical  fur- 
naces etc.,  440,  455,  457,  458,  514, 
524,  526 

Frick  furnace,  455,  456 

Fuel  cells,  209  ff,  heating,  168  ff,  209 

Furnace  efficiency,  8,  170,  475,  478, 
494,  500,  gases,  172, 174,  437,  439,  440 

Furnace  electrodes,  consumption,  438, 
439,  441,  446,  448,  450,  460,  461,  466, 
467,  514,  515,  517,  518;  cooling, 
175,  440,  446,  447,  449,  450,  478, 
490,  499,  513,  514,  517  ;  regulation, 
438,  440,  446,  447,  449,  465,  466, 
467,  474,  476,  496,  515 

Fused  quartz,  496 

Galena,  306 

Gall-Montlaur  process,  338 

Galvanic  deposits,  308  ff 

Garuti  electrolyser,  387 

Gas  cells,  212  ff,  constant,  41,  elec- 
trodes, 99,  ions,  183,  189,  524,  power, 
10,  385,  435 

Gases,  electrical  discharges  in,  183,  in 
steel,  444,  459 

Gay  Lussac's  law,  42 

Gelatine,  added  during  forming,  226, 
effect  on  cathodic  deposits,  126, 
269,  302  ff 

Generator- gas  cells,  211 

Gibbs-Helmholtz  equation,  83,  85,  231 

Girard-Street  furnace,  468 

Girod  ferro-alloy  furnace,  468,  steel 
furnace,  444,  445,  449,  450,  457,  461, 
462,  465,  466 

Glow  discharge,  185,  186 

Glue,  303,  304 

Glycerine,  206,  208,  226 

Gold;  anodes,  273,  274,  314,  anodic 
behaviour,  139,  140,  271,  276,  314, 
315,  cathodes,  274,  cathodic,  273, 
278,  cathodic  deposition,  273,  314, 
chemical  solution,  276,  compounds, 
271  ff,  277,  279,  314,  extraction, 
154,  276  ff,  ores,  277  ff,  plating,  314, 
refining,  271  ff 

Goldschmidt  detinning  processes,  288, 
291,  385,  thermite  process,  1,  462 

Graphite  anodes,  151,  152 ;  anode 
effect,  164 ;  chlorine  overvoltage, 
153,  345,  378;  cathodes,  151,  166; 


542 


SUBJECT  INDEX 


electrodes  for  furnaces,  175,  448 ; 
electrodes,  manufacture,  495  ;  elec- 
trode furnace,  495  ;  found  in  car- 
borundum furnace,  490  ;  furnaces, 
493,  495 ;  hydrogen  overvoltage 
at,  119;  in  solid  depolarisers,  198, 
207,  208,  236  ;  manufacture,  492  ff  ; 
resistor,  489 ;  thermal  conduc- 
tivity, 174 

Gravity  cell,  347,  371 

Griesheim-Elektron  cell,  152,  153,  347, 
355,  356,  362  fl,  383,  385 

Grove  cell,  115,  201 

Haanel  reports,  437 

Haas-Oettel  cell,  380,  334,  335 

Hall  cell,  425 

Hargreaves-Bird   cell,    151,  376,    379, 

383 

Hasse  process,  284 
Hayden    process,   253,   257,   258,   327 

(also  Series  system) 
Hearth  electrodes,  438,  445,  449,  450, 

457,  465  ff,  473  ff 
Heat  losses   from   furnaces,  172,  173, 

175  ff 
Heating   of   alkali-chlorine   cells,  364, 

374,  375,  377 

Helfenstein  furnaces,  174,  467,  478 
Helmholtz-Thomson  rule,  78,  85,  335, 

383,  411,  429 
Henry's  law,  21 
Heroult  ferro-alloy  furnace,  466,  467, 

steel  furnace,  174,  442,  444,  447.  449, 

450,  459  ff,  aluminium  process,  425, 

427,  430 

Hessberger  furnace,  517,  519,  521 
Heterogeneous  equilibrium,  16,  system, 

13 

Higgins  furnace,  496 
High  tension  arc,  185,  188  ff,  505 
Hiorth  furnace,  455 
Hoepfner  processes  for  extraction   of 

copper,  264,  nickel,  296,  zinc,  286 
Hoff,  van't,  factor,  47,  48,  70 
Homogeneous  equilibria,  14,  system,  13 
Horizontal  diaphragms,  370,  375 
Horry  furnace,  475 
Horse  power,  6,  hour,  7,  year,  9 
Hot  galvanising,  310 
Hydrazobenzene,  127 
Hydrochloric  acid,  190,  202,  219,  226, 

2.v>.    2r,r,,   271,   274,   287,   309,   338, 

340,  345,  362,  365, 385,  404,  418,  422, 

425,  433 

Hydrofluoric  acid,  304,  338,  404 
Hydrofluosilicic  acid,  302  ff 

^en;    action   on   nitrogen,   385, 
I7't;     anodic    solution,    219, 

chlorin-     cell.     82,    83,    219. 

electrode,  95,  105  ;  electrolytic  pro- 
duction, 386  ff;  in  cathodic  iron, 


300  ;  in  anodic  chlorine,  349,  353  ; 
oxygen  cells,  213  ff  ;  peroxide,  277, 
283,  402,  403,  404 ;  scale  of  potential, 
95  ;  utilisation,  385 

Hydrogen  evolution,  117  ff  ;  in  accumu- 
lators, 228,  238,  electrolytic  reduc- 
tion, 128,  metal  deposition,  125,  278, 
311  ff,  Castner  sodium  cell,  409  ff, 
mercury  cells,  348  ff 

Hydrogen  ion;  discharge  from  brine 
solution,  318,  effect  on  passivit}', 
140,  292,  299 

Hydrogen  overvoltage,  118  ff,  129, 
198,  201 

Hydrolysis  of  halogens,  319 

Hydrosulphites  (see  Hyposulphites) 

Hydro xyl  ion  ;  discharge  from  brine, 
318,  343  ff,  361  ;  migration,  347, 
355,  356,  399  ;  velocity,  367  ff 

Hypochlorites ;  cathodic  reduction, 
'322  ;  chemical  decomposition,  320, 
321,  327,  331  ;  in  alkali  chlorine 
cells,  343  ff,  372,  380  ;  ions,  anodic 
discharge,  323  ff,  336  ff,  343  ;  techni- 
cal production,  327  ff ;  theory  of 
formation,  318  ff,  321  ff 

Hyposulphites,  404  ff 

Impedance,  182,  187,  188 

Induced  current,  451,  electromotive 
force,  451 

Inductance,  180,  181,  187,  188,  451, 
457,  511,  517,  526 

Induction  furnace,  7,  167,  444,  450  ff, 
460,  461,  heating,  167,  451 

International  electrical  units,  5 

lodate  from  hypoiodite.  320,  394,  395 

lodic  acid,  146,  397 

Iodine,  319  ff,  393 

lodion  discharge,  132,  134 

lodoform,  393  ^ff 

Ion,  29,  166,  183 

Ionic  conductivity,  66,  67,  migration, 
49  ff,  mobility,'  71,  transport,  49  ff, 
velocity,  50 

Tonisation,  69,  by  impact,  184,  191,  524 

I  ion  ;  accumulator,  221,  234  ;  affects 
zinc  deposition,  282  ;  effect  on  fused 
/inc  chloride  electrolysis,  425,  on  nickel 
deposition,  294;  electrolytic,  124, 
299  ff.  31<>  :  electrolytic  deposition, 

121,  20!);    electrothermal,  437,  438, 
441  ;       electrothermal      production, 
437    ff ;     for    ferro-alloys,    4<>:*     IT  : 
furnaces,  172,  435,  437  ff  ;    impurity 
in  zinc  extraction,  285,  286  ;   oxides. 
243,'   435,   443,   444,   463,   464,   468 
(see   also  ferrous  and  ferric   oxides, 
tnaqnetite}  ;      refining,     29!)  ;      salts, 

122,  162,  216,  227,  260  ff,   375,  417 
(see  also  ferrous  and  ferric  xultx) 

Iron  anodes- ;  oxygen  overvoltage,  131, 


SUBJECT  INDEX 


543 


132,  145,  386,  401  ;    passivity,  139, 

140,   199,  243,  289,  290,  299,  406; 

technical,  151,  154 
Iron  cathodes  ;   hvdrogen  over  voltage, 

118,   119,   282,   290,   299,   343,    351, 

386  ;   technical,  151,  166 
Irreversible  cell,  80 
'  Irreversible  '  reactions,  12 
Isothermal  process,  79 

Jablochkoff  cell,  210 

Jacques  cell,  214,  216 

Jaice  cell,  353 

Joule,  6 

Jullien  and  Dessolle  process,  316 

Jungner  cell,  211,  215 

Keith  processes  for  copper,  261,  lead, 

302 
Keller   ferro-alloy   furnace,    438,    466, 

steel  furnaces,  449,  450 
Kellner  cells  for  alkali  and  chlorine, 

351  ff,  383,  hypochlorites  (horizontal), 

332,  334,  387,  hypochlorites  (vertical), 

328,  334,  387 
Kieselguhr,  174 
Kilogram-calorie,  7 
Kilowatt,  6,  hour,  6 
Kjellin  furnace,  168,  442,  454,  456  ff 
Kossuth  process,  392 

Lalande  cell,  203,  221 

Laszczynski  processes  for  copper,  261, 
zinc,  285  - 

Laval,  de,  furnace,  167,  503 

Lead;  accumulator,  82,  83,  221, 
222  ff  ;  cathodic  deposition,  302,  303  ; 
compounds,  166  ff,  222  ff,  260,  302  ff, 
388,  389  ;  electrolytic,  124,  302,  304  ; 
electrometallurgy,  301  ff ;  electro- 
plating on,  309  ;  extraction,  305  ; 
hvdrogen  overvoltage,  97,  118,  119, 
130,  227,  230,  342,  343,  386  ;  oxygen 
overvoltage,  386 ;  -sodium  alloys, 
342,  414  ff,  432;  (see  also  chrome 
yellow,  galena,  red  lead,  white  lead) 

Lead  peroxide  ;  anodes,  131,  154  ;  as 
depolariser,  199,  221,  222  ff;  elec- 
trolytic preparation,  388,  389 

Leblanc  process,  384 

Leclanche  cell,  206 

Lime,  189,  208,  262,  285,  444,  483; 
for  carbide  furnaces,  472  ff 

Limestone,  521 

Limiting  yield  in  ozonisers,  522  ff 

Liquid  depolariser,  197  ff,  potential 
difference,  90,  104 

Lithium  salts,  236,  483 

Lixiviation  of  ores,  difficulties,  263, 
265,  281,  285,  287,  297 

Load  factor,  8 


Local  action,  198,  200,  204,  209,  222, 
226 

Low  tension  arc,  185,  187,  189,  .'ill! 

Lowering  of  vapour  pressure,  23 

Luckow  reactions,  388 

Luminous  phenomena  in  gaseous  dis- 
charges, 184,  186,  523 

Lyon  furnace,  439 

MacArthur-Forrest  process,  276,  279 

Macdonald  cell,  380,  384 

Magnesia,  172,  174,  208,  285,  392, 
furnace  lining,  450,  503  (see  also 
magnesite) 

Magnesite  (calcined),  172,  431,  440,  446, 
447,  454,  455 

Magnesium,  407,  416  ff 

Magnesium  salts,  199,  201,  226,  334, 
390  ff,  416  ff,  433;  in  brine  for 
alkali-chlorine  cells,  375,  376,  for 
bleaching  liquors,  327,  334,  for 
chlorates,  340 

Magnetic  field,  510,  512,  513,  leakage, 
452,  457,  losses  in  induction  furnaces, 
452,  455,  properties  of  pure  iron,  299 

Magnetite,  435  ff,  anodes,  151, 153,  279, 
285,  339,  345,  346,  361,  362,  363 

Manganese  ;  compounds,  206,  214,  227, 
396,  443,  444;  impurity  in  zinc 
extraction,  286  ;  steel,  462 

Manganese  dioxide,  398,  399,  anodes, 
151,  154,  285,  depolariser,  199, 
206,  208 

Mansfeld  experiments,  261 

Marchese  process,  260 

Mass  action  law,  14,  72,  102,  192,  520 

Maximum  external  or  useful  work,  78 

Meidinger  element,  201 

Mercury,  cathodes,  129,  130,  151, 
350  ff,  cells,  342,  347  ff,  compounds, 
216,  221,  226,  234,  243,  351,  hydrogen 
overvoltage,  118,  119,  122,  130,  132, 
198,  226,  343,  347,  348,  oxygen  over- 
voltage,  226,  351 

Metal  deposition,  120,  fog,  160,  161, 
419,  420,  422,  428 

Metallic  conductors,  26,  415 

Metaphosphoric  acid,  500 

Migration  of  ions,  49  ff,  355  ff,  ratios, 
(see  Transport  numbers) 

Moebius  process,  258,  266,  269 

Molecular  concentration,  14,  osmotic 
pressure,  42,  weight  of  dissolved 
substances,  determination,  43  ff 

Molten  electrolytes,  158  ff 

Mond-Langer  cell,  217 

Monochloracetic  acid,  73,  385 

Monox,  486,  487 

Multiple  system,  253  ff,  259 

Multipliers,  111 

Nathusius  furnace,  450,  457 


544 


SUBJECT  INDEX 


Negative  characteristic,  185,  187,  189, 
electrode  or  pole,  27,  plates,  225,  226, 
231,  232,  234,  243 

Neotherm  cell,  203 

Nickel ;   alloys,  454,  anodic  behaviour, 

292,  as  catalyst,  385,  cathodes,  297, 
298,  339,  399,  400,  408,  compounds, 
134,  221,  234  ff,  292  ff,  320,  327,  339, 
401,     electrolytic,     124,     136,     294, 
295,     297,     298,     300,     electrolytic 
deposition,    121,    126,    293,    electro- 
metallurgy,    292     ff,     fusion,     454, 
hydrogen   overvoltage  at,  118,  119, 

293,  343,  matte  anodes,  298,  plating, 
309,  311 

Nickel  anodes,  292,  295,  296,  299,  311, 
312  ;  oxygen  overvoltage,  131,  132, 
134,  145,  passivity,  139  ff,  154,  292, 
408 

Nickel  ores,  electrothermal  treatment, 
438,  439,  504,  extraction,  292,  296  ff 

Nitrates,  129,  283,  479 

Nitric  acid  as  forming  agent,  224,  225, 
from  air,  479,  505  ff,  from  nitrous 
gases,  519  ff,  in  primary  cells,  199, 
201 

Nitric  oxide  equilibrium,  24,  506, 
formation  in  electrical  discharge,  186, 
505  ff,  oxidation,  519  ff,  technical 
formation,  510  ff 

Nitrites,  479,  505,  520 

Nitrobenzene,  127,  129,  385 

Nitrogen  ;  absorption  by  carbide,  472, 
479  ff;  fixation,  478;  in  blast 
furnace,  435 ;  in  fuel  combustion, 
168;  oxidation  in  arc,  479,  506  ff ; 
oxidation  in  silent  discharge,  190, 
507,  524;  oxides,  190  ff,  479,  507, 
519, 520, 524  ;  reduction  to  ammonia, 
385,  479 

Nitrosobenzene,  127 

Nitrosyl-sulphuric  acid,  216 

Nitrous  gases,  519  ff 

Noble  potential,  96 

Nodules  on  cathodes,  150,  255 

Normal  electrodes,  95,  105 

Oerlikon  electrolyser,  387 

Oettel  chlorate  process,  339 

Ohm,  6 

Ohm's  law,  5,  75 

Osmometer,  39 

Osmotic  pressure,  38  ff 

Otto  ozoniser,  527 

Outhenin-Chalandre  cell,  365,  383 

Overvoltage,  118,  129,  145,  bromine, 
132,  chlorine,  132,  hydrogen,  118, 
oxygen,  130,  theory,  133  ff 

Oxidation,  electrolytic,  144  ff,  of  nitro- 
gen, 505  ff,  -reduction  cell,  102, 
-reduction  electrode,  101,  196 

Oxidising  agent,  103 


Oxygen,  combination  with  nitrogen, 
505  ff,  discharge  at  porous  anodes, 
345,  disturbs  zinc  condensation,  502, 
electrode,  131,  electrolytic  produc- 
tion, 154,  386,  from  alkali-chlorine 
cells,  343  ff,  361,  in  anodic  chlorine, 
362,  383,  overvoltage,  130  ff,  145, 
ozonisation,  522  fi 

Oxygen  evolution,  130  ff,  in  accumu- 
lators, 228,  238,  in  electrolytic  oxida- 
tion, 144 

Ozone,  522  ;  anodic  formation,  131,  387, 
402,  403;  equilibrium,  191,  522; 
formed  in  silent  discharge,  186, 522  ff  ; 
oxidises  nitrous  gases,  521  ;  techni- 
cal production,  525  ff ;  thermal 
production,  522  ;  tube,  523 

Ozonisers  (technical),  525 

Parallel  connections,  148 

Parkes  process,  284,  301,  302,  305,  503 

Partition  coefficient,  22 

Passivity,  139  ff 

Pasted  plates,  223,  225 

Pattinson  process,  301 

Pauling  process,  512,  514,  516,  519,  520 

Perchlorates,  341,  401,  403 

Perchloric  acid,  224,  225,  401,  402 

Periodicity,  181 

Permanganates,  398  ff 

Permeability  of  diaphragms,  156,  369  ff, 
376 

Persulphates,  402  ff 

Persulphuric  acid,  283,  285,  402  ff 

Petroleum  coke,  494,  495 

Phase,  16,  difference,  180,  511,  rule,  17 

j8-phenyl  hydroxylamine,  127 

Phosphorus,  electrothermal  produc- 
tion, 497,  498,  500;  furnaces,  500, 
501,  503  ;  in  ferro  alloys,  462,  ferro- 
silicon,  464  ;  removal  from  steel,  443, 
459 

Pinch  effect,  453 

Plante  plates,  223 

Platinised  platinum  electrodes ;  me- 
chanically weak,  151,  152,  329,  345  ; 
overvoltage  at,  131,  132,  133,  318 

Platinite  solution,  315 

Platinum  ;  black,  213,  217,  339,  345  ; 
catalyst,  404 ;  cathode,  151  ;  elec- 
trolytic, 124 ;  hydrogen  overvoltage, 
118  ff,  151,  342  ;  -iridium  electrodes, 
151,  329,  331,  333  ;  oxide,  131,  133, 
390  ;  plating,  315 

Platinum  anodes,  acid  formed  at,  367, 
attached  by  anodic  gases,  151,  152, 
274,  285,  404,  bromine  overvoltage 
at,  132,  390,  chlorine  overvoltage  at, 
132,  133,  153,  318,  345,  oxygen  over- 
voltage  at,  131,  132,  154,  318,  345, 
402,  passivity,  139,  151,  152,  274, 
technical,  151 


SUBJECT  INDEX 


545 


Poggendorff  cell,  202 
Polarisation,    106   ff,   discharge  curve 
113,  228,  in  conductivity  measure- 
ments, 60,  voltage  or  E.M.F.,   113 
256,  267,  304,  348,  383 
Pole,    27,    plates,    457,    458,    reagent 

paper,  28 

Porosity  of  carbon  anodes,  153,  331, 
345,  depolarisers,  198,  diaphragms 
156,  370,  376 

Positive  column,  186  S,  523,  electrode 
or  pole,  27,  plates,  223,  225,  232, 
235,  240 

Potassium  chlorate,  364,  402,  electro- 
lytic production,  338  ff 
Potassium    chloride,     104,     162,    338, 
340,  349,  364,  402,  417,  422,  electro- 
lysis of,  344,  356  ff,  361 
Potassium  chromate,  prevents  cathodic 
reduction,  322,  327,  340,  394,  397,  403 
Potassium  compounds,  57, 146, 159, 199, 
202,   285,   312,   313,   314,   319,   340, 
391,  393,  394,  395,  398,  399,  401,  402, 
404 
Potassium  cyanide,  270,  276  ff,  312  ff, 

398,  480 

Potassium   hydroxide,   221,   234,    236, 
386,    388,   electrolytic   liquors,    364, 
376,  electrolytic  production,  342  ff 
Potential  difference,  5 
Power,  5,  factor,  179,  production,  8  ff 
Preheating  of   charge,  438,    439,   440, 

499,  503,  514,  517 
Primary  cells,  3,  195,  circuit,  451 
Producer-gas  cells,  211 
Purification  of  brine,  375,  380 
Pyridine,  130 
Pyrrhotite,  438,  439 

Quantity  of  electricity,    5,    measure- 
ment, 31 
Quartz,  496 
Quartzite,  464 
Quinone,  130 

Radiation  losses  from  furnaces,  172,  518 

Rapid  forming,  224 

Rathenau  furnace,  465 

Rathenau-Suter  sodium  process,  408, 
412 

Reactance,  182 

Reaction  resistance,  12,  121,  136,  144, 
146,  250,  velocity  effect,  114,  121, 
128,  139,  141,  142,  201,  206,  395 

Red  lead,  226 

Reducing  agents,  103 

Reduction,  electrolytic,  126  ff 

Refractory  ores,  169,  442,  504 

Regeneration  of  chromic  acid,  395, 
depolariser,  199,  203,  electrolyte,  257, 
267,  269,  270,  274,  279,  284,  291, 
304,  340,  397 


Regulation  of  arc  discharge,  187 

Resinate,  323 

Resistance,  5,  determination,  59,  60,  de- 
polarisers, 198,  207,  236,  diaphragms, 
157,  369,  375,  electrolytes,  58  ff, 
furnaces,  167,  170,  174,  heating,  167, 
169,  171,  primary  cells,  198,  201  ff, 
208,  209,  secondary  cells,  229,  237, 
technical  cell,  150 

Resorcinol,  126 

Reversible  process,  78 

'  Reversible  '  reaction,  12 

Rhodin  cell,  354 

Rochling-Rodenhauser  furnace,  450, 
452,  454,  456  ff,  459  ff 

Roesing  process,  284 

Rossler-Edelmann  process,  284 

Rotating  arcs,  510,  512,  cathodes,  270, 
286,  287,  316,  317,  furnaces,  446, 
475,  magnetic  field,  510 

Salom  process,  306 

Sault  Ste.  Marie  experiments,  438,  442 

Savelsberg-Wannschaff  process,  297 

Schmidt  electrolyser,  387 

Schonherr-Hessberger  process,  512,  516, 
518,  519 

Schoop  electrolyser,  387,  388 

Schuckert  electrolyser  for  hypochlorites, 
331,  334,  for  hydrogen  and  oxygen, 
388 

Secondary  cells,  220  ff,  circuit,  451,  457 

Self  discharge,  226,  227,  238,  induction, 
181,  452,  457 

Semi-permeable  membrane,  38 

Series  system,  253,  258,  387 

Short  circuiting,  222,  302,  428 

Shunt  circuit,  351,  current  losses,  258, 
327,  329,  331,  340,  387 

Siemens-Halske  processes  for  copper, 
262,  264,  284,  gold,  277,  zinc,  284 ; 
ozoniser,  525,  527 

Silent  discharge,  188,  507,  522 

Silica,  172,  174,  447,  463  ff,  486  ff, 
493,  496 

Silicon  carbide,  486,  487,  488  ff; 
furnaces,  488 ;  monoxide,  486, 
487,  490  ;  oxycarbide,  486  ;  techni- 
cal production,  487 

Siloxicon,  172,  486  ff,  492,  494  ff 

Silundum,  492 

Silver  anodes,  266,  269,  313,  cathode, 
266,  cathodic,  125,  266,  267,  268,  269, 
compounds,  159,  221,  252,  266  ff, 
274  ff,  313,  coulometer,  32,  266, 
electrolytic  refining,  265  ff,  extraction, 
270,  hydrogen  overvoltage,  118, 
plating,  313,  -zinc  alloy,  284,  503 

Slimes,'  277 

Sodium  ;  alloys,  d42  ;  amalgam,  122, 
342  ff;  bicarbonate,  338,  378; 
bichromate,  202,  338;  carbonate 


546 


SUBJECT  INDEX 


355,  377,  383,  384,  389,  393,  395, 
412,  433,  483,  520;  chlorate,  319, 
338  ff,  388,  389,  402;  chromate, 
329,  389  ;  deposition  from  aqueous 
solution,  122,  342,  343,  347  ff,  from 
fused  caustic,  408  ff,  from  fused 
salt,  413  ff;  hypochlorite,  199, 
319  ff ;  hyposulphite,  404  ff ;  -lead 
alloy,  342,  414,  415,  432  ;  nitrate, 
159,  408,  478,  520  ;  phosphate,  146, 
398 ;  resinate,  323,  332  ;  salts,  57, 
199,  226,  315,  393,  398,  401  ff,  410, 
415,  429,  480,  520,  521  ;  sulphate, 

310,  349,  389,  390,  406;    sulphide, 
287,  298,  305;    sulphite,  312,  404; 
technical  production,  408  ff 

Sodium  chloride,  162,  255,  286,  298, 
338,  363,  389,  391,  393,  416,  417, 
421  ff,  427,  480,  483,  489  ff ;  caustic 
soda  from,  342  ff,  431  ;  chlorate  from, 

335  ff  ;  electrolysis  of,  318  ff,  342  ff, 
413  ff,  431  ff ;    electrolysis  of  acid 
solutions,  338,  of  alkaline  solutions, 

336  ff;    hypochlorite  from,  318  ff ; 
sodium  from,  413  ff 

Sodium  hydroxide,  199,  203,  288  ff, 
334,  386,  426,  520,  521  ;  diffusion, 
361  ;  molten,  408  ff,  414  ff,  431  ff, 
electrolysis,  408  ff ;  technical  pro- 
duction, 347  ff,  362  ff,  371  ff,  431  ff 
Solid  depolariser,  197,  198,  solution,  18, 

121,  133,  134,  136  ff,  241,  492 
Solubility  product,  73 
Solution  laws,  39  ff 
Solvay  cell,  352,  354 
Specific  conductivity,  58  ff 
Stable  arc,  production,  187,  516 
Standard  cell,  6,  electrode,  104 
Stannic  compounds,  56,  288  ff 
Stannous  compounds,  288,  289 
Stassano  iron  furnace,  437,  steel  fur- 
nace, 167,  444,  446,  461,  503 
Steam,  363,  374,  377,  432,  480  ;  power, 

10 

Steel;    electric,  434,  442  ff;    electric 
refining,  434,  442  ff,  459  ff  ;  furnaces, 
7,  172,  174,  444  ff,  454  ff,  459  ff; 
losses  in  steel  furnaces,  447,  448,  450, 
455,  459,  460  ;   refining  process,  443 
Sublimation  pressure,  18 
Sulphate  ion,  134,  349,  397,  403 
Sulphating,  231 
Sulphide  anodes,  260,  261,  298 
Sulphur,  499, 500,  chloride,  385,  dioxide, 

215,  224,  262,  272,  488 
Sulphuretted  hydrogen,  306 
Sulphuric  acid,  202,  255,  274,  306,  309, 

311,  315,  386,  387,  395,   396,  520, 
for    hydrogen    peroxide,    404,    for 
lixiviating  ores,  262,  284,  in  chromic 
acid     regeneration,     396,    397,     in 
Jungner  cell,  215,  in  lead  accumu- 


lator,  221  ff,  in  primary  cells,   199, 
201,  202 
Swinburne- Ashcroft  process,  421,  423 


'  Tailings,'  277 

Tantalum,  143 

Tapping  furnaces  (carbide),  470 

Tapping  of  furnaces  (electrical  method), 
467,  476,  488 

Taylor  furnace,  498 

Technical  electrolytic  bath,  148  ff 

Temperature  coefficient  of  electro- 
motive force,  83,  of  Daniell  cell, 
84,  200,  of  fuel  cell,  212,  of  iron 
accumulator,  237,  of  lead  accumu- 
lator, 227 

Temperature ;  effect  on  ammonia 
equilibrium,  23,  anode  effect,  164, 
165,  bath  voltage,  150,  carbide  equili- 
brium, 470,  471,  chlorate  formation, 
337,  338,  combination  of  nitric 
oxide  and  oxgyen,  519,  condition  of 
cathodic  deposits,  125,  copper  re- 
fining, 246,  256,  electrolytic  disso- 
ciation, 69,  electrolytic  oxidation, 
145,  electrolytic  reduction,  129,  130, 
equilibrium,  23,  furnace  efficiency, 
170,  hypochlorite  concentration,  325, 
326,  337,  molten  sodium  hydroxide 
electrolysis,  408,  415,  nitric  oxide 
equilibrium,  24,  506,  overvoltage, 
120,  132,  133,  ozone  production, 
522  ff,  passivity,  140,  271,  quality  of 
anodic  chlorine,  346,  specific  con- 
ductivity of  electrolytes,  64,  158, 
steel  refining,  443,  444,  459  ;  of  arc 
discharges,  189,  190,  506,  508,  509, 
of  electric  heating,  169,  of  fuel 
heating,  169 

Testing  of  carbon  anodes,  346,  of 
diaphragms,  156 

Thermal  efficiency  of  electric  furnaces, 
170,  of  fuel  fired  furnaces,  170,  of 
zinc  distillation,  501 

Thermite  process,  3,  462 

Three-phase  currents,  439,  440,  446, 
448,  450,  452,  458,  466,  477,  478,  518 

Tin,  138,  250,  288,  309;  anodic 
behaviour,  289,  cathodes,  129,  288, 
342,  electrolytic,  124,  125,  291, 
electrometallurgy,  287  ff,  hydrogen 
overvoltage  at,  118,  119,  290,  342, 
ores,  504 

Titantferous  iron  ores,  438,  443 

Titanium  compounds,  130,  199,  426,  438 

Total  energy  decrease,  78,  of  fuel 
combustion,  209 

Townsend  cell,  155,  375,  378,  383,  384 

Transformer,  451,  losses,  451,  452 

Transport  of  ions,  49,  numbers,  52  ff, 
528 


SUBJECT   INDEX 


547 


Transportable  accumulators,  230,  234, 

238 

Trinitrochloro- benzene,  385 
Tudor  plates,  224,  226 
Two-phase  currents,  441,  499 
Tyndall-de  Vries  ozoniser,  526 

Unsaturated  fatty  acids,  385,  glycerides, 
385 

Vanadium  salts,  130,  199,  216,  323 

Vertical  diaphragms,  370 

Volt,  6 

Voltage,  6,  dependent  on  furnace  charge, 
464  If,  473,  gaseous  discharges,  185  ff, 
losses  in  leads  and  contacts,  150,  256, 
424,  474,  measurement,  111,  series, 
96,  technical  electrolysis,  148,  149 

Voltage  gradient  in  alkali-chlorine 
cells,  367  ff,  diaphragms,  369,  electric 
gaseous  discharges,  186,  189,  191,  509 

Voltmeter,  111,  calibration,  111,  for 
current  measurement,  31 

Volume  changes  in  accumulator  plates, 
225,  226,  234 

Vosmaer  ozoniser,  526 


Water ;  action  on  nitrous  gases,  519  ; 
coulometer,  33,  36 ;  dissociation 
constant,  117  ;  electrolysis,  132,  386  ; 
power,  9 

Watt,  6,  second,  6 


Wavellite,  500 

Welsbach  accumulator,  221 

Wet  galvanising,  310 

Wheatstone's  bridge,  59 

White  lead,  302,  303,  388 

Whiting  cell,  352,  383 

Wildermann  cell,  854,  383 

Wohlwill  process,  271  fl,  275,  278 

Working  cell,  conductivity,  62,  voltage, 

110 

Wright  electricity  meter,  34 
Wrought  anodes,  135 
Wiinsche  cell,  391 


Zinc,  250,  277  ff,  302,  309,  404  ff,  502, 
503,  amalgamated,  36,  198,  201, 
208,  221,  anodes,  198  ff,  221,  284, 
cathodes,  36,  129,  130,  284,  334,  340, 
cathodic  deposition  from  aqueous 
solution,  120,  122,  126,  280  ff,  chemi- 
cal extraction,  281,  497,  501,  chloride, 
199,  206  ff,  286,  310,  418,  421  ff, 
compounds,  133,  199,  200,  221, 
283  ff,  310,  313,  404,  421,  424,  502, 
503,  distillation,  502,electrodeposited, 
282,  284,  285,  286,  287,  electrolytic 
extraction,  154,  284  ff,  421  ff,  electro- 
thermal production,  498,  501  fl,  from 
fused  zinc  chloride,  407,  421  ff,  fur- 
naces, 503,  504,  hydrogen  over- 
voltage,  118,  282,  405,  refining,  284, 
vapour  condensation,  502,  503 


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